Specifying Trees has become the “go-to” book when it comes to landscape architecture. This is the second blog regarding the Specifying Trees book and we will be focusing on above-ground specifications. Since trees are above the ground, the specifications are generally effective. The above-ground quality discussion covers the following aspects of quality trees:
● True to type
● Health and vigour
● Freedom from pests and disease
● Free from injury
● Self-supporting
● Stem taper
● Pruning.
● Apical dominance
● Crown symmetry
● Trunk structure
● Included bark
● Trunk position
● Compatibility of graft unions
● Indication of the north
In order to ensure that the implementation of the design is accurate, the trees supplied and planted must be exactly what was ordered. If a tree has specified problems it can seriously compromise the landscape project.
The designer may specify by
● species;
● species plus cultivar or variety; or
● species plus provenance (e.g. locally indigenous).
Trees must also be clearly labelled so that there is no confusion upon delivery.
True to type assessment criteria:
● Trees supplied must be true to type.
● Individual trees and batches must be clearly labelled.
In order to ensure that the trees supplied are able to continue to grow successfully, they must be healthy and vigorous at the time of delivery.
Health and vigour assessment criteria:
● Health i.e. the foliage size, texture and colour at time of delivery must be consistent with the size, texture and colour shown in healthy specimens of the selected species.
● Vigour i.e. the extension growth must be consistent with that shown in vigorous specimens of the species selected.
If the trees are infected by disease or under attack by insects when they are being planted, they will be vulnerable and may result in the transfer of the pests or disease to other plants in the project area. Generally speaking, trees should not show any evidence of pests or disease. However, a small amount of insect attack is usual for indigenous species.
Freedom from pest and disease assessment criterion:
● Foliage should be free from attack by pests or disease. This means that native species with a history of a previous attack by native pests must be restricted to less than 15% of the foliage. There must be no insects that are actively feeding on the trees.
Free from injury assessment criterion:
● Trees must be free from injury. The trees should be able to support themselves at the time of planting, this ensures good nursery practices. It also allows us to avoid excessive use of stakes.
When it comes to self-supporting assessment criterion:
● Trees must be self-supporting.
Stem taper can be defined as the increase in caliper down the tree stem (refer to Figure 1.1). The laying down of woody tissue in the lower section of the stem causes stem taper. This is a response to the tree’s physical movement while it is growing. This can also be seen under the Pruning assessment criteria.
In order for trees to develop adequate stem taper, they need space in the nursery and freedom from long-term support such as stakes. Stem taper allows us to measure the practices used in the nursery.
Stem taper assessment criterion:
● The caliper on the stem must be greater than the caliper at any higher point on the stem. Pruning practices in the nursery may impact the structural integrity, health, and appearance of the tree. The tree grower’s pruning practices should benefit the tree’s development.
The pruning history criteria are designed to ensure the following:
● The trees must not be pruned into a saleable shape just before the shipment. For example, if a project requires a specific clean stem height, it is imperative that the tree has been grown to roughly conform with that requirement rather than having the lower branches removed just before the shipment.
● If the crown of the tree is not lifted too high, stress on the trunk can be reduced, healthy growth rates and adequate stem taper can be maintained. (refer to Figure 1.2).
- All pruning should allow the pruning wound to be sealed quickly to avoid long-term damage to the tree. The designer should describe the specific shaping and pruning needed to achieve the required tree form.
Pruning assessment criteria:
● Specify clean stem height and any specific form required
● Clean stem height must not exceed 40% of the total tree height
● Fresh pruning wounds must be confined to less than 20% of the total tree height
● All pruning must be clean-cut at the branch collar
● The diameter of any wound must not exceed 50% of the caliper immediately above the point of pruning
Many species, by their nature, have an excurrent form. These species should be specified as having a central leader at the time of delivery.
