Tech News

Buying the right quality tree.

James Will
Senior Lecturer, Burnley College, The University of Melbourne

Recommendations

  • Choose a complete growing system that will give you the appropriate root quality, free from root deformation
  • Make certain that the root mass is appropriate to support the canopy (and establish quickly in the landscape)
  • Check the canopy development to assure easy tree management after planting without immediate pruning
  • Check the trunk taper to assure that the plant will require staking only for protection.

The growing system

Trees can be produced in Victoria using these growing systems: bare-root production, ball & burlap (hessian) techniques, container-grown and modified-in-ground production.

Bare-root production requires careful undercutting of stock in the nursery, precise timing for lifting of trees in winter and almost immediate planting. Since the trees are lifted without any soil, concerns about the root system and post-lifting care are paramount. Also, some trees will respond well to bare-root production (deciduous trees including most Fraxinus and some Quercus) while other varieties will die after typical bare-root production. If well-produced, the tree will be undercut to develop a multi-branched, fibrous root system: undercutting is essential to ensure adequate root mass. Bare-root production can save money in the price of each tree, shipping and installation, but post-planting maintenance must be ideal for success.

Ball & burlap production is possible for most trees. Like bare-root production, undercutting in the nursery is of paramount importance in producing a high-quality tree. Also, root balls must be appropriately-large for the size of the canopy, otherwise the tree will have inadequate root mass for rapid establishment. Soil balls can be damaged in transit and once they are damaged, the root system can be severely affected. Ball & burlap systems have a limited time each year when lifting can be done properly, and planting should occur within a few weeks post lifting. Like bare-root production, ball & burlap systems require a thorough knowledge of the nursery’s methods to ensure correct quality of the root systems.

Container production is the most common production method used in Victorian tree production. Container growing allows harvest of all roots, easy shipping/handling, and harvesting throughout the year. For success, container production requires:

  1. Containers large enough for the canopy size,
  2. Containers designed to reduce root spirals, and
  3. Container profile broad and shallow.

Container size

The root mass of the tree must be in balance with the size of the canopy. You cannot grow a 4m tree in 20 litres of soil mix and establish it quickly in the landscape: this root-to-shoot imbalance will produce a difficult-to-establish tree. We recommend the following container sizes to canopy height to give the correct root-to-shoot ratio:

Container Maximum Height
15-20 litre container 1.5 to 2.0 m maximum height
40-50 litre container 2.1 to 3.0 m maximum height
75-100 litre container 3.1 to 4.0 m maximum height

Container design

In round pots with straight, slick sides, roots that grow to the edge of the pot will circle. When planted in the landscape, these spiralled roots will restrict the size of the tree’s root system. Because of this, containers should have vertical scallops or ridges that will slow this spiralling. Unfortunately, these ridges alone will not stop root spiralling; air pruning or copper pruning is needed to stop root spiralling. These two root control systems also stop roots as they reach the pot walls and encourage root branching within the root mass. Both SpringRing® (air pruning) and SpinOut® (copper pruning) will successfully control root spiralling and give high root mass within the container.

The root systems of all trees are not the same.

  1. Lagerstroemia root system that shows the vigorous surface roots of this genus
  2. Acer, showing typical surface and support roots
  3. Quercus, showing the sparse rooting associated with this genusNo growing system will be best for all tree genera.Tree root systems, a, b and c

Adapted from Gilman, E.F., 1997, Trees for urban and suburban landscapes. Used with permission.

Container profile

When trees are grown in the ground, the majority of the roots are in the top 40cm of soil. Most pots above 40 litre volume will be over 40cm deep, encouraging root growth where it would not normally occur. Ideally, containers should be as broad as possible and no deeper than 40cm. This will give the greatest mass of roots in this optimal area, and allow best establishment. Also, containers that are broad and shallow (a) will not require placing in the subsoil when planted, and (b) will not require as much staking in the nursery to keep them upright.

Canopy form

When buying planting stock, consider buying stock that has the correct form to develop into an ideal tree rather than looking like a fully-formed tree in miniature. This will minimise pruning in the landscape. These factors should be noted:

  1. no bifurcations on any street tree stock,
  2. radial branch attachment around the trunk,
  3. strong branch attachment (>45º angles to trunk),
  4. trees appropriately limbed-up to above 1.8m, and
  5. twiggy, over-formed canopies should be avoided.

Trunk taper and staking

Although you will want straight trees to plant into the landscape, these trees must also have trunks strong enough to support the canopy weight. If the canopy is appropriately pruned to reduce over-twigginess, all tree taxa can support canopy weight without staking. Weak trunks result from over-staking in nurseries, when trunks never develop to maintain tree weight. When trunks are staked, they form a cylinder that is not particularly strong. When the stakes are removed, often these trunks will bend over, not able to support the canopy weight. If stakes are removed early, as soon as the trunks are straight, the trunks will begin growing naturally – larger diameter at the base than at 1.4 to 1.8m tall. These tapered trunks will support canopy weights without bending.

Trees with strong, tapered trunks will not require staking in the landscape for establishment. Also, if the soil balls are broad and shallow, the trees do not require staking to keep the roots from moving. Staking is necessary only to reduce vandalism and lawn mower damage. Stakes only need to be 60cm high, and ties to these stakes should be loose enough to permit trunks to move in the wind.

You can easily check the trunk taper of a containerised tree by simply bending the trunk slightly. The “good tree” will flex in the container, while the “bad tree” will pull the roots at the surface of the container without flexing.

Adapted from Gilman, E.F., 1997, Trees for urban and suburban landscapes. Used with permission.

Final notes

  • Remember that faults can begin very early in the life of the tree, so get as much information about total production as possible, and
  • Check the overall health of the tree; slight damage (such as lerp damage on eucalypts) is inconsequential, but major damage (such as serious leaf necrosis) can indicate major health problems.

Disease problems in the Genus Platanus

Mr John S. Brereton

In Melbourne as in other parts of the world various forms of Platanus are widely grown suiting the parks, boulevards and avenues to be found in urban areas where large ornamental trees (25-30m height, 20-25m width) are preferred. The Plane often tolerates polluted air and compacted soil conditions making an ideal hardy choice for roadsides and streetscapes. The Plane generally exhibits a degree of drought tolerance under Australian conditions but is sometimes a problematic tree selection due to outbreaks of Microsphaera alni, Powdery Mildew and Apiognomonia veneta (asexual: Discula platani) Plane Anthracnose. The latter disease problem is more serious and apart from the obvious aesthetic problems post infection the disease also places the trees under stress. Stressed trees are likely to be more susceptible to other infectious and non-infectious problems including insect attack.

