The Biggest Mistake You Can Make Pruning Shrubs

For most of us, pruning shrubs is a do-it-yourself project. Unfortunately, the results are not always satisfactory. This is especially true in the long-term: often a shrub looks good right after pruning but becomes progressively less attractive in the following seasons. What went wrong?

One key mistake is usually responsible: not knowing your plant. 

You don’t need to be a botanist or an arborist to understand the basic requirements of your shrubs. Observation, coupled with a little research, will provide you with a pruning plan that will enhance the appearance of every shrub in your landscape, while promoting their long-term health.

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One of the first things to learn is when to prune. Late winter is the ideal time to prune most shrubs—but there are exceptions such as lilacs and azaleas. The rule of thumb is to prune in winter unless the shrub flowers in June or earlier; these should be pruned after flowering. You only need to do the research on your plants once, then you’ll have a pruning calendar for the future. For most landscapes, two or three pruning times a year will meet the needs of every plant.

Another important thing to know is what your shrub or tree should look like. Unless you are an extremely skilled and committed pruner, you will want to enhance the natural shape of the shrub rather than coerce it into an artificial form. Even in something as common as a hedge, maintaining a formal shape is much more labor- and knowledge-intensive than maintaining an informal one that allows for the natural forms of the shrubs.

Plants such as roses, forsythia and viburnum, which are so common in our gardens, are often pruned incorrectly. A few seasons of shaping forsythia into a ball by shearing off the ends will leave you with a shrub that has few flowers and many brown, woody stems. Pruning canes from the base (the correct method) will lead to a beautiful drift of yellow flowers.

In fact, there are really no plants that can be simply sheared into the shapes we want. For hedge plants such as boxwood, arborvitae and yew, which are often sheared, pruning is necessary as well. Is the hedge or ornamental beginning to show woody dark places instead of lush greenery? This will only get worse, unless you let some light inside the hedge and prune to promote new growth. (The need for sun is also why the top of the hedge should be narrower than the base.) Ideally, after shaping the plant each year, you will judiciously prune some branches, promoting new inside growth. If this hasn’t been done in several years, it may be time for a serious rejuvenation pruning.

Pruning evergreens, like pine and spruce, requires hand pruners rather than shears. These trees and shrubs should not be pruned beyond the current year’s growth. If you wish to maintain their size or shape them, careful annual pruning by trimming the new candles is a must.

Woody deciduous ornamentals, such as lilac and Japanese maple, require gentle pruning to refine their shape. Some, such as lilacs, will need to be “de-cluttered” of the nest of crossing branches that can limit airflow and encourage diseases. Others, like Japanese maple, will need an occasional snip to correct any branches that depart from the desired shape. For an ornamental such as laceleaf Japanese maple, begin by envisioning what the tree should ultimately look like: a series of fans, gently overlapping but maintaining separate layers. Carefully trim out any limbs that touch the ground or fall on the layer below. If you’re not sure, wait until next year and see how it looks then—you get another chance.

Although learning to prune properly can seem daunting, it is also rewarding. Mistakes are rarely fatal to your shrubs. Observe your plants and see how they respond to your pruning. Every year, you will improve—and so will your landscape. And of course, if this isn’t a job you want to do yourself, an arborist can help you keep your landscape in top shape.