The usual goals of pruning shrubs are to keep them at the same size (controlling size), make them smaller (reducing size), increase their flowering, encourage colorful bark, or to make them more shapely. This section describes how to achieve each goal.
Pruning Shrubs to Control Size
Despite their beauty and diversity, most deciduous shrubs grow beyond the space allotted for them in the landscape. For this reason alone, be prepared to prune. Common shrubs such as lilac, viburnum, and privet can grow surprisingly large, sometimes reaching 20 feet tall when mature. Few shrub species sold in nurseries will remain under 3 feet tall, Dwarf and compact-growing cultivars stay more in scale with the home landscape, but still require pruning to control size in many garden locations.
Some shrubs grow quickly, others slowly. Choose the right shrub for the space or reserve enough space to accommodate the shrub as it grows, and there will be no need for concern about growth rate. Unfortunately, availability, price, and flowering effect — not mature size — are usually the reasons for selecting particular shrubs. Consequently, large-growing shrubs often end up in small spaces.
Most homeowners — and even many landscapers — are concerned that a new planting not look sparse, so they plant shrubs close together. In the space of several years, the shrubs begin to crowd each other. In this case it is better to remove or transplant every other shrub rather than prune them all severely year after year.
Only minimal pruning will be needed if shrubs are spaced so that their branches mingle slightly when they reach their mature size. Where space is limited, however, fast-growing shrubs need annual pruning. Slow growers, which make considerably fewer demands on your time, may not need yearly attention.
Pruning Shrubs to Reduce Size
Thinning cuts are used to greatly reduce the size of a shrub without changing its natural branching structure. Prune a main branch back to a lateral branch with loppers or hand pruners. Cut the branch off where it forms a Y with the side branch, being careful not to leave a stub. The lateral branch should be at least two thirds the diameter of the main branch so that apical dominance and natural growth habit are maintained. Thinning cuts mimic and accelerate nature’s way of shedding old, dead, and dying branches.
Don’t head branches to reduce shrub height. Heading — that is, cutting individual branches off above or below a bud or at a small-diameter lateral branch — causes many vigorously growing stems to arise directly below the cut. This can produce an unnatural flat-topped shape and crossed branches. Along with masking the branching characteristics, this type of pruning doesn’t reduce the size of the shrub for long. The new growth arises quickly, soon growing beyond the pruning point. As a result of heading, a shrub actually increases gradually in height. This happens because a heading cut goes only partway into the previous season’s growth, allowing the shrub to slowly grow larger. Since few, if any, new shoots will arise from the base of the shrub, the existing branches become older and less productive, and the shrub gradually declines in vigor. The dense growth at the top of the plant also reduces light penetration and slows interior growth, often leaving the inside of the shrub bare and full of dead twigs.
Pruning Shrubs to Increase Flowering
Pruning increases flowering by encouraging new wood to form and by removing older, less-productive branches. Most deciduous flowering shrubs have multiple stems and send up new growth from the ground. Soon after these shoots develop into branches, they reach their flower-bearing prime. With age they grow less energetically. Keep flowering shrubs in a vigorous condition indefinitely by constantly removing old stems.
In the late summer, when growth begins to slow and sugars accumulate in woody branches, most deciduous flowering shrubs set flower buds for the following year. The more sugars accumulate, the more flower buds are set. Severe pruning in late winter or spring may remove so much growth that all the plant’s energy goes into producing stems and leaves, with little energy left to create flower buds for the following season. Heavy pruning in late summer removes many of the flower buds that have just been set, resulting in a poor floral show the next year.
Although deciduous shrubs can renew themselves even when large quantities of old wood are removed, it is best to leave some mature flowering wood. For the showiest flower display, prune a little each year instead of waiting for the shrub to grow into a tangled mass that must be cut back severely. Thin regularly to renew the shrub, always maintaining a healthy stand of prime branches for a lively show of flowers.
Pruning Shrubs to Encourage Colorful Bark
Some deciduous shrubs have attractive gray, green, red, or yellow twigs that become darker and turn brown as they age. Encourage showy new growth by thinning the oldest and darkest wood. Shrubs with vividly colored stems such as the red-osier dogwood need regular removal of old, dull-barked branches to be their most colorful.
Other shrubs such as oak-leaved hydrangea bear interesting flaking or exfoliating bark on older wood. In this case the pruning goal is just the opposite. Maximize the exfoliating effect by retaining mature branches and remove some lower branches to reveal the attractive stems.
Pruning Shrubs to Maintain Shape
Try to use natural forms of shrubs to fulfill specific functions in the landscape. For example, stiffly upright shrubs can serve as space dividers, a hedge, or a living wall. Low, horizontally spreading shrubs can cascade over a bank or wall, or cover the ground. Rounded shrubs make a good background for most landscape plantings.
To enhance and maintain the inherent form, it is important to learn to prune properly and to respect the natural branching habit of the shrub. If pruning alters the shape of a shrub, the entire landscape design could be affected. Thoughtful pruning will emphasize the natural branching pattern of each species. Most shrubs have a rounded and upright, but gradually spreading, form and are easy to prune.
Shrubs like forsythia (Forsythia x intermedia) are upright when young but their branches gradually relax and cascade softly with age. Shrubby dogwoods with many small twigs give a delicate, fine-textured winter effect when massed. The horizontally spreading but somewhat stiff branches of the double file viburnum (Viburnum plicatum tomentosum) make a strong, bold showing in the shrub border. Dwarf winged euonymus (Euonymus alata ‘Compacta’), with its distinct, upright, vase-shaped branching, grows so slowly that it seldom needs pruning.