Specifying Trees – Below Ground

Specifying Trees has become the “go-to” book when it comes to landscape architecture. This is the third blog regarding the Specifying Trees book and we will be focusing on below-ground (the root system) specifications. Since we hardly see the root system, the specifications are generally hard to define. Below-ground specifications are still just as important as the tree above-ground specifications and require the same amount of attention. The root system of a landscape tree must have enough surface area in order to take up water and nutrients. It must also be well structured so that it can grow on and support the tree permanently.

Two key elements define a quality root system:
● Division
● Direction

The division of roots is generally stimulated by root pruning, therefore, structured and repeated division of roots is the result of a good root pruning process and potting sequence. When it comes to above-ground criteria, an assessment of the root division and direction is an indirect evaluation of the tree nursery’s practices. It is also imperative that the tree nursery uses practices that produce rootballs that are compatible with soil found on-site. They also need to promote the rapid development of the root plate (i.e. are well structured and relatively shallow).

The discussion of below-ground quality, with appropriate assessment criteria, covers the following aspects of tree quality:
● Root division
● Root direction
Rootball occupancy
Rootball depth
Height of root crown
Non-suckering rootstock

Two important aspects of root division are:
● Total division
● Pattern of division

A root system’s ability to take up nutrients and moisture is directly related to the surface area of the root system, which in turn is directly related to root division. Roots need to divide and divide again (total division). Root systems with insufficient division suffer from reduced surface area and also offer too few points for new roots to develop.

Roots should be repeatedly divided, but the pattern of division is also important. The division should be progressive (primary division) in order to ensure a strong structural base for any later root development. If trees are held too long in containers (above ground or in-ground) they may exhibit too much secondary division. Secondary roots add little to the final root system. In extreme cases, secondary division can make the rootball hydrophobic, causing watering problems at planting.

The entire rootball in smaller trees must be occupied by fibrous roots. When it comes to larger trees it is possible to use a range of techniques in the early and intermediate stages of growth, provided that there is sufficient division within the rootball to support the tree at the time of sale.

For instance, deciduous exotics may be grown in-ground to a certain size and then potted through a series of containers. These trees will have little division in the centre of the rootball, but sufficient division for the balance of the root volume. In order to allow for varying production techniques and combinations of techniques, for larger trees, the assessment criteria focus on root division in the outer section of the rootball (refer to Figure 2.1).

Root division assessment criteria:
● When it comes to trees in containers smaller than or equal to 45L or ex-ground trees with a size index less than or equal to 70, roots must have been subjected to major division at not more than 100mm intervals.
● When it comes to trees in containers greater than 45L or ex-ground trees with a Size index greater than 70, roots within the outer 50% of the rootball must also have been subjected to major division at not more than 100mm intervals.

Roots grow by extension from the root tips. Once the tree is planted all newly established roots will be extensions of the root system contained within the rootball at planting. Hence, the root system in the rootball becomes the core of the root system of the mature tree. If while planting the tree, the landscape tree has any root distortion or confusion that problem may become a structural weakness in the established root system.

It is important to remember that
● problems caused by poor root direction may not become clear for years; and
● confused roots may not be apparent when potting-on or planting. However, as the roots expand those confused roots can cause major problems e.g. spiraling roots in small trees can strangle the developing root system and cause the tree to fail.

Roots of a mature tree generally grow outwards or downwards, so roots within the rootball of a landscape tree should also usually grow outwards or downwards (refer Figure 2.2).

Given that the internal root direction is satisfactory, some nonconforming (confused or circling) roots at the outside edge of a container can be removed prior to planting (provided that these roots represent a small margin of the total root system) thus eliminating potential problems at a later stage.

When it comes to larger trees, the root development in the lower section of the rootball may not be critical, as it will be the new roots generated from the upper section of the rootball that become the dominant roots of the maturing tree. So, if the rootball is greater than or equal to 500mm depth assessment of root direction at the base of the rootball is not critical.

Root direction assessment criteria:
● Roots, from the point of initiation, must generally grow outwards or downwards. Any deviation from that outwards or downwards direction must not exceed 45 degrees.
● When it comes to trees with a diameter (at ground level) of less than 40mm, the diameter of any nonconforming roots at the extremity of the rootball must not exceed 25% of the trunk diameter. When coming to trees with a diameter (at ground level) of 40mm or greater, the diameter of any nonconforming roots at the extremity of the rootball must not exceed 10mm.

It is imperative that the root system fully occupies the volume of the rootball in which it is being purchased, otherwise, the rootball will not stay together during the time of transportation, planting and other root checks.

Rootball occupancy assessment criterion:
● On shaking or handling the unsupported rootball at least 90% of the soil volume must remain intact.

It is commonly accepted that tree roots grow at the top of the soil profile (the root plate). It is therefore imperative that the root systems generated by tree nurseries in the production of landscaping are also relatively shallow.
Generally, urban planting sites have very shallow soil available for planting, hence shallow rootballs are suitable (refer to Figure 2.3).

In order to ensure that air-filled porosity is not enormously reduced in small containers, the depth of such containers may need to exceed their width. Such containers are rarely deep enough to cause problems with typically shallow site soils. Therefore, the criteria set apply to larger containers/rootballs. Certain planting situations will place restrictions on rootball depth e.g. services in footpath areas may restrict the rootball depth to 450mm for street plantings.

Rootball depth assessment criteria:
● Containers/rootballs 45L or larger must
– have a depth of less than or equal to the maximum depth specified;
– have a diameter greater than or equal to their depth; and
– no rootball (irrespective of size) shall exceed 550 mm in depth.
The root crown is the joining point between the below-ground parts and above-ground parts of the tree. The root crown belongs at the surface of the rootball irrespective of the size of the tree. Burying the root crown can lead to fungal infections, serious disruptions of the root system’s structure and function, or even cause the tree to die (refer Figure 2.4).

Height of root crown assessment criterion:
● The root crown must be at the surface of the rootball.
If the tree species required for a project are naturally suckering, the designer may need the grafted trees onto a non-suckering rootstock.
The rootstock options for particular varieties may also include suckering and non-suckering species. The designer may also require a non-suckering option. (Suckering from root systems can also be caused by disruption to roots on-site.)
Non-suckering rootstock assessment criterion:
● When it comes to grafted cultivars/varieties, the trees supplied must be grafted onto non-suckering rootstock.

We hope this series of blogs regarding Specifying Trees has been informative and will be helpful to you when sourcing the best trees for your landscape project. The Specifying Trees book is available for purchase at www.natspec.com.au.


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