Planes are generally known world wide as being susceptible to Plane Anthracnose a fungal disease caused by Apiognomonia veneta. This disease problem is also refered to in the United States as sycamore anthracnose. The Sycamore (Platanus racemosa) specifically the endemic American form, is generally considered to be the most susceptible to anthracnose. The London plane (Platanus x acerfolia) is less susceptible and Oriental Plane (Platanus orientalis) exhibits a high degree of resistance by comparison. Cultivars of Platanus x acerfolia ‘Liberty’ & ‘Columbia’ are found to be faster growing and more resistant to anthracnose than the parent.

Anthracnose symptoms expressed include the following:
Spring twig blight destroys previous years growth at the tips;

Destruction of buds;
Blight of new growth and young leaves;
Cankers may form in older more mature trees growing at the base of twigs and girdling new shoots;

Major leaf blight causing leaf distortion and marked brown necrotic areas on leaves crossing over the veins. These areas enlarge and will often cover most of the leaf surface. Leaf drop may follow in severe cases. This disease in leaves should not be confused with scorching from heat and drought stress. The distinctive elongated brown lesions on leaves crossing over veins should avoid any confusion with drought stress.

In young trees under nursery growing conditions it may also be more difficult to maintain strong apical dominance as terminal buds are destroyed.

Planes receiving adequate water and nutrients usually re-foliate in summer.

Climatic conditions favouring outbreaks of anthracnose. Outbreaks of anthracnose are more likely when during periods of wet weather in spring and early summer, with the disease surviving and over-wintering

The fungal organism over winters in cankers in twigs and branches

During the spring, spores which lie within infected tissue are produced and released to be spread by wind and rain or water splash.

Management of anthracnose.

  • The maintenance of tree vigour is important with adequate nutrient and water supply.
  • Removal of infected twigs and branches by pruning is beneficial. This will be usually confined to peripheral growth resulting in limited control of the disease problem
  • Under nursery conditions do not use overhead irrigation methods that will result in long periods of leaf wetness and high humidity levels
  • Maintain adequate air circulation between trees.
  • Raking up infected leaves twigs which have fallen may have little impact on disease control as parts of the tree are already infected and a source of infective spores.
  • Applications of fungicides such as copper formulations, triadimefon, chlorothalonil and mancozeb have assisted to contain but not eradicate this disease problem. Fungal applications are usually applied at approximately 10 to 14 day intervals from bud swell. The application must be strictly in accordance with all label instructions. The application method must ensure adequate coverage particularly under nursery conditions.

Essential establishment irrigation information

A strategy for successful tree establishment

Mr Geoff Connellan
Principle Lecturer, Burnley College , The University of Melbourne
Consultant Engineer

 

Successful tree establishment and continued growth is the result of sound planning and ongoing careful maintenance. It should not be a matter of luck or chance. There are numerous factors that can damage or kill newly planted trees. These include climatic conditions, mechanical damage (above and below ground), pests and diseases, compaction, water logging and lack of water.

Access to adequate soil moisture is the single most important factor that will determine the successful establishment of many urban trees.

It is critical that new tree plantings have a watering program that will give the tree the greatest chance of success. Key issues including how much water to apply, how often to apply water and where to apply it all need to be considered.

Encouragement of the development of the root system so that maximum utilisation of the surrounding soil volume should be the key strategy in tree establishment. This approach, which maximises access to soil water, will provide the greatest chance of success.

Tree Water Use

In order to establish and successfully manage a tree, it is important to have an appreciation of the amount of water required by the tree.

The rate at which trees use water depends on the water use characteristics of the plant, the stage of development and the prevailing environmental conditions. When considering the water use rate for a particular species, it is the changing climatic conditions that will have greatest influence on the water use rates. Extreme climate conditions of high ambient air temperature, low relative humidity, high wind speed and high levels of solar radiation are all associated with high water consumption by trees.

The rate of tree water use can be estimated by taking into account:

  • Total leaf area of the tree – water can be evaporated/transpired from all of the leaves (Represented by Leaf Area Index value)
  • Evaporation capacity of the atmosphere
  • Water use characteristics of the particular species (high or low water user – Crop Factor value)
  • Available water in the soil

A guide to water requirements of trees in Melbourne of selected dimensions and properties is presented:

Table 1: Weekly water use rates of selected trees in Melbourne .

Crown Diameter Summer h Autumn/Spring h Winter h
500 mm 10 L 5.5 L 2 L
1.0 m 40 L 22 L 8.5 L
2.0 m 158 L 87 L 34 L
3.0 m 366 L 201 L 78 L
6.0 m 1426 L 784 L 305 L>

Notes:

  • It has been assumed that the tree has a Leaf Area Index of 2.0
  • A Crop Factor of 0.6 has been assumed.
  • Average evaporation data for Melbourne has been used.

The total foliage area (total surface area of all of the leaves, one side only) needs to be taken into account as this reflects the total potential transpirational area of the tree. The Leaf Area Index (LAI), which is the ratio of the total leaf area of a plant to the projected plan area (C A), is used to calculate the total leaf area. LAI values are typically in the range of 2.0 to 5.0 for trees. A Crop Factor value for the tree can be used in conjunction with an evaporation value to determine the water use rate. For established trees, values of crop factor are generally in the range of 0.3 to 0. 8.

How Much Water To Apply

The amount of water that needs to be applied through irrigation is dependent on the size of the deficit between the water available in the soil through rainfall and the total water used by the tree.

When a new tree is planted the amount of water available from the soil/media is relatively small and hence frequent applications of water is required to ensure the root system, which often has been damaged, has access to water on a continuing basis.

Table 2: Guide to water available in various size soil volumes

Soil/Root System Diameter Soil Depth Water Available (20% of total volume)
200 mm 200 mm 1.2 L
300 mm 300 mm 14.2 L
500 mm 300 mm 47.1 L
1.0 m 500 mm 314 L
2.0 m 500 mm 629 L
3.0 m 500 mm 1414 L

Notes:

  1. The total soil volume has been determined using a cylindrical shape.
  2. It has been assumed that the available water is 20% of the total soil volume.

Reference to Table 2 shows that only very limited amounts of water are available from soil storage with newly planted trees (For example, a soil root system 300 mm diameter, 300 mm deep, available water is 14.2 Litres)

It is therefore very important that:

  1. Water be supplied regularly to newly planted trees to ensure survival maintain growth and
  2. Development and expansion of the root system is encouraged so that the tree gains access to increasing volume of water in the surrounding soil.

