Tree work may be necessary for several reasons, for example:
- Ensuring the tree is in a safe condition
- Removing dead or dying branches
- Promoting growth and general tree health
- Regulating size and shape
- Improving the quality of flowers, fruit or timber
- For aesthetic or amenity reasons
Poor quality tree maintenance techniques not only cause trees to become unsightly but can also lead to disease and decay, and can potentially result in the tree becoming dangerous.
It is therefore important to ensure that you know what sort of tree work operations you want to undertake on your trees, and to understand what work a tree professional is recommended.
We have put together this brief guide that defines the most common tree work terms to help you.
British standards for tree work
The British Standards with relevance to tree work and arboriculture are:
- BS3998: 2010 Recommendations for Tree Work
- BS5837: 2912 Trees in Relation to Design, Demolition and Construction – Recommendations
Pruning cuts – the correct method
Using the correct pruning cuts are as important as the tree maintenance techniques themselves. This is because each time a tree is pruned, the cut inflicts a wound. The size, angle and position of the cut relative to the parts of the tree not being pruned, affects the trees ability to withstand the wound and sustain healthy growth.
A general rule-of-thumb is to remove branches at the point of their attachment to the tree, or if you are shortening, to adopt a drop crotching technique (see other useful terms). It is important that all cuts are kept as small as possible.
Tree maintenance – the three main techniques
There are three main techniques used in tree maintenance: crown thinning, crown lifting and crown reduction. Each technique is described in detail below:
Crown thinning – this is the methodical removal of a portion of the smaller branches at the outer crown, throughout the tree to an agreed percentage. Generally, crown lifting should not exceed more than 30 percent.
The result is foliage with a uniform density and a branch structure that is evenly spaced throughout the tree allowing more light to pass through. Crown thinning also reduces wind resistance and the trees overall weight, and can be used to control epicormic growth. It is important to note that crown thinning does not alter the overall size and shape of the tree.
Crown thinning is commonly used on broad-leaved species and is often undertaken on a regular, cyclical basis to keep the tree in a good condition.
Crown lifting or crown raising – this is the removal of the lowest branches in order to increase the transmission of light throughout the tree. The technique also enables access to the crown and can also be used to refer to the act of preparing lower branches for future removal.
Crown lifting or crown raising should be restricted to no more than 15 percent of the crown height to ensure the crown remains at least two-thirds of the total height of the tree. Ideally, crown lifting or raising should be specified with reference to a fixed point, such as the number of metres clearance required above ground level, for example: crown lift to give a 4 metre clearance above ground level.
Crown lifting or crown raising should not include the removal of large branches growing directly from the trunk as this can cause problems with decay and overall stability. It is also advisable to avoid crown lifting older, more mature trees, or, at the very least, restricting the method to only the secondary branches.
Crown reduction – this is the reduction in height and/or spread of a tree’s crown to ensure the ratio between the tree’s roots and its canopy are balanced. The cuts should not exceed more than 100 millimetres in diameter, save for exceptional circumstances, and all reductions should be specified by a measurement where possible both in terms of the end result and the length of the areas that need to be removed. For example: crown reduce the height of the tree by 1 metre and its lateral spread by 0.5 metres all round to meet the finished crown dimensions of 15 metres in height by 9 metres in spread.
The aim of this technique is to reduce the overall levels of stress placed on the tree. Crown reduction can also reduce the effects of shading and light loss in the environment. It is important to remember that the result does not necessarily need to be symmetrical but should retain the main structure of the crown, albiet with a smaller outline. Not all species are suitable for crown reduction, ask your tree professional for advice before beginning.