Landscape - Understanding Pruning Series

The following is the first in a series on understanding pruning. Types of pruning are covered.

Types of pruning

Five basis techniques are used for pruning shrubs: pinching, heading back, thinning, renewal pruning, and shearing. Some plants require more of one method than another, but good pruning is usually a combination of several methods.

Pinching - is the removal of the terminal portion of a succulent, green shoot before it becomes woody and firm. Pinching can greatly reduce the need for more dramatic pruning later on. Whenever (except late summer) you see a shoot becoming excessively long simply pinch or cut the shoot to reduce its length and to promote side branching. Long, vigorous shoots should be cut back into the canopy instead of cut at the outer limits of the existing foliage.

Heading back - involves removing the terminal portion of a woody branch by cutting it back to a healthy bud or branch. Heading back will stimulate shoot growth below the cut thus making the plant more dense. The shape of the plant can be influenced by cutting to inward or outward growing buds. The top bud should be located on the side of the branch that faces the direction new growth is desired. Some plants will have two buds opposite each other on the stem. When such stems are cut, remove one of the buds if you need to control the direction of new growth. If both are allowed to grow, a forked and often weak stem may develop. Repeated heading back with no thinning cuts results in a top heavy plant. Dense top growth reduces sunlight and results in the loss of foliage inside the plant canopy.

Thinning - is the least conspicuous method of pruning and results in a more open plant without stimulating excessive new growth. Considerable growth can be cut without changing the plant’s natural appearance or growth habit. With thinning cuts a branch is cut off at its point of origin from the parent stem, to a lateral side branch, to the “Y” of a branch junction, or at ground level. A good rule-of-thumb is to prune to a lateral that is one-third the diameter of the branch being removed. Thin out the oldest and tallest stems first, allowing vigorous side branch development. This method of pruning is best done with pruning shears, loppers, or a saw --- not hedge shears. Plants pruned by thinning include crapemyrtle, magnolia, viburnums, spireas, smoketree, and lilac. Repeated thinning with no heading back results in plants with long spindly branches. The entire plant may take on a straggly look.

Renewal pruning (rejuvenation) - involves removing the oldest branches of a shrub by prunig them near the ground, leaving only the younger, more vigorous branches which may also be cut back. Small stems (less than pencil size) should be removed. Plants pruned by renewal include abelia, deutzia, forsythia, mockorange, spirea, and weigela.

A variation of renewal pruning involves cutting all branches back to a predetermined height each year. Butterfly bush is often pruned back to woody framework. With time the framework becomes congested and will require some slight thinning. Yellow and red twig dogwood and beautyberry are severely pruned almost to the ground each year to promote the growth of more colorful twigs or berries.

Shearing - involves cutting the terminal of most shoots with shearing or hedge clippers. This method should not be used on foundation plants but should be restricted to creating formal hedges. Shearing destroys the natural shape of the plant. It causes a thick profusion of growth on the exterior of the plant that excludes light form entering the center of the plant (Figure 15-30). Foliage on the interior of the plant dies. The natural renewal growth from within the plant in prevented.

Extracted from "Pruning Shrubs" from North Carolina State University Cooperative Extension.
http://www.ces.ncsu.edu/depts/hort/consumer/factsheets/shrubs/text/pruning.html#broadleaf