Prune trees, shrubs carefully

By LSU AgCenter Horticulturists Dan Gill, Kyle Huffstickler and Allen Owings

Pruning is one of the activities that many home gardeners have questions about. When to prune? How to prune? Why prune?

To get started, we need to remember that no specific set of rules will cover all pruning. The important consideration should be preserving the natural form of a particular species.

The extent of annual pruning depends on each plant. Some shrubs may require the removal of a considerable amount of wood each year, while others require little pruning. It is much better to prune lightly each year rather than severely “butcher” a plant after several years of growth.

When you prune, first remove weak and spindly wood inside the plant or near the ground. Next, reduce the plant’s height to the desired level by making cuts at various points, always keeping in mind the natural form of the plant. One rule for cane-type plants like nandina and mahonia is to remove one-third of the oldest and tallest canes near the ground each year. This will keep the plant’s height at a reasonable level.

Experts used to believe that pruning cuts, especially large ones, needed to be painted with special pruning compounds to prevent the entry of insects and diseases. We now know that these compounds can cause the plant more harm than good.

When you prune, make all large cuts on the outside of the shoulder wrinkle to promote callus and healing. Cuts made flush with a primary branch or central trunk remove this shoulder wrinkle, and healing over never occurs, leaving your plant susceptible to insects and diseases.

Several special plant types or categories need special treatment to train them properly. These include espaliered plants, topiary work or “poodled” plants and other landscape oddities.

Pruning flowering shrubs depends on the time of the year they bloom. Prune late-winter- and spring-flowering shrubs after they flower. Pruning spring-flowering shrubs during winter removes the flower buds. Examples in this category include azalea, spirea, mock orange, quince, hydrangea, weigelia, forsythia, gardenia, camellia, viburnum, deutzia and flowering almond. For azaleas, make sure you finish pruning by late June or early July.

Prune summer-flowering shrubs from mid- to late winter, before spring growth begins. Some plants in this group are crape myrtle, oleander, vitex and althea. Most evergreens not selected for flowering should be pruned in the dormant, winter season, but some pruning may be done throughout the year.

For trees, young ones need to be pruned and trained properly to develop growth for the future, and mature trees may need an occasional pruning to maintain plant health and vigor.

Improper pruning of shade trees results in growth patterns that lead to structural weaknesses and disease and insect entry. In addition, branches not pruned properly can hamper human and vehicular activity, present problems with utility lines and interfere with buildings and related structures.

Root establishment is critical during the first three years after planting a young tree, and this determines whether the tree will survive. The amount of pruning and whether pruning is done correctly will be major factors in how that young tree will develop in the later years.

First, be sure that the tree has a dominant central leader, like a trunk or major branch. Next, select the permanent branches that will be the tree’s major structural framework in future years. These branches should have a wide angle of attachment to the main trunk for greatest strength. Also, they should be uniformly distributed around and up and down the trunk of the tree. This distribution up, down and around the tree is referred to as vertical and radial branch distribution.

While selecting the permanent branches, you may need to leave a few temporary branches to help the tree grow. These temporary branches can be removed after two or three years – as soon as the permanent branches have filled out.

Mature trees need dead wood removed occasionally. This process is called “dead wooding” or “cleaning out.” Crown thinning is another pruning technique and involves “opening up” the canopy of deciduous trees.

Another popular pruning technique for mature trees is thinning out. This reduces the height and spread of the tree while maintaining the natural shape. It is very important to prune mature trees properly. Such pruning needs to be done in most cases by a licensed arborist. Never “top” a tree.


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