Water Management Strategies for Newly-Planted Trees

  • Maintain adequate soil moisture – match supplemental irrigation water needs to climate conditions and available soil water
  • Water effectively – apply water so that it reaches the tree root system with minimal losses
  • Encourage extension of the root system – apply water to extremity of root system and beyond
  • Remove competition for water – mulch around the tree

Implementing the Water Management Strategies

Maintain soil moisture

The care of trees in the early stages of transplanting is critical. The tree canopy continues to demand water and if adequate amounts are not available, from the root ball, then the tree will become stressed.

In addition to the relatively small volume of water available, the tree often has diminished capacity to absorb water due to root damage and transplant shock.

Daily monitoring of soil moisture is recommended for the first couple of weeks.

The variation in demand by the tree for water is large. A period of windy conditions with hot dry air can very quickly increase water demand by a factor of several times. Soil water reserves can be quickly depleted.

Water supplied through irrigation should be matched to the weather conditions and stage of growth of the tree. It should not be done on a purely fixed period (eg. once every two months) basis throughout the whole year.

Water effectively

Delivering water to the tree root zone, with minimal losses, should be the aim of irrigation.

There are a number of characteristics and properties of trees that present challenges in achieving high irrigation efficiency. These include:

  1. Roots located within a relatively confined ground area
  2. Roots located deep within the soil profile – water needs to be delivered at depth
  3. Compacted soils (low infiltration rates) at some sites

The choice of irrigation method and the operation of the system is very important.

A sound recommendation for the watering of trees is that the watering should be slow and deep. Water should be applied slowly so that runoff does not occur and deep so that the soil volume, where the bulk of the roots are located, is watered rather than just the top layer of soil.

It is advisable that some form of piped irrigation system be installed to aid the establishment of newly planted trees. The system may only be used for the first two or three years and then discarded.

Manually based watering systems are often prone to failure due to missed waterings, inappropriate and ineffective applications. If manual watering is all that is available, then planting must include some provision for water direction; either an earth berm should be formed around the planting hole with the backfill soil, or one of the proprietary root collars should be installed. Use of aggie pipe to “channel water to the rootball” does not fulfil the slow, deep water requirement.

Encourage root extension

Watering of a newly planted tree really has two purposes. One is to satisfty the immediate water needs of the tree and the second is to provide a moist soil environment into which the roots will develop and extend.

Both the irrigation technique and the control of the application of water need to be considered. The applied water needs to penetrate the soil profile to an appropriate depth. Slow application rates over long periods of time are generally best. Drip irrigation is well suited to this situation.

Water needs to be applied beyond the tree dripline to encourage root into this soil volume.

As a guide water can be applied to a circular area defined by a radius of 50% of the dripline and 150% of the dripline.

Irrigation drip emitters should be positioned to encourage roots to develop away from the trunk. In the first season of growth the emitter may be placed between the trunk and dripline and then, following establishment, moved beyond the dripline to the surrounding soil. This may involve installing drip emitters, using flexible tubing, to allow the emitter to be moved as the tree root system develops.

Remove competition

The ground area around the tree should be mulched. A depth of 75 mm or greater depth is generally recommended. By reducing the water evaporation from the soil and eliminating water wastage, through prevention of weed growth and grass intrusion, the mulch provides several water efficiency benefits.

An area of mulch around the tree also provides a protection zone, which allows maintenance, both mechanical (mowers and trimmers) and chemical to be carried out with minimum risk to the tree.

Another benefit of the mulch is the modification of the soil temperature so that the tree roots experience more stable temperatures and hence provide greater opportunity for healthy tree growth.

Due to the water absorbent properties of organic mulch, it is important to recognise that both rainfall and irrigation water can be absorbed within the mulch and may not penetrate to the soil.

Selecting an irrigation technique – Critical factors

Each irrigation method has its own characteristics and features.

The key performance characteristics to consider are:

  1. Wetted area
  2. Precipitation rate
  3. Efficiency of application
  4. Flexibility in application
  5. Robustness and sensitivity to vandalism

The following are key or critical features that are relevant to the selection or exclusion of a particular irrigation method for urban trees.

Sprinklers

  • wetted radius large – large area covered
  • top soil layer wetting only
  • system precipitation rate suitable to most soils

Sprays

  • wetted radius small
  • top soil layer wetting only
  • system precipitation rate very high
  • need to ensure water stream is free of obstruction eg. turf

Bubblers

  • high flow rate, high precipitation rate
  • surface installation – impractical for site use and maintenance

Microsprays

  • medium to high precipitation rate
  • small drops, wind drift, evaporation – excessive losses
  • surface installation – vulnerable to interference or damage

Drip (surface)

  • low precipitation rates – suitable for soils of low infiltration rates
  • multiple point source application – many drippers required for large areas
  • surface installation – impractical (unless covered in mulch)

Drip (subsurface)

  • low precipitation rates – suitable for soil of low infiltration
  • multiple point source application
  • damage to root system during installation of established trees
  • risk of emitter blocking and pipe constriction by roots
  • no visual assessment possible to check operation

Some watering don’ts

  • Don’t apply water to the trunk of the tree.
  • Don’t overwater a tree so that the soil remains waterlogged for long periods and the soil is depleted of oxygen.
  • If runoff occurs when irrigating, stop watering. Cycle the application of water.
  • Don’t apply shallow applications that only add moisture to the top soil layer (eg. 50 mm)
  • Don’t continue to apply water inside the dripline once the tree has become established.

References

Handreck, K. A. and Black, N. D. 2001. Growing media for ornamental plants and turf, 3 RD Edition. New South Wales University Press, Kensington , Australia.

Homoclimes (Similar climates)

Dr. David Symon
Senior Botanist Curator Waite Arboretum, Urrbrae SA (1956 – 1985)

Homoclime is not a widely used word. It is derived from homo meaning the same and cline a tract or region of the earth. It is used to describe regions of the globe that have similar climates. Many of the European and North American trees commonly grown in South – East Australia have their origins in very dissimilar climates. The originating tree material would have been imported without thought of matching Australian climates and most likely on visual characteristics. This has proven to narrow the use of such taxa (ie Aesculus hippocastanum) which originally would have been selected from a Northern European ecotype when more suitable ecotypes to suit Southern Australia may have been found in Southern Europe. Significant selection and investigative work in the field of tree selection exists in the Waite Arboretum collection, located in Urrbrae South Australia. This work is ongoing and uses specific scientific principles.

