by Gabriel Hemery, Northmoor Trust / Peter Savill, University of Oxford / Gary Kerr, Forest Research
Objective
This leaflet aims to provide practical advice on the formative pruning of young broadleaved trees. It does not deal with high pruning, which is the subject of another leaflet in the series.
What is formative pruning?
The aim of any form of pruning is to produce a single straight stem of at least 6 m in height. Formative pruning is carried out on young trees up to 3 m tall. It involves the removal of multiple leaders and unwanted large branches to promote their potential to achieve clear straight stems
(Figure 1). When carried out correctly, formative pruning can be the most effective pruning that a tree can receive. It is not always essential but it is required when the leader has been lost following damage by browsing, bird-perching, insects, frost and wind. Young trees are particularly susceptible to these hazards.
Figure 1 : Improvement of a forked ash:
1 before pruning, 2 immediately after pruning and 3 two years later
Why prune?
Formative pruning is done while the tree is still young to:
Favour a single leader
Reduce the number of large branches
Remove steeply ascending branches
The need for formative pruning can be significantly reduced by using close-spacings (greater than 2500 stems per hectare) and good genetic stock.
Time of year to prune
For most species, formative pruning is best done in late spring, just before trees come into leaf. This ensures that wounds heal quickly. With the trees free of leaves it is also easier to see the crown architecture and to choose the best branches to prune. However, cherry and walnut must be pruned in late June or early July. In cherry this is to minimise the risk of bacterial canker and silver leaf disease, and in walnut, to avoid excessive bleeding.
When to begin formative pruning
Formative pruning should start as soon as the trees are well-established. This is usually between the second and fourth seasons after planting. Ideally the branches removed should not exceed 2 cm diameter at the point of intersection with the stem, small enough to cut with a pair of secateurs. Formative pruning should be continued at annual or two-yearly intervals until the tree is 3 m tall, after which high pruning may be necessary. Different tools and techniques are then required.
What to remove in formative pruning
Figure 2 : Pruning candidates: (a) forked, (b) competing leader and (c) steep and large branches. The form of (d) is so bad that it requires stumping rather than pruning (see text).
Forks in the main stem to favour a single leader (See Figure 2)
Any branches that are excessively large (more than 50% of the thickness of the stem) or growing at too steep an upward an angle, wherever they are on the stem. Having carried out these two steps, if there are a number of large branches, then remove the lowest ones first.
Never remove more than 5 large branches in any given year.
Always maintain as many live branches as possible in the top third of the tree.
How many trees should be pruned?
Formative pruning can usually be carried out very simply and cheaply. Aim to prune a maximum of half the number of trees planted, concentrating on those with greatest timber potential.
Species differences
Ash and sycamore are frost-tender species and though their apical dominance is usually very good, they also quite often lose their leaders from frost. The arrangement of buds in these two species (Figure 3) usually results in the production of two new leaders from the same point on the stem, one of which should be removed by pruning.
Figure 3 : The arrangement of buds on ash, like sycamore, will lead to forking if the terminal bud is damaged
Wild cherry generally has strong apical dominance but at low densities it will develop heavy branches that need removing.
Beech, oak and sweet chestnut usually lack apical dominance and are most likely to require formative pruning.
Walnut is also particularly frost-sensitive and damage-prone.
Most other species fare better when the terminal shoot is damaged because a single bud lower down the stem then sprouts to become the new leading shoot. Oak will tolerate the removal of larger branches than sycamore.
Pruning practice and risks of diseases
When a live branch is removed, a wound is created on the stem of the tree that is a potential infection route for diseases so it is important that steps are taken to minimise this risk. To promote fast healing:
Branches that have to be removed should ideally be cut off before they reach 2 cm in diameter at the point of intersection with the stem. They should certainly not be more than 4 cm diameter.
Always cut as closely as possible to the outer edge of the branch-bark ridge, but do not cut into or behind the ridge (Figure 4).
Do not use flush cuts. Cutting too close to the main stem causes an excessively large wound and removes the natural tissues and mechanism that promote healing.
Do not leave branch stubs. These will have to decay and fall off, or the diameter of the trunk will have to increase sufficiently to cover the stub.
Do not risk tearing the bark below the cut when removing larger branches.
In rare cases when the form of trees is beyond improvement (Figure 2d), stumping should be considered. This involves cutting the stem at 2-5 cm above ground level and subsequently singling new shoots that arise.
Figure 4 : Position of the branch-bark ridge and the correct pruning cut
Tools for pruning
Sharp secateurs are best for removing small branches of up to 2 cm in diameter. Only if formative pruning has been delayed, and the branches are too big for secateurs, should loppers or handsaws be used. All tools should be kept very sharp. Health and safety guidelines must be followed when pruning.
Other educational leaflets are available on request covering Establishment of Broadleaves and High Pruning techniques, please email enquiries@woodlandheritage.org.uk for more information.
A PDF of this leaflet can be downloaded here.