Apical dominance assessment criterion:
● For species with an excurrent form, trees supplied must have a defined central leader with the apical bud attached. The symmetry of the crown is imperative to the appearance of the tree in the landscape project. This also allows us to indirectly measure the growth of the tree.
Each tree needs to be planted in an area with enough space in order to prevent crowding from its neighbours. This will produce well-balanced, stable, and sun-hardened trees. Crowded nursery trees are most likely to have asymmetrical crowns (refer to Figure 1.3).
Crown symmetry assessment criterion:
● The difference in crown distribution on opposite sides of the stem axis must not exceed 20%. For excurrent species, it is imperative that there is a strong central trunk. This trunk must be thicker and upright than the branches at the point of attachment (refer to Figure 1.4a). For decurrent species, division of the central stem is part of the natural development of the crown. However, the stem junction at any such division must be sound (see discussion of Included bark) and any such division must be above any clean stem requirement (refer to Figure 1.4b).
When it comes to all tree species, the trunk(s) need to be significantly thicker than the branches at the point of attachment. (refer to Figure 1.4c).
Trunk structure assessment criteria:
● If the tree species have an excurrent form, there must be a single trunk roughly in the centre of the tree and any deviation from the vertical must be less than 15 degrees.
● If the tree species have a decurrent form, the central stem must not be divided at any point lower than the clean stem height specified. The stem junction at the point of division must also be sound. This can be seen in the bark assessment criterion.
● All tree species must not have a branch greater than one-half of the trunk diameter when measured directly above the branch junction. Structural weakness in the branch or stem junction is caused by included bark in crotches and can result in breakages during storms. However, some tree species are prone to include bark and rarely fail e.g. Tuckeroo.
The difficulty arises when the bark is folded into the joint as the tree grows. This creates a faulty junction between the stem and branches or at the junction between codominant stems. This problem is commonly associated with acute-angled crotches, but the real cause is the shape of the branch bark ridge at the junction between the stem and branches or the stem bark ridge at the junction between codominant stems. If the branch/stem bark ridge is outwardly pointing the bark will be pushed out as the tree grows. Nevertheless, if the branch/stem bark ridge is inwardly pointing, the bark will be folded inwards. This can be seen in Figure 1.5.
Poor pruning practices and regrowth after damage cause Included/ingrown barks. Luckily, the problem can be detected at a relatively early stage since it seems to be genetically controlled.
Included/ingrown bark assessment criterion:
● The branch/stem bark ridges at junctions between stems and branches and between codominant stems must be outwardly pointing (convex). The exception to this criterion is tree species that are prone to included/ingrown barks. It is important that the trunk of the tree is roughly in the middle of the root ball in order to allow the development of the root system to be symmetrical.
Trunk position assessment criterion:
● The distance from the centre of the trunk to the extremity of the root ball must equal the radius of the root ball. It needs to be approximately 10%. (refer to Figure 1.6).
It is important that the graft union is sound and that the scion and rootstock are compatible should you decide to buy named cultivars propagated by grafting. If the graft is unsuccessful there will be a structural weakness at the base of the stem, which may cause the tree to die. The failure can be credited to the poor union or a secondary infection through the fault.
It is essential that the rootstock and scion grow at comparable rates.
Compatibility of graft unions assessment criteria:
● The union between the rootstock and the scion must be sound for the entire perimeter of the graft.
● The diameter of the rootstock right above the graft must be equal to the diameter of the scion right below the graft. It needs to be approximately 20%. The correct orientation of a tree (especially large trees) is important to the success of that tree in its new location.
Indication of north assessment criterion:
● If the trees are in containers larger than 100 L or of Size index larger than 140 (e.g. 3.1 m high x 45 mm caliper), the northerly aspect in the nursery must be clearly indicated in such a way as to not be erased or misplaced in transit.
We hope the above information will be helpful to you with regards to ensuring that the trees you purchase meet the necessary industry standards thus increasing the probability of fruitful/successful trees. Specifying Trees is available for purchase at www.natspec.com.au.