Various climatic indices may be used. J. A. Prescott & C. E. Lane Pool, in their study of the introduction of pines of the Mediterranean region to Australia, used rainfall, mean annual – temperature, temperature – amplitude and temperature – phase (the lag of temperature to day length). It should be noted that soils were not included and nor was altitude. The Authors compared these climatic values with those determined for the original sites of Pinus canariensis, P. pinaster and P. radiata. They were able to find close matches of the climate for these three tree species in the South West of Western Australia, Kangaroo Island, the South East of South Australia, parts of Victoria, the Islands of Bass strait and the North East corner of Tasmania. The major populations of P. radiata pine are now found in these areas with some extending North into New South Wales. In a later study of the climatology of the grape vine in Europe, Prescott used the temperature of the warmest month only, a far simpler index.

These general principles can be used to guide the introduction and suitability of new plants to Australia. Soil types may be important and watering regimes will allow many plants to survive that would normally have perished under normal rainfall conditions. The effect of water withdrawal can be seen in some trees that are slowly senescing after irrigation of the Waite Arboretum ceased in the 1960′s.

Day length, dependant on latitude, can be a strong factor in the control of bud burst timing within deciduous trees. With climates similar to South Australia, with hot – dry Summers, the earlier in spring the trees shoot, the better they can make use of available winter rainfall. Examples can be compared of the early growth (and flowering) of the Southern European Pyrus spp., Fraxinus spp., Aesculus californica and with the late shooting of Fraxinus excelsior ‘Aurea’. Within the Waite Arboretum, the A. californica often will have extension growth of as much as 15-20cm before plants originating from Northern Europe begin to grow.

A great many of our early plant introductions came from Northern Europe and North America, with most not thriving in the Mediterranean type climate (Geographic areas of Adelaide & Melbourne). For example many Ulmus spp., Betula spp., Fagus spp. and Tilia spp. will only survive and thrive with copious watering. I expect that all could be successfully re-introduced by locating new plant material of the same taxa from more southern eco-types.

The concept of homoclimes was used to guide the search of new trees for the Waite Arboretum in the late 1960′s. Some success from this period can be seen in the following list of specimens:

Southern Europe

Acer monspessulanum
Acer obtusifolium
Crataegus azarolus
Junipers phoenica
Phoenis theophrastii

Many Pyrus spp. (Including P. amygdaliformis, P. tadshikistanica)

South Africa

Celtis africana
Curtisia dentata

Cussonia spp.
Euclea pseudebunuus
Nuxia floribunda

Similar concepts can be applied to other parts of Australia, please use the concept of homoclime as a guide in the successful introduction of new and interesting or better – suited trees for the Australian urban landscape.

Local Provenance and the Urban Tree

Mr James Will

Senior Lecturer, Burnley College , The University of Melbourne

Using information provided by
Mr Lincoln Kearn
Managing Director, Practical Ecology Pty Ltd

When bushland areas are revegetated, horticulturists and the community recognise the importance of planting new plants that are genetically similar to those that might have grown in the site previously. Revegetation programs almost always require use of “local provenance” indigenous plants. Why? Most agree that in revegetation projects we want to replace plants that can grow and reproduce in the same way as the plants that were lost. We are also trying to re-create some sort of wilderness that reflects what the landscape was like before human intervention.

In urban conditions, we most often plant trees to survive in areas that have been modified by human intervention; we have designed our cities to have roads, structures and services. These interventions make the revegetation approach inappropriate, since we can never re-create a pre-human or pre-urban wilderness. So what local provenance issues should urban horticulturists concern themselves with?

  • When planting trees in the streetscape or in most parks, we are concerned about the living individual, not that individual’s potential to breed or regenerate from seed.
  • We have to understand that we cannot create an urban wilderness, and that the urban environment has many limitations that make this creation impossible, un-sustainable, or extremely expensive.
  • Horticulturists need to understand the surrounding endemic vegetation to assure the community that urban trees will not affect the genetics of those plants that will breed and regenerate on local sites.
  • Local provenance can give definite variation for site suitability, especially in reactions to soil, tree size and foliage adaptation. With many wide-spread species, using a foreign provenance can cause problems 10+ years after planting.

Localised provenance and regeneration

When managing reserves or revegetation plantings, horticulturists must assess the existing vegetation and understand the extent of the populations of plants that are present. Often, a species will occur in limited pockets where the plant has begun breeding within the pocket rather than outside. These pockets, or remnants, can become quite different from other plants of the species outside the remnant population. In some instances, it may be valuable to maintain the specialised genetics of a remnant; in others, by re-enforcing breeding only within the remnant, we may be magnifying the genetic characters that will lead to that species’ extinction. In the early 1990s, concerns about genetic restriction were examined, and the book Genetics and conservation of rare plants (Falk, D.A. & Holsinger, K.E., eds., Oxford University Press, 1991) addressed many of these issues. Horticulturists considering revegetation sites should be aware of the issues presented in chapters 1-3.

As eucalypts are one of the most widely distributed tree taxa in Australia , most questions revolve around their provenances. Through many years of significant research, many questions about eucalypts have been answered. We now know that most eucalypts will breed with genetically-different-others (outbreed) at a higher rate than they will breed with genetically-similar-others (inbreed). Still, as widely-distributed populations become remnants, research has shown that many eucalypts will become successful inbreeders. The consequence of this inbreeding may limit the vigour of seedlings, with those seedlings showing the most genetic variation (and typically the least inbreeding) succeeding at the highest rates.

Developing an Australian Landscape

Remember, there is no tree indigenous to a nature strip or a degraded landfill site. For any successful landscape, the plants must grow in the soils available, and with the resources that can be obtained. Also, there are other bottlenecks that will limit the benefits of using local provenance material. As an example, in providing food sources to local birds by planting indigenous trees, you still may not get any more local birds in the planted area. You may not get any more local birds because the Indian Mynahs may predate the local bird species eggs to such a high level that your indigenous tree planting does not give you the result you’re after.

