Much has been written about pruning trees and shrubs, to the point of confusion for many! What, why, when, how…..? This week, Joyce Hostyn outlines the basics of pruning and reassures us that learning to do so, while a bit daunting to some, is part of the gardening journey!

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What is Pruning Anyway?

Pruning woody plants refers to the removal of a part of the plant, resulting in a change to the direction and amount of new growth. Before you begin, it helps to know a little about how trees grow. Spend some time looking at your tree and observe where buds emerge – you’ll find them at the end of a branch (terminal) with others along the sides of the branch (lateral), either opposite each other or alternating. These buds are destined to become new leaves, branches or flowers. 

Tree growth is regulated in part by hormones. For example, the terminal bud contains a hormone that suppresses the growth of the lateral buds and allows the tree limb to grow out from the end. Removing the terminal bud at the end of a branch will allow those lateral buds to grow.

Image credit: UGA Extension

Why Prune?

Before you begin pruning, Joyce asks you to consider the purpose.  To paraphrase Canadian gardening guru Ed Lawrence, considerations when pruning are damage, disease, danger and desire.  And, there are various approaches when it comes to pruning. Some gardeners are minimalists and will prune only to avert danger (e.g. dead branch overhanging a structure) or to remove a diseased limb. Others may prune to more easily access fruits, and still others may want to control a straggly bush that has become unattractive (at least to some eyes; the birds may love it)!

Form and Flowering

If you decide to prune, learn the growth and flowering habit of your tree or shrub before you begin.  Each plant wants to follow its natural form – is is a mounding shrub? Canes? Or tree-like? Does it flower in spring, summer or fall? Understanding this will help you decide when and how to prune.

The Pruning Budget

Joyce thinks of a pruning “budget” – and no, this is not the amount of money you’ll spend on secateurs and pruning saws!  Instead it helps to define how much and what can be pruned from a tree.

The pruning budget is made up of three components: the type of cut, (heading cut, or a thinning cut), the size of the cut, and the total amount of foliage removed. At a certain point pruning should stop, since the limiting factor is the pruning budget. Exceed that budget and the tree will suffer potentially long lasting damage

  • “Thinning” cuts remove entire branches at the branch collar and are usually the recommended type of cut
  • “Heading” cuts remove only part of a branch and encourage vegetation growth below the cut. These are less common

If you decide that pruning is needed, the cuts should be selective, that is, the pruner thinks about the impact of each cut before making it. Cutting plants can cause them to increase their growth rate, as well as splitting the growth into more and more branches.  For this reason, “topping” a tree (lopping off the entire top), should be avoided.

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The amount removed will depend on the species, but for example, a mounding shrub  such as boxwood or Spiraea may have a budget in which pruning reduces the foliage by up to one third, and the overall size up to one quarter. 

What to Prune

The above diagram illustrates the removal of suckers and waterspouts, branches that are crossing or rubbing and thus damaging the bark, and those with narrow, weak crotches. Limbs that are spaced very closely may be thinned to allow more air and light into the tree.

When to Prune

There is no single answer to this question, as it depends on the species to be pruned and intended result! Always consider the plant. Some work may be best done in late winter when a tree is dormant (before bud break).  This will have an invigorating effect, promoting new growth. On the other hand, pruning done later in June after peak growth periods, tends to slow growth.

For ornamentals, understand when the plant flowers. Those that flower in spring, on one year old wood, will result in loss of blooms if pruned in early spring.  If you you don’t want to miss spring flowers, then prune after flowering is finished.

However, in the words of gardening writer Christopher Lloyd:

The wrong time may be  the only opportunity and a preferable alternative to not doing  something at all. ….. You’ll make mistakes but you’ll perhaps learn not to mind making them….

Making the Cut

Always think about the response of the tree buds and shoots to your cuts. For example, cutting above a bud facing the outside the plant will cause the branch to grow in that direction. When removing a branch, cut close to, but not into, the collar. This will help the tree to heal itself – no sealant on cuts is required. Cut on a moderate slant where possible.

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Use sharp tools designed for the job at hand. Stand back and look at the tree after each cut to assess whether and where additional cuts should be made.

Resources

There is an abundance of literature on pruning and Joyce suggests consulting research based and current guidance. University Extension offices and Master Gardening websites will often have well researched answers for many gardening questions. A certified arborist can also provide professional advice. 

University of Minnesota Extension Pruning Trees and Shrubs.

Turnbull, C. Guide to Pruning: What, When, Where, and How to Prune for a More Beautiful Garden. Sasquatch Books, 2006

Pruning Wisteria (Cass Turnbull)