Sustainable Landscape News Distributed 02/26/10

By LSU AgCenter Horticulturists Dan Gill, Allen Owings and John Young

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

To start, 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 plant.

The extent of annual pruning depends on the plant. Some shrubs may require the removal of a considerable amount of wood each year, while others require little pruning. It’s 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, lower the height of the plant to the level you want by making cuts at various levels, 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 height of the plant at a reasonable level.

People once believed 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 these compounds can cause the plant more harm than good.

When you remove branches, make all large cuts on the outside of the shoulder “wrinkle” to promote callus and healing. If you remove this shoulder wrinkle by cutting flush with a primary branch or central trunk, the wound will never heal, leaving your plant susceptible to insects and diseases.

Several special plant types or categories need special treatment. These include espaliered plants trained to special shapes on wire, topiary or “poodled” plants and other landscape oddities.

For flowering shrubs, pruning depends on the time the plants bloom.

Prune late-winter- and spring-flowering shrubs after they flower. If you prune spring-flowering shrubs during the winter, you’ll remove 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, complete pruning by late June or early July.

Prune summer-flowering shrubs from mid- to late winter, before spring growth. Some plants in this group are crape myrtle, oleander, vitex and althea.

Most non-flowering evergreens should be pruned in the dormant, winter season, but some pruning may be done throughout the year.

For trees, young trees 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.

Improperly pruning shade trees results in growth patterns that lead to structural weaknesses and entry points for diseases and insects. In addition, improperly pruned branches 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 it determines whether the tree will survive. The amount of pruning and whether pruning is done correctly will be the major factor in how that young tree will develop in the later years.

First, be sure the tree has a dominant central leader, like the trunk or major branch. Next, identify the permanent branches that will be the major structural framework of the tree 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 both around and up and down the trunk of the tree. This distribution – up, down and around – is referred to as vertical and radial branch distribution.

While selecting the permanent branches, you may need to leave a few branches temporarily 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 occasionally need to have dead wood removed. This process is called “dead wooding” or “cleaning out.” Crown thinning is another pruning technique and involves “opening up” the canopy of deciduous trees that lose their leaves in winter.
 
Never “top” a tree.

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.

Visit LaHouse in Baton Rouge to see sustainable landscape practices in action. The home and landscape resource center is near the intersection of Burbank Drive and Nicholson Drive (Louisiana Highway 30) in Baton Rouge, across the street from the LSU baseball stadium. For more information, go to www.louisianahouse.org and www.lsuagcenter.com/lyn.

Rick Bogren