Prune liriope in late February prior to new growth. (Photo by Allen Owings. Click on photo for downloadable image.)

Topping is an extreme pruning method, shown here on waxed-leaf ligustrum. This is not recommended. (Photo by Allen Owings. Click on photo for downloadable image.)

Boxwood often is pruned formally to achieve a particular look. (Photo by Allen Owings. Click on photo for downloadable image.)

News Release Distributed 01/25/13

By Allen Owings
LSU AgCenter horticulturist

HAMMOND, La. – No specific set of rules will cover all pruning. The important consideration should be preserving the natural form of a particular species.

For shrubs, the extent of annual pruning depends on the plant. Some shrubs may require removing 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 pruning, first remove weak and spindly wood inside the plant perimeter or near the ground. Next, reduce the height of the plant to the desired level by making cuts at various heights, 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.

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.

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.

Several plant types or categories need special treatment for the purpose of specific training. These include espaliered plants, topiary 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. If you prune spring-flowering shrubs during winter, you will be removing flower buds. Examples in this category include azalea, spirea, mock orange, quince, hydrangea, forsythia, gardenia, camellia, viburnum, 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.

Prune ground covers, such as liriope, before growth commences in late February or early March.

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 disease 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 influence how a young tree will develop in the later years.

First, be sure the tree has a dominant central leader, like a trunk or major branch. Next, select 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 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 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 occasionally need 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. 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.

You can see more about work being done in landscape horticulture by viewing the LSU AgCenter Hammond Research Station website. Also, like us on Facebook. You can find an abundance of landscape information for both home gardeners and industry professionals.

Rick Bogren