Protecting endemic vegetation

Unlike mammals, tree species can frequently interbreed. Further, if trees of the same species from one provenance are planted close to those from another provenance the resultant seedlings will most likely be related to both provenances. The pollen from street trees can strongly affect any indigenous trees that are within a pollination distance. For eucalypts this pollination distance can be a few kms, for acacias, it tends to be smaller distance. Responsible streetscape and park managers will select tree taxa that do not have a chance of breeding with any of the endemic flora: e.g., it would not be responsible to plant Eucalyptus leucoxylon ssp. megalocarpa in areas where the local E. leucoxylon ssp. connata is endemic; nor would it be responsible to plant Acacia melanoxylon from Otways seed in the north-west area of Melbourne where there is a local provenance.

If there is an endemic species, sub-species or remnant, responsible environmental management indicates that the horticulturist streetscape manager chooses a tree variety where cross-breeding with these locals will not occur. There are enough tree choices available to make this possible and easy.

Finally, the benefits of local provenance

As a species or subspecies has evolved in a specific environment, the more successful offspring for that environment will thrive and reproduce. Hence, over many generations, a stable, specialised form, or ecotype, of that plant can emerge that shows tolerances different to that which is generally shown by that species. Frequently, these ecotypic differences will include:

  • Change of plant form, including reduced height;
  • Ability to grow in low-nutrient or compacted soils;
  • Ability to grow successfully in soils with unusual pH or high salinity;
  • Adaptation to changes in water — either waterlogging or drought; and
  • Tolerance to salt spray, or other climatic conditions.

These adaptations can be very useful for urban horticulture, as the sites in which most trees are planted are not ideal. So by choosing a provenance or ecotype with adaptations to harsh soil and climatic conditions, you can have a better chance that the tree will survive and grow well in harsh urban conditions.

Metropolitan Tree Growers Pty Ltd grows a number of provenances of trees indigenous to the Melbourne area. Also, we grow known seed sources of many “exotic” Australian trees that are useful in Melbourne . We grow either the Bodalla Forest or Mottle Ranges provenances of Corymbia maculata because these ecotypes have better tolerances for growing under Melbourne conditions.

For different areas of Melbourne , we can suggest the following indigenous selections:

In areas of Basalt Plains

Many local government areas include these soils, including parts of Darebin, Hobsons Bay, Hume, Maribyrnong, Melbourne, Melton, Moonee Valley, Moreland, Wyndham and Yarra, although there is local significant variation in soils in various areas of these municipalities.

This region has relatively rich soils is the result of many lava flows in the last million or so years from volcanoes to the north and west of Melbourne . These lava flows pushed various streams including the Plenty River, Darebin Creek, Yarra River up against the Silurian hills of northeast and east Melbourne that form this region’s eastern boundaries. The soils are heavy clays, easily waterlogged and often have many lava rocks on or close to the surface. These soil characteristics do not support trees well through extended drought periods because of their immense adsorptive capacity. 25% of the fully wetted soils can be water and when these soils dry out, air replaces the soil and major cracks form. Root tips in these cracks can dry out quickly.

Trees that occur on these basalt soils include:

  • Acacia implexa,
  • Acacia melanoxylon,
  • Allocasuarina verticillata,
  • Eucalyptus camaldulensis
  • Eucalyptus leucoxylon ssp. connata
  • Eucalyptus melliodora

In the Silurian Hills, east or north-east of Melbourne

The localities of Boorondara, Glen Eira, Maroondah, Nilumbik Stonnington, Whitehorse , as well as parts of Melbourne and Whittlesea are found in this region. Again there is local significant variation in soils in various areas of these municipalities.

This region is the rolling hills of north-eastern and eastern Melbourne . They are derived from ancient marine sediments that have been uplifted and twisted over 450 million years. The clay soils have been leached over time and are very poor in nutrients. In many areas they are not much more than “dust and gravel”. The boundaries of this region tend to be the basalt flows to the west, the higher mountains to the north and east and the sand belt to the southeast. Although the soil type is similar across a wide region the topography causes a large variation in soil conditions and the retention of rainfall. North-facing slopes are very dry, and south-facing slopes support species that grow better in slightly more water. Lower areas can become very waterlogged and be frost affected. This soil and climate variation makes tree selection a very site-specific choice.

Trees that occur in these soils include:

  • Acacia implexa,
  • Allocasuarina littoralis,
  • Allocasuarina verticillata
  • Eucalyptus camaldulensis
  • Eucalyptus leucoxylon ssp. connata
  • Eucalyptus melliodora
  • Eucalyptus polyanthemos
  • Eucalyptus radiata
  • Eucalyptus tricarpa

Areas of Tertiary Sands

The cities of, Bayside, Frankston, Greater Dandenong, Kingston, Monash, Port Philip and parts of Glen Eira are found in this area, although there is local significant variation in soils in various areas of these municipalities.

This region is the open plain of south-eastern Melbourne . It is predominantly sand or alluvial silts. The sandy areas are a product of the dominant western winds over Port Phillip Bay and the rises and falls of sea level over the millennia. As sea levels changed, sand was deposited over different areas to the east of Port Phillip Bay . Sandy soils supported heath woodland. Heath woodlands were mostly a diverse scrubland and only had a few over storey trees. There are also extensive areas of old wetlands with alluvial soils, dominated by a Red Gum Woodland. These areas had an over storey of Red Gums and an open grassy aspect. These two dominant soil types are a complex mosaic across the region. It is one clear region but tree selection must be fairly site-specific.

Trees that occur in these tertiary sands include:

  • Acacia melanoxylon
  • Allocasuarina verticillata
  • Banksia integrifolia
  • Eucalyptus camaldulensis
  • Eucalyptus ovata
  • Eucalyptus pauciflora
  • Eucalyptus pryoriana

Coastal Areas

Many of the municipalities listed in other soil type have coastal areas where the soils become beach sands. Coastal areas have unique characteristics whatever their geological origins. The effects of salty winds and water influence all vegetation within a kilometre or more of the coasts. Much of the soils is sandy and requires very specific tree selections.

Trees endemic to these coastal areas include:

  • Allocasuarina littoralis
  • Allocasuarina verticillata
  • Banksia integrifolia

Ornamental Tree and Shrub disease problems

Mr John S. Brereton (BSc Dip Ag Sci Grad Dip Ed Admin)

For the 2006–2007 Metro Tree Handbook we have invited John Brereton to write articles on specific disease problems involving Public Open Space and Nursery crops. This edition we will target Phytophthora cinnamomi.

