How to prune your plants and trees

There are three basic techniques for pruning plants: thinning, heading back, and shearing. It's important to understand the difference between these techniques, because the easiest method, heading back, is seldom the best. Here are some more tips on properly pruning your plants and trees.

Thinning is what you do when you follow a branch or stem back to where it sprouts, and prune it at its point of origin. With trees, this is usually where it emerges from a larger branch or the trunk.

• In the case of long-limbed shrubs, such as forsythia and mahonia, most stems originate right at the ground.
• Spend a few minutes studying where branches originate and you will know exactly how to do this type of pruning.
• Always make pruning cuts at an angle to the stem to allow water to drain from the cut surface to keep it from rotting.
• Although thinning is often a slow and awkward way to prune, it is the best way to preserve the natural shape of any woody plant.

Heading back is a technique that involves pruning off the tips of branches, which causes buds farther down the branch to develop into new stems and flowers. This type of pruning increases the number of branches, making a plant bushier.

• Heading back is used mostly for plants that bloom on stem tips, such as buddleia, rose, and most perennial vines.
• Deadheading, or removing the spent flowers, of annuals and perennials uses this same principle.
• Because heading back is easy compared to thinning, many gardeners mistakenly head back azaleas, forsythia, and hydrangea, robbing the plants of their natural beauty as well as many future flowers.

Shearing is wholesale heading back, and its use should be limited to plants you wish to grow as dense hedges, for example barberry, boxwood, holly, and other shrubs that have a tight, compact form.

• Keep in mind that just because a shrub can be sheared does not mean that this is necessary.
• When dwarf cultivars of shrubs are chosen, often little or no pruning of any kind is needed to keep them shapely and attractive.

If you look closely at the place where one tree branch joins a larger one, including the trunk, you will see a wrinkled raised collar of bark. Strive to make pruning cuts just outside this collar, without cutting into it. This is because the collar contains chemicals that the tree uses to wall off the injury (your pruning cut), making it naturally impervious to invasion by pests and diseases.

1. If the limb is a large one, begin by cutting it off about 15 centimetres (six inches) from the branch collar.
2. To keep the weight of the limb from binding the saw blade, first make a small cut on the underside of the branch.
3. Cut off the limb starting from the top edge of the branch, about two centimetres (one inch) beyond the cut on the branch underside.
4. Remove the stub by making a single downward cut just outside the branch collar.

Thinning removes up to a third of the oldest shrub branches to promote colourful new growth. Tree experts no longer recommend painting a pruning wound with tar or paint, which can trap moisture, inviting rot and disease.