Tree selection – species and size
Long term viability of trees is dependent on tree structure and tree health. Diseases and pest infestations are usually a secondary issue brought about by the underlying conditions that contribute to stressed, vulnerable plants. Selecting the right plants for a site is critical, there are two primary factors to consider:
- Plant selection – Trees and shrubs should be either a resilient species or indigenous to the area, an indigenous tree should be naturally well adapted to the local climate and soils. When purchasing plants you should use local, reputable growers, select specimens that have healthy root structures that are not root bound, natives generally do better if planted as a seedling rather than a more mature plant.
- Site conditions – Plants will do better in groups, with a range of plant types (trees, shrubs, herbs, grasses) rather than as isolated individuals. Soil condition is critical; soil can be protected by maintaining defined garden beds which excludes some or all traffic from within the drip-line or Tree Protection Zone (TPZ) of a tree, and by placing mulch. Large trees in open areas of school grounds or public spaces are problematic due to the constant foot traffic. If soil conditions are poor and a trees health declines this increases the chances of a failure and with the high use of the area tree related risk becomes very high.
Tree planting – how to establish a tree
Though it may seem counter intuitive, when establishing some Australian natives, the younger the plant you start with, the faster and healthier the plant will grow. Ideally you would direct seed however if this is not practical use tube stock. Planting out more mature specimens can be traumatic to the plant and they can take some years to establish a healthy root system, they are also more prone to insect and disease attack.
Planting in autumn allows roots to become established prior to facing the rigours of summer. Planting at other times is fine as long as water is available throughout that establishment period.
- Clear the immediate area to be planted of any grass or weeds
- If you are removing older plants to create space check if they are a suckering variety, if they are, then kill them with a stem injection treatment well before removal, this will eliminate regrowth and competition to new plantings, this needs to be done before the plants are removed to enable systemic chemicals to move throughout the entire plant, this is only relevant for trees that sucker or grow vigorously when coppiced such as English Elm.
- Ensure the ground is not too dry
- Ensure the soil is suitable for the selected species considering soil type, pH (acidity), drainage etc
- Different themed garden areas may be planned such as ‘indigenous’ or ‘fauna habitat’ or ‘threatened species’, whichever type, there should be adequate space for individual trees: consider the mature size of the canopy and the issues around competition for water and nutrients.
- Before planting keep the root ball moist at all times as young fibrous roots can be dried out and damaged in minutes
- Disturb the root ball as little as possible however ensure the small, fine, outer roots are free and able to integrate into the new soil. Also make sure there are no girdling roots, these will in time strangle the tree as they increase in girth.
- Prepare a hole as wide as practical to allow lateral root development, the hole should only be as deep as the potted plant and in a bowl shape (picture a cooking wok). Ensure the side of the planting hole is roughed up and not glazed to avoid girdling roots.
- Well decomposed organic matter is an appropriate soil enriching medium, only use a small amount per plant; a couple of handfuls.
- Don’t plant the root ball too deep or cover the plant stem with soil, place the top of the root ball equal with ground level
- Immediately upon planting flood the plant and ensure the entire root ball and surrounding area is well soaked, do this before starting on the next plant
- Establish a bank or well around the plant (at the juncture of potting mix and soil) to ensure watering is directed to the root ball and young roots
- Maintain about 7 cm of mulch around each plant, keeping the stem/trunk clear for ventilation, mulch of irregular sized pieces better allows gas and water transfer
- If using freshly chipped material for mulch add a small amount of high nitrogen fertilizer (slow release ideally) to the soil to counteract nitrogen drawdown from decomposing mulch
- Ensure soil moisture is maintained particularly during the first summer (roots require oxygen: do not keep soil saturated)
- Use deep intermittent watering rather than constant, light, surface watering this trains the roots to grow more deeply
- Maintain a weed and grass free environment to minimise competition
- Avoid staking if possible as the tree may become dependant and not put on structural growth
- Stakes should be east and west of the new tree to avoid shading and attached loosely so as to catch the tree in windy conditions but not impact or support in calm conditions
- Trees should be inspected and often require ‘formative pruning’ during the juvenile years (2-8yrs) This is the time to choose a central leader, remove damaged or crossing limbs and thin the canopy if needed. Of particular importance is pruning to avoid included bark unions which are more prone to failure later on
Soil compaction near trees
Soil compaction reduces the number and size of soil pores, subsequently reducing the available water and oxygen to a tree, this then impedes a trees ability to respire (consume reserved energy supplies) and increases stress. Subsequent symptoms may include thinning or dying crown, increased deadwood, reduced or no seasonal extension growth, increased susceptibility to decay, limb shed or death of the tree. Insect infestation may increase as a trees natural ability to withstand pests is diminished.
Compaction problems can be alleviated by excluding some or all traffic from within the drip-line or TPZ of a tree and placing mulch. Large trees in school grounds or public spaces are problematic due to the constant foot traffic combined with the high risk factors. Parking cars under trees will have a measurable adverse impact on tree condition.
Planting out the TPZ with shrubs and tall grasses, particularly prickly species, helps with de-compaction and reducing traffic.
Mechanical damage to trees
Physical damage to tree parts, particularly the trunk, is unsightly and provides entry points for pests and diseases such as fungal infections. This may cause long-term decay and can lead to partial or complete tree failure and death.
Alterations to soil levels over root zones
Alteration of soil levels around trees will affect the root zone and stability of a tree as well as tree metabolism. This may result in reduced tree health, excessive deadwood, thinning foliage and poor vigour; it can take some years for the impact to become evident at which time it is normally irreversible.
Where soil is banked against a trunk the bark and lower trunk is likely to rot introducing decay into the basal area.
Trees impacting buildings
Trees impact on buildings in a number of ways including exerting a mechanical force on foundations via lifting or pushing that can damage foundations and walls.
They may also cause damage to buildings in reactive (clay) type soils through the uptake of moisture from the soil and the subsequent dehydration and uneven shrinking of the soil profile.
When buildings are damaged in this manner, signs may include sinking foundations and cracked wall sections as well as a horizontal movement at the top of a wall in the direction of the dehydrated soil as the soil shrinks unevenly and allows the foundation to roll.
Pruning vs tree lopping
An Australian standard exists to give guidance on pruning of trees.
It is important that all remedial works are carried out by a competent contractor in accordance with the Australian Standard.
(AS. 4373 2007 – Pruning of Amenity Trees)
Tree Lopping as defined within the Standard AS-4373, is detrimental to trees, often resulting in decay and poorly attached epicormic shoots. Natural Target Pruning methods should be used wherever possible when removing sections from trees.
Wounds in trees are permanent, they don’t heal. A wound my callus over with subsequent years growth which is good for tree structure however this external callus wood has no impact on the spread of decay internally.
Above are some basic notes on trees: selection, planting and care, this type of information will underpin the conclusions and recommendations made within arborist reports particularly where they relate to tree assessments and protection on development sites.