Tree and shrub die back caused by Phytophthora cinnamomi

Many Australians have now heard of the “dieback” disease which has devastated some of our valuable heath land and forested areas as well as causing significant horticultural production problems in some areas. In some cases the impact of the presence of this disease can be extreme with very large losses of susceptible species. For example thousands of native plants died in the Canberra Botanic Gardens at Black Hill between 1969 and 1972 when an outbreak of Phytophthora cinnamomi occurred there. In the Brisbane ranges in Victoria the understorey of the dry sclerophyll woodland areas has been decimated with the losses of shallow rooted species such as Xanthorrhoea australis. Many dead stumps of this highly susceptible plant can be seen on walking through this area. Other significant areas affected by this disease include parts of the Grampians, Wilson’s Promontory area in Victoria and Jarrah forest areas in Western Australia. In addition to the immediate problems caused by Phytophthora cinnamomi longer term ecological problems often occur as native flora changes over time accompanied by plant and animal habitat change and possibly water table changes.

Phytophthora cinnamomi is one of more than 17 significant fungal species and is recognised as the most widely distributed species in Australia with an extensive plant host range including both native and exotic plants. Particular interest in this disease problem has occurred because of problems in Australia with commercial crops such as avocadoes, pineapples, proteas and other proteaceous plants.

Phytophthora cinnamomi is a microscopic, soil inhabiting, root rotting fungal organism causing dieback in native and exotic vegetation. In Victoria commercially significant trees affected by the organism include Eucalyptus obliqua, E sieberi and E. mullerana.
Early attempts in Australia to diagnose this problem of dieback in native vegetation confused the symptoms of this disease with drought stress. Growers of trees and shrubs should be aware that when plant roots become stressed or diseased for a variety of reasons the early symptoms are often expressed as dieback in the crown particularly when the trees or shrubs are drought stressed. Initially this will commence with wilting and gradual dessication of foliage.

It is for this reason that in the absence of a pathological examination that this disease problem can be confused with drought problems.

Methods of infection

Phytophthora is a root invading pathogen that develops rapidly in the presence of susceptible roots and warm moist soil. The disease causing organism will attack fine feeder roots and destroy them so that water uptake is diminished. Symptoms of the disease normally appear when the plant becomes stressed such as under hot dry conditions. As infection takes place the root cells collapse in advance of the invading fungal organism. The extent of injury and speed with which the host plant dies depends on whether the Phytophthora has a root pruning affect or is spreading rapidly within root tissue.

The disease causing organism will be transmitted rapidly in water, along drainage channels and natural water courses. The fungus will spread via soil, infected roots and fungal-growth itself into the soil. Warm, wet to water logged soils are ideal for the development of the disease

The most important methods of dissemination are through movement of contaminated soil, water and plant material particularly infected roots. Other methods of distribution include vehicles, footwear, roadside grading and trenching equipment. Persons engaged in the selection and planting of trees and shrubs should source plants carefully to avoid the introduction of this problem into a new growing sites.

Species susceptibility

Susceptibility of trees and shrubs both exotic and native varies. As far as Eucalypts are concerned the subspecies monocalyptus is particularly susceptible. Under story trees and shrubs in open areas are often shallow rooted and highly susceptible. The fungal organism is actually attracted to the roots of many native plants including a wide range heathland species, hakeas, proteas, banksia spp, proteaceus plants generally and Xanthorrhoea spp.

Soil conditions that are conducive

Detrimental conditions include soils that are shallow and poorly drained or subject to intermittent waterlogging; often duplex-soils where the topsoil has poor infiltration characteristics. With increasing topsoil depth and increasing levels of organic matter susceptibility may decrease.

Suppressive soils

Suppressive soils are characterised by high levels of organic matter, nitrogen in the ammoniacal form and high cation exchange properties, Such conditions may be found on red basaltic soils. Significant improvement in soil conditions to make them more suppressive to the disease causing organism can be achieved through the addition of organic matter such as composted pine bark. The basis for suppressive soils is related to the presence of competitive and antagonistic micro-organisms in the soil, which act against the Phytophthora sp.

Strategies for control in nurseries include:

Careful sourcing of plant material and growing media,
Heat treatment of potting mixes,
High levels of hygiene,
Testing of water supplies, water disinfestation via filtration, UV light treatment, chlorination, ozone treatment of contaminated water supplies,
Avoidance of over watering,
Isolation when outbreaks occur,
Use of composted growing media,
Appropriate use of eradicant fungicides and disinfection of equipment with quaternary ammonia compounds.

Strategies for open spaces include:

Avoiding movement of infected plants, soil and water into clean areas,
Monitoring runoff water,
Improving drainage conditions,
Public education to avoid moving of infected plant material or soil,
Planting more tolerant species or dilution of plant population which consist of highly susceptible species,
Maintenance of high organic matter levels where possible,
Monitoring of soil and water for the presence of the disease causing organism

Where outbreaks are suspected to have occurred accurate diagnosis is important, persons should contact a plant health services laboratory for testing and confirmation of the presence of this organism. Portable field test kits are also available. This is a disease problem to avoid.

Post-planting – Maintenance for Best Establishment

James Will

Senior Lecturer, Burnley College, The University of Melbourne

Recommendations

  • Differentiate between those trees that will establish quickly (in 1 year) and those that will require 2 years’ maintenance to establish correctly
  • Budget for as much irrigation as possible each week
  • Make certain that water is appropriately applied to wet the rootball and surrounding soil.
  • Re-mulch when necessary to improve water-holding capacity and weed suppression of the site
  • Control weed competition

The Establishment Period

The commonly-held wisdom dictates that trees are planted in late winter or early spring, and then will establish over the first summer post-planting. For this reason, most establishment maintenance is contracted for 6 to 9 months. With fast-growing trees, this one-summer maintenance will establish the tree, but with slow-root-growth varieties, such as Ginkgo, Magnolia and others, it may take 2 spring/summer periods for the tree to become established. Trees that fail in the second year after planting may be failing because they require an additional year of establishment watering and care. Further, many large, super-advanced trees will require at least 2 years’ maintenance.

Water: the Essential Factor

Dr. Peter May is famous for saying, “Identify the irrigation that you believe will be adequate, and then double the figure.” Water is the most important factor in establishing plants in the landscape. North American authors suggest rates of at least 4 litres water/week/25mm trunk diameter. In south-eastern Australia, rains occur only sporadically in summer, and you must expect that added irrigation is the only way for watering trees.

Using the 4 litres/week/25mm trunk diameter (at 100mm above ground level) as a starting point, these trees will need:

Container size Tree height Ave. trunk diameter Litres of water/week
15-20 litres 1.5-2.0m 30mm 5
40-50 litres 2.1-3.0m 50mm 8
75-100 litres 3.1-4.0m 75mm 12

There are few instances where this water requirement is met.

