What is pruning?
In a nutshell, pruning is the removal of part of a plant to influence its subsequent growth. Woody plants – trees and shrubs – have a clever system of growth. The ‘terminal bud’ – the bud at the very tip of a shoot – produces growth inhibiting chemicals, which it passes down the stem to discourage the buds lower down from growing. This makes sure that the plant can grow upwards towards the light, where more food is to be found via photosynthesis.
If we cut off the shoot tip, no growth inhibitors are passed down the stem, with the result that the lower buds begin to grow. This is why we pinch out shoot tips, or cut them off, to encourage bushiness in all kinds of plants.
So, one reason for this sort of pruning is to encourage a bushier, fuller plant, but depending on the tree or shrub in question, there are other reasons for cutting bits of it off:
◼ To remove dead, diseased or damaged shoots, which would otherwise help to spread infection into healthy tissue – known as ‘the three Ds’.
◼ To make a plant shapelier if it has branches or shoots growing out at odd angles or where we do not want them.
◼ To increase air circulation and light penetration, which will help to maintain plant health.
◼ To encourage the production of more
young and vigorous growth.
◼ To remove a portion of older wood and keep a tree or shrub youthful and longer-lived.
◼ To encourage the production of flowers
and fruits, rather than leafy growth.
All of these aims can be achieved by adjusting the kind of pruning we carry out. There is no ‘one-size-fits-all’ and that is why pruning techniques are often confusing. They needn’t be if your aims are clear from the outset.
Not all woody plants need pruning. Some, such as magnolias and Japanese maples, are happy to be left to their own devices, unless there is a particular branch that grows where you do not want it. With all such trees and shrubs, the most important thing is to plant them where they can have room to grow to their ultimate potential. Information is always available on height and spread after 10 or 20 years and it is worth checking this out before you plant. There is no sadder sight than a butchered magnolia that has outgrown its available space.
Trees are generally grown on a single stem and in the formative years some lower stems may need to be snipped off to allow for a clean trunk in maturity.
So when is the job best undertaken? Without a doubt the majority of pruning is best carried out when the tree or shrub is dormant and the sap is not rising. If you prune birch trees in spring, for instance, they will ‘bleed’ as the sap is pushed through the cells of the cut surface. Prune them in autumn and early winter and you will have no such problem.
Spring-flowering shrubs, which flower on wood that was produced the previous year, are best pruned immediately after flowering when a portion of the older, flowered wood is removed to encourage the production of stems during the summer. These, with any luck, will flower the following year. The older wood tends to fade in its flowering capacity as it ages. There are exceptions – the delightful spring-flowering Viburnum plicatum f. tomentosum increases its spread annually and manages to carry on flowering year after year with no pruning other than the removal of dead wood – in my garden at any rate!
For our guide to what to prune in spring, summer, autumn and winter, see page 74.