Buying the Right Quality Tree

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

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:

  • Containers large enough for the canopy size,
  • Containers designed to reduce root spirals, and
  • 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.

  • Lagerstroemia root system that shows the vigorous surface roots of this genus
  • Acer, showing typical surface and support roots
  • 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:

  • no bifurcations on any street tree stock,
  • radial branch attachment around the trunk,
  • strong branch attachment (>45º angles to trunk),
  • trees appropriately limbed-up to above 1.8m, and
  • 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.