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Pruning deciduous trees

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Paper birch (Betula papyrifera)
Photo: Jardin botanique de Montréal (Gilles Murray)
Betula papyrifera.

These few recommendations tell you how and when to prune your deciduous trees. The tree's age and the time of year are two important things to consider when pruning.

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.

Thus, the larger the diameter of a wound, the more slowly it will be covered by woundwood, which increases the risk of infection. Therefore, it is best to prune trees when they are young so that you only have to cut small diameter branches (less than 4 cm).

Pruning severity

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. By limiting pruning in this way, we ensure its proper development and maximize its life.

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

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. Forks cause structural problems that can lead to considerable breakage. 
  • 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.), maintain the central leader throughout the tree's formative years. For rounded or spreading species, such as Norway maples (Acer platanoides), 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.

Ideal structure

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.

In some cases, a low branch could grow long and block a crossing or path. You have to anticipate the degree of nuisance of a branch and remove it when its diameter is small. Avoid waiting as it may become harmful.

 

Maintenance pruning

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 they appear.  
  • 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

Recommended pruning times

In Québec, pruning is ideally done from mid-June to the end of July. The tree then has sufficient time to initiate the closure of pruning wounds and activate its natural defense mechanism (compartmentalization) aimed at isolating the injured area from pathogens (diseases).

Very small branches (less than 1 cm in diameter) can however be pruned at any time, including in the spring, before the leaves appear, or at the beginning of branch elongation. The structure is clearly visible at those times and makes it possible, in particular, to intervene if a fork is spotted in a young tree (see the Formative pruning section).

Dead, diseased, broken or damaged branches may be removed at any time.

Note : pruning times may be different for fruit trees.

Pruning times to avoid

In general, tree pruning should not be done at the start of the growing season, when tissue building is highly active. Pruning during this period can lead to abundant sap flow which impairs the tree's reserves and also increases the risk of disease.

In addition, pruning large branches (more than 4 cm in diameter) at the beginning of the season very often leads to the production of new shoots around the wound, which means that another intervention will be needed.

Avoid pruning trees during extremely hot or dry spells.

Pruning in the fall is not recommended as compartmentalization does not occur quickly enough at this time of year and the pruning wound would be subject to winter cold.

Pruning during very cold periods (January and February) is also to be avoided, because unprotected tissues are exposed to frost.

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