Irrigation is most important in late spring through early autumn. Deciduous trees may have shut down growth earlier in the season, and irrigation after 1 March may not be necessary, but Australian trees and other evergreen trees will require irrigation at the rates indicated above until 1 April.

With homeowners, you frequently find that over-watering is a major reason for tree death, but with urban trees this is less likely. Seemingly the only times when trees die from too much water is when the site is originally waterlogged and not corrected by pre-planting preparation.

Fertilisation in the establishment phase is not critical unless the trees are in a luxuriant state. As noted in the previous article, fertilisers will not aid in root growth during the establishment phase unless irrigation and soil conditions are ideal.

Note that weekly irrigation is best to keep soil moisture at the critical level for best root growth; irrigating twice as much each fortnight will not give the same benefit to the root system. Further, be concerned with low-infiltration-rate soils; trees planted in these soils will require careful irrigation to assure that water ends up in the rootball.

Reducing Competition

If trees have been planted properly, a mulch layer of between 75 to 100mm will have been installed at planting. This mulch layer, preferably of a moderately-coarse composted bark, woodchips or prunings, will help insulate the root zone, and reduce evaporation from the planting hole. This mulch will also reduce weed competition around the root zone of the newly-planted tree.

If the mulch layer is too thin, top this mulch up to at least 75mm during the establishment period to assure that the benefits possible from mulching can be realised.

Weed competition will severely limit the root growth of newly-planted trees. Weeds will use available water and compete for nutrients that should be available to the tree. Newly-turned soil will expose the soil-seed-bank that exists in most soils, and after planting a tree these weed seeds will grow. A layer of mulch will restrict weed growth, but clean-up with a herbicide or by hand-weeding when weeds are visible.

Recommended Tree Planting Detail

Drawn K.D. Smith, not to scale. Used with the permission of the author.

The author gives permission to copy as necessary for specifications.

Recommended design for a typical uncovered individual tree pit or planting hole. Adapted from
Craul (1992).

 

Site Preparation and Tree Planting Detail

Dr. Peter May
May Horticulture Services

Successful tree planting depends on the ability of the tree to rapidly initiate root growth, into the backfill of the planting hole, and then into the soil beyond the planting hole.

Recommendations

  • Check physical and chemical restrictions of the site before planting; improve if possible or select a compatible tree.
  • Improve sites where access is readily available by improving water penetration and oxygenation.
  • For inaccessible sites, ameliorate drainage and compaction by planting technique.
  • In planter boxes, use a high mineral content mix to lessen compaction promoted by organic matter decomposition.
  • Plant trees only as deep as the rootball; make the hole 2x as wide as the rootball; do not “fluff-up” soil under the rootball; do not amend backfill; mulch after planting.

Background: physical soil properties and tree growth

The main soil properties influencing planting success are physical, primarily being the availability of water and the levels of oxygen to the soil. The other important physical soil property is soil strength: how difficult or easy it is for roots to physically penetrate the soil.

Water availability is essential for rapid root growth and tree survival. Even where the tree has been selected to match the patterns of natural water availability of the site, most often trees will perform much better if supplemented with water in the year following transplanting; the late spring, summer and autumn periods are the most critical. The practices for ensuring adequate water are dealt with in the next article. Pre-planting treatment of the site and planting practice should ensure that rain or irrigation can penetrate the root ball and backfill soil.

Oxygen availability is primarily determined by the water content of the soil. Soils which are too wet contain little oxygen and this will reduce root growth or kill root tissues (and even lead to tree death). Site preparation that ensures that the backfill and site soil have open structure (porosity) and allows reasonable drainage of the planting hole will result in better tree establishment.

In many urban sites, development activities can lead to serious soil compaction. Compacted soil has poor water movement rates/aeration and is difficult for roots to penetrate. These effects can result in root growth restricted to the planting hole.

Background: chemical soil properties and plant growth

Soil pH should be checked before selecting trees. Extremes of pH can occur and should be remedied before planting. In some circumstances (for example highly alkaline soils) it may be better to select for tolerance of the property, rather than trying to amend the soil.

Woody plants are not highly responsive to plant nutrition and unless soil is very infertile, the use of fertilizers may not be an essential part of tree establishment. Research indicates that fertilizer responses may only occur where growing conditions are very good.

High levels of salt in soil will affect tree performance and if a problem is suspected, a soil test should be done before species selection is made. Use of tolerant species is the best approach to this problem.

Site Preparation: readily-accessible sites

Assess the site to establish whether any conditions will limit growth. If the soil is compacted, cultivate the surface soil to alleviate compaction. Where dense subsoils exist, these should be ripped to provide drainage channels in the subsoil. Stabilize the soil with gypsum if soil tests indicate it is needed. Incorporating composted organic matter into the surface 100mm of a poor soil will improve plant growth. Suggested rates of organic matter incorporation are 10% by volume.

The planting hole should be wider than, but not deeper than, the rootball. Backfill the planting hole carefully with the soil dug from the planting hole, ensuring that it is not compacted in the process. Water in well.

Where machine access is more difficult and compaction cannot be readily alleviated, research has shown that a planting hole with sloping sides gives better establishment.

Limited sites: e.g., street trees, tubs or planters.

Provide as large a root volume as possible. In sites with limited root volumes trees are often badly stressed through water shortage. Ensure that the planting hole or planter has drainage. It may be necessary to install a drain at the base of the planting hole. For street trees the site soil is the preferred backfill. For planters or tubs, use a planting mix with a moderate drainage rate. These mixes are preferably mostly soil rather than highly organic planting mixes which will lose volume and porosity through decomposition.

Tree selection and performance in relation to sub-regional variation in climate

Dr. Peter May
May Horticulture Services

Summary

  • Cities are warmer, especially at night, presenting planting opportunities.
  • Exposed walls and paving can be very hot, this may cause heat damage or increased water use by trees.
  • Paving can change the amount of rainfall being intercepted for tree use, this will affect the taxon selection.
  • City development produces complex shade and wind environments for trees, plan for these effects carefully.
  • Roof gardens are windy and exposed, tree selection and planting must reflect this.

