Start pruning early
The size of pruning wounds affects the speed at which they seal over – the larger the diameter of a wound, the more slowly it will be covered by woundwood.
You should never remove more than 20% of the crown of a tree each year, to encourage it to grow properly and maximize its life expectancy.
Topping is to be avoided. This sort of severe pruning:
- totally destroys the tree's appearance;
- weakens the tree's structure, by encouraging heavy growth of water sprouts;
- makes the tree more vulnerable to insect pests and disease;
- considerably reduces the tree's life expectancy.
Pruning young deciduous trees
Pruning at transplanting
At this point, remove only any dead, diseased, broken or damaged branches.
Formative pruning should start in the year after planting and continue for about five years. The aim is to give the tree a sturdy, balanced structure, while maintaining its natural shape.
Maintain a central leader
- A tree's central leader is the main upright stem. To ensure that the tree grows upright, with a straight trunk, maintain this central leader by pruning back or removing competing branches. The central leader must always be at least 15 cm longer than the other branches.
- If the central leader is divided (with a fork at the top), keep the branch that is growing in the best direction (usually the one that is most centred over the trunk) and cut back or remove the other branches.
- If the central leader is broken, damaged or very weak, or has a dead terminal bud, choose a vigorous lateral branch to replace it. If the best-located lateral branch is growing at an oblique angle, cut back the damaged leader as close to this branch as possible. Place a stake in the ground to support the lateral branch. You will be able to remove the stake after two or three years. If the tree has an almost vertical lateral branch, cut back the damaged leader just above that branch, which will straighten up and eventually take over from the original leader.
- For erect or conical (pyramidal) species, such as lindens (Tilia spp.) and ashes (Fraxinus spp.), maintain the central leader throughout the tree's formative years. For rounded or spreading species, such as Norway maples (Acer platanoides) and American elms (Ulmus americana), maintain the central leader until the base of the crown is the desired height.
- Never cut off a tree's central leader without a very good reason, for you are apt to weaken its structure and destroy its natural shape.
Replacing a damaged leader (with stake)
Replacing a damaged leader (without stake)
Choosing and developing scaffold branches
- Choose 4 to 10 branches – these will be the tree's main branches. This selection process may take several years.
- Choose branches that are evenly spaced (about 30 cm apart) and arranged radially around the trunk.
- If several branches are located at the same height on the trunk, the tree will be weaker and less balanced. Keep the branch that is growing in the best direction and gradually remove the others.
- Remove any branches growing at too sharp an angle to the trunk (less than 30°), unless the tree is a columnar or fastigiate species, like a 'Fastigiata' English oak (Quercus robur ‘Fastigiata'). Such branches could compete with the central leader, rub against other branches or break off.
- Branches that are too large (more than half the trunk diameter) should be removed. Proceed gradually, pruning them back a bit at a time. This will avoid weakening the tree by removing too much of its foliage.
- When pruning the scaffold branches, be careful to maintain the tree's natural shape. As a rule, the lower branches should be kept longer than those at the top of the crown, to ensure a strong structure and allow sunlight to reach the foliage.
Top view of the ideal structure
Height to the base of the crown
A tree's branches always remain at the same height from the ground; they do not rise as the tree grows. Pruning allows you to decide where the crown will start.
For the strongest trunk, it is best to remove all branches growing on the bottom third of the tree, but never more. A full-grown, wide-spreading tree should have at least 2 m of bare trunk.
Gradually remove the tree's lowest branches. Branches and twigs growing under the scaffold branches help the trunk grow thicker and protect it from sun or mechanical injury. Keep these temporary branches short enough that they do not compete with the permanent limbs.
This type of pruning is done periodically to keep a tree healthy and vigorous.
- Remove any dead, broken, damaged or diseased branches and any stubs (broken or cut base of a branch projecting from a tree trunk) as soon as you notice them.
- Remove any water sprouts and suckers as quickly as possible.
- Prune any inward-growing branches and any that are rubbing on or interfering with other branches.
- Prune any branches and stems that are too closely spaced.
Pruning mature deciduous trees
Well-chosen and well-trained trees require little pruning. Normally, all you should need to do is remove dead, diseased, broken, weak or interfering branches, along with stubs, water sprouts and suckers.
When to prune young and mature trees
It is easiest to prune trees when they are dormant, either in early spring (before the buds burst) or in late fall (about a month after the leaves have dropped). Their structure is clearly visible at those times. They can also be pruned during the growing season, except during extremely hot or dry spells. No pruning should be done in the depths of winter (January and February).
Special advice for certain trees
Some species bleed sap when pruned in early spring. These include birches (Betula spp.), maples (Acer spp.), walnuts (Juglans spp.), elms (Ulmus spp.), poplars (Populus spp.), willows (Salix spp.) and lindens (Tilia spp.). It is best to wait until some leaves are fully open before pruning these trees.
It is preferable to prune trees in the Rosaceae family (crabapples, mountain ashes, hawthorns, etc.) in spring, because pruning in fall can interfere with their hardening off in readiness for winter.
Dead, diseased, broken or damaged branches, along with stubs, water sprouts and suckers, may be removed at any time.