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When should you prune? These guidelines can help you decide

One reason to prune is to improve the shape and vitality of your shrubs and trees.
One reason to prune is to improve the shape and vitality of your shrubs and trees. Centre Daily Times, file

In my last column I talked about pruning to repair winter damage. In this column I am going to continue talking about pruning for shape and when is the best time to prune.

Another reason to prune is to improve the shape and vitality of your shrubs and trees. If you have little or no winter damage, you may still need to prune to change plant density. Those that are too thick and full can be thinned in the center to allow more air and light to enter. Plants that are “leggy” can be pruned to stimulate a fuller and denser growth.

When you prune, remove individual stems and branches. Shearing (lopping off several stems at a time,) may produce a compact dense layer of growth on the surface which may be desirable for hedges and formal designs, but not for balanced growth. Pruned plants keep their shapes longer than sheared ones because pruning from selected locations allows you to stimulate growth from the center of the plant instead of just on the surface.

Selected removal of the longest or most crowded stems also allows plants to retain their natural growth pattern and shortens that unattractive period of looking “freshly pruned.”

When the subject of pruning is brought up among gardeners there are always questions about using tree paint or tree-wound dressings. I believe that research has shown that there is no evidence that these materials help in the healing process. Where large areas of wood are exposed, there may be some cosmetic value in using paints and dressings if they are applied lightly to color the wood. However, heavy applications are suspected of trapping moisture and decay organisms against the wood and may cause more problems than they solve.

Deciding when to prune may seem to be a problem for the novice, but it’s really not too difficult if you follow these guidelines.

1. Non-flowering evergreens such as yew, juniper, arborvitae and Japanese Holly can be pruned just about anytime while they are dormant.

2. Flowering plants — rhododendron, azalea, viburnum, crabapple and forsythia, for example — should be pruned immediately after they flower. If you do, you will have the pleasure of this season’s flowers. After pruning, plants still have enough growing time to set flower buds for the next year.

3. Plants that normally flower late in the season or even in the fall, can be pruned in the spring since they have most of the summer to set buds.

4. If your plant is large, old and overgrown it’s probably best to prune it early in the spring before active growth begins. Then you can see the shape as you cut. As long as you leave at least one-third of the plant intact, you can cut away tangled side shoots, old wood and prune for shape as much as you like.

Don’t be afraid to prune your plants, but do so for a good reason. Don’t prune just because you’ve put the wrong plant in the wrong location. Prune to keep your landscape plants healthy, beautiful and growing strong.

Bill Lamont is a professor emeritus in the department of plant science at Penn State and can be reached by email: wlamont@psu.edu.