Introduction

As Dr David Symons has shown in the previous section of this Tree Handbook, regional climatic variation plays an enormously important role in tree selection. However, there may also be climatic variations at a sub-regional scale that can also be taken into account when deciding on the suitability of a particular tree for a project. These sub-regional variations can be caused by small-scale local variations in geography but they can also come about as a result of human modification of the landscape. This section concentrates on this latter category. In the gardening literature this small-scale variation is often called microclimatic variation but this is not technically correct. The microclimate is the climate within or very close to the plant canopy. For the variations dealt with in this section, mesoclimate is preferred.

The urban “heat island” effect

Concentrations of human activity result in environmental temperature increases. There are a number of causes of this but they include fuel consumption and the use of building materials with the ability to store heat (such as concrete and masonry). These temperature increases are primarily the result of increased night temperatures rather than day temperatures. The effect of these temperature increases on trees will not be great but one would expect that in highly urbanized areas there is a reduced risk of low temperature injury as compared to the suburbs or surrounding rural land. This may allow a slightly broader palette of trees to be cultivated. In the case of Melbourne, sub-tropical species may be more successful in the CBD than in the outskirts of the city.

Trees that will benefit from a heat island effect include:

Arbutus canariensis (Canary Islands Madrone)
Ficus microcarpa var. hillii (Hills Weeping Fig)
Magnolia grandiflora and cultivars (Bull Bay Magnolia)
Yucca elephantitis (Spineless Yucca)

Effects of paved surfaces on water availability

In built-up areas the use of impermeable paving surfaces is very common. This can result in changes to the water budget of trees planted in or near paving. In some cases paving will impede water infiltration into the soil, resulting in a climate that is, in effect, drier than the regional climate. When coupled with higher air temperatures from radiant heat and possibly lower relative humidity, this may require the selection of tree species that are suited to climates more arid than the one being considered.

Consider these trees that may manage paved surfaces and lower water availability:

Acacia pendula (Weeping Myall)
Brachychiton acerifolia and hybrids
Ceratonia siliqua (Carob)
Dracaena draco (Dragon’s Blood)
Geijera parviflora (Wilga)
Jacaranda mimosifolia (Jacaranda)
Olea europaea ‘Tolley’s Upright’ and O. europaea ‘Swan Hill’ (Olive)
Ulmus parvifolia ‘Burnley Select’ and U. parvifolia ‘Yarralumla’ (Chinese Elm)

The opposite can also occur. In some planting situations, (roadsides for instance), rainfall is collected by the surface and directed to soil adjacent to the paved surface. This, in effect, results in an increased rainfall for that small zone. This would allow the culture of trees with higher moisture requirements. It may also necessitate the selection of trees with higher tolerance of reduced soil aeration. Trees that may manage sites of different levels of compaction and low soil oxygen:

Quercus bicolor (Swamp Oak)
Quercus palustris and Q. palustris. ‘Early Defoliant Form’ (PinOak)
Taxodium distichum (Swamp Cypress)
Tristaniopsia laurina (Water Gum)
Waterhousea floribunda (Weeping Lilly Pilly)

Effects of high-rise urban development

The profoundest effects of humans on small-scale climatic variation are found in the high-rise parts of cities. In these environments new climate zones are created by the construction of buildings that block sunlight and modify wind speed and direction. In the early 1980s Kjelgren and Clark coined the term “urban canyons” to describe these new environments. Their characteristics include the creation of a number of planting zones with variations in light intensity and duration. Depending on street orientation and building height, trees can experience from complete shade to rapid change from shade to full sun. Other variations can include changes in relative humidity, wind speed and direction, rainfall and even day length. Perhaps the most difficult environments found in the urban canyon will include zones of permanent shade and wind tunnel areas.

Shade effects on trees

The highest levels of shade will be found on the north-facing sides of east-west streets. In this case a tree may get no sunlight at all except perhaps in late afternoon in summer. The effects of shade on plants are complex. The first response will be reduced flowering. For street trees this may not be significant. The next level of response will be canopy quality. Shaded canopies will be less dense with thinner leaves. For deciduous trees this may not be critical but the appearance of evergreens may suffer. Conifers in particular often lose quality when shade grown.

Wind tunnels

While cities are generally less windy than outlying countryside, tall buildings will create very windy locations within cities by deflecting wind downwards or by concentrating it along particular streets. Such wind pockets will damage wind sensitive trees and can cause increased evapo-transpiration. In Melbourne, the Docklands and South bank precincts will become very windy as high-rise development continues and tree selection may have to change to reflect this.

Trees on buildings: effects of building height

In the broader sense, altitude effects on climate fall under the consideration of macroclimate. In the built environment however, altitude comes into play when trees are placed on top of buildings. The construction of roof gardens is becoming more common in our cities and many designers want to include woody vegetation in their planting palettes. How then does the climate on a roof vary from that at ground level? The major variation is in wind speed. As height increases, wind speeds typically increase (doubling height increases wind speeds by 10%). Trees with known sensitivity to wind damage (structural faults, poor branch attachment, large leaves, delicate leaves) should not be used in such applications. Trees with tough, small foliage will be more resistant, as will trees that grow in naturally windy locations such as cliff tops. Planting relatively small trees is important to reduce wind loads on canopy and rootsystem. The presence of neighboring tall buildings may add further to wind loads by the creation of wind tunnel effects.

Consider the use of the following trees for windy roof top situations:

Acacia boormanii (Snowy River Wattle)
Arbutus canariensis (Canary Islands Madrone)
Ceratonia siliqua (Carob)
Dracaena draco (Dragon’s Blood)
Geijera parviflora (Wilga)
Olea europaea ‘Tolley’s Upright’ and O. europaea ‘Swan Hill’ (Olive)
Ulmus parvifolia ‘Burnley Select’ and U. parvifolia ‘Yarralumla’ (Chinese Elm)

Coppiced or multi-stemmed specimens (or Mallee forms) could be considered:

Corymbia eximia (Yellow Bloodwood)
Eucalyptus kitsoniana (Gippsland Mallee)
Eucalyptus risdonii (Risdonvale Gum)
Lagerstroemia indica x fauerii cvs. (Crepemyrtle)
Tristaniopsis laurina (Watergum)

Please note that these trees would require a maintenance regime to remove extension growth on an annual or bi-annual basis.

 

Further reading

Craul, P. 1999 Urban Soils: Applications and practices, Wiley. Detailed treatment of heat loads on and evapotranspiration of urban trees. Also a good discussion of wind loads on trees.

Harris, R., J. Clark and N. Matheny. 2003 Arboriculture: Integrated Management of Trees, Shrubs and Vines (4th ed.) While North American in focus, a good discussion of climate effects on trees with some information on urban mesoclimates.