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Prune to keep plants healthy

One of the most important cultural practices for maintaining woody ornamental plants is pruning. It helps to produce attractive, well-formed plants.

Pruning is the process of removing plant parts to improve its shape, to reduce plant size and to remove damaged or diseased parts. Each plant species in the landscape has its own growth habit and specific pruning requirements. Some shrubs have slower growth habits and may never require pruning, while vigorous shrubs may require frequent attention.

Pruning is both an art and a science. It is an art in making the pruning cuts properly and a science in learning the proper methods and timing for pruning in order to gain the best results. Unsightly plants often result from not pruning or pruning incorrectly. There are classes gardeners can take to gain the knowledge of correct pruning techniques for their landscape plants.

Proper pruning requires a basic understanding of how plants respond to pruning. The terminal bud on the end of the branch secretes a hormone that suppresses the growth of the lateral buds. When the terminal is removed, the lateral buds and shoots begin to grow. The most vigorous new growth occurs within 6 to 8 inches below the pruning cut. Heavy pruning causes the plant to respond by increasing the re-growth of stems and leaves which is the plant's response to restore a balance between the top part of the plant and the root system.

Often, a shrub is pruned by shearing the new growth to control its size and shape. However, the continual shearing results in dense, thick, new growth being produced near the outer portions of the canopy. Less light reaches the interior portions of the plant, leading to sparse foliage with a leggy or hollow appearance which can increase the risk of harm from environmental stresses and pests.

Thinning involves removing an entire shoot or limb to its point of origin on the main branch or lateral branch, while not pruning some of the shoots, allows for the plant to develop a more natural growth form. Thinning cuts shorten the branches, improve light penetration and direct the growth of shoots or limbs. It encourages new growth within interior portions of a shrub, reduces the plant size and creates a denser, more attractive plant. Proper pruning techniques will keep a plant at a chosen size, shape, and density.

Often a shrub is overgrown, and it needs renewal or rejuvenation pruning to bring the plants back under control. Renewal pruning involves heavily pruning the plant back several feet or close to the ground. Early spring, late February into March, is the best time of the year to do it. As the weather warms up, the plants put on a huge flush of growth which will quickly cover the bare branches.

Do not renewal prune in fall or winter as this will cause the shrubs to have an ugly, bare appearance until the new spring growth occurs. It can also make the plant more susceptible to cold injury. Most broad leafed ornamentals, like ligustrums, hollies, crape myrtles, and cleyera respond well to renewal pruning. However, boxwoods recover slowly and can die when subjected to severe pruning. Narrow-leafed evergreens, like junipers, Leyland cypress, and arbovitaes should never be pruned in such a manner since they do not regenerate new growth from the old wood and could perish. Renewal pruning is often only a temporary solution to overgrown shrubs as they may need to be continually pruned. Consider removing such a plant and replacing it with a slower, lower growing species.

A hedge should be pruned into a trapezoidal shape with the narrowest part of the hedge at the top tapering to a wider base. This allows for adequate light to reach the lower portion of the hedge. If the widest part of the hedge is at the top with it being narrower underneath, the undergrowth will be shaded out causing the shrub to have a thin and aesthetically unpleasant appearance.

The best time to prune varies with plant species and should be done at times that complement the growth characteristics, flowering and other objectives you desire.

Prune spring flowering shrubs, like forsythias, flowering quince, and azaleas after flowering since pruning before flowering will remove the flower buds. Summer-flowering plants, such as vitex (chaste tree), tea olive, roses and crape myrtles can be pruned in the dormant season since they flower on the new growth.

Pruning plays an important role in the development and maintenance of woody plants. Developing clear pruning objectives is important, and by combining them with a basic understanding of pruning and the response of plants to it, you can get the results you desire.

Timothy Daly, MS is an Agricultural and Natural Resource agent with Gwinnett County Cooperative Extension. He can be reached at 678-377-4010 or timothy.daly@gwinnettcounty.com.