When To Prune
The best time to prune many shrubs usually is late winter, when they are not actively growing. You should prune out diseased or dead branches on any shrub any time of the year.
If you have flowering shrubs, be aware that spring-flowering and summer-flowering shrubs must be pruned at different times. Prune spring-flowering shrubs like azaleas, deutzias, forsythias, rhododendrons, spireas, and weigela, right after they bloom in late spring. If pruned too late in the summer, these shrubs will lose next year’s flower buds, which are set shortly after this year’s bloom. Pruning spring shrubs does not have to be an annual ritual; prune them only if they’ve gotten too large or if there is dead wood to be cleaned out. Summer- and fall-flowering shrubs such as abelia, buddleia, hibiscus and oleander do not make their buds ahead of time and keep them through the winter. They bloom after August and form their buds the next spring on new branches. They should be pruned in the fall or very early the following spring before growth starts.
Tip: If you live in the far West and South, your shrubs will only be only partially dormant. If you prune them during the winter, they may be severely shocked because they have not yet finished pulling all their nutrients from the stems for storage in the roots. Instead, prune in early spring just before growth starts again.
There are two main methods of pruning shrubs: Thinning and heading. Each of these can be applied to multistem or single stem shrubs.
Thinning involves completely removing a twig or branch down to the ground, as with a multistemmed shrub, or back to another main branch or trunk. Care must be taken not to leave a stub. Thinning is best used with shrubs that are too dense. By cutting off many of the inward growing branches just above an outward growing bud, you can influence the remaining branches to grow new twigs toward the outside of the shrub and open up its structure.
Heading involves cutting branches back to healthy buds. This leaves a cut close to a bud from which new growth develops. Heading is especially useful for filling in “leggy” shrubs as it stimulates dense growth of many smaller buds and twigs lower on the branch. You can control the shape of the shrub somewhat by noting the bud's location on the branch at the end of the cut. An inward pointing bud will stimulate growth toward the center of the shrub making it a denser plant; an outward growing bud will do the opposite.
Tip: Plants that winter kill, such as crape myrtle and hydrangea, should be thinned down to ground level in early spring. Dead wood should be cut down to the soil level, and live wood should be cut down to 6 or 12 inches. This will increase flower production and grow stronger stems.
Making The Cut
The key to making a good pruning cut is to make it cleanly, so that the wound or cut end will heal properly in the shortest possible time with the least risk of pest or disease attack. A correct pruning cut is made at a 45 degree angle, about ¼ inch above the bud. If the cut is too far from the bud, a stub will result. Stubs do not heal over properly and will eventually dry and crack, providing a source of entry for disease and insects. If the cut is too close, the bud may be damaged. If the cut is slanted too sharply, more wood tissue will be exposed to damage.
To avoid leaving a stub that could sprout or invite rot, place the hook of a bypass-bladed pruning tool or loppers around the branch with the large cutting blade against the parent stem. Then cut from the bottom of the branch junction. Otherwise the blade may get wedged in a narrow crotch and make a ragged cut. Always keep your pruning tools well sharpened.
Tip: To avoid the risk of spreading plant diseases (such as bacteria and fungi), sterilize your pruning tools before and after the job. Disinfect them by dipping them for 5 minutes in a container filled with a solution of 1 part household bleach (5.25% sodium hypochlorite) to 9 parts water. Or spray the tools with Lysol aerosol spray.
How Not To Prune
Remember that a shrub’s height and general shape are genetically determined. Whenever you cut a branch off, all the buds below the cut will start growing as the plant tries to replace its lost growth.
Careful pruning works because it utilizes and guides this result. The common practice of “topping” a shrub, or simply lopping off the outlying branches to reduce the plant to the desired height, produces weak, unsightly new growth that is clustered near the top of the shrub. You will actually end up doing more work to correct the original mistakes.
A variant of topping, called shearing, is actually quite useful with certain evergreen shrubs, especially those you want to train to form a hedge. To be successful, shearing requires that you clip tender new growth every year in early spring.