Pruning should be done with a purpose in mind. In the early years of a shrub’s development, pruning is about developing a good structure for the shrub. This structural pruning serves to maintain a good shape and size, within the limits of a plant’s normal size range. The best results are attained through selective pruning, rather than shearing. Selective removal of certain branches allows us to remove weaker branches and maintain those that are stronger. Selective pruning does take more time, but it yields better results. It allows us to make healthier cuts and work with the natural shape of each plant.
Proper pruning of shrubs helps to maintain vigor, remove damaged or diseased branches, and improve flowering and fruiting. There are four basic pruning techniques used for maintaining shrubs: heading back, renewal, rejuvenation, and shearing. For information on pruning specific shrubs, download the following list.
Time to Prune
Pruning time for deciduous shrubs depends on when the shrub flowers and when the new flower buds are produced. Spring-flowering shrubs will produce new flower buds in early summer after flowering ends. When the next spring arrives, we say that the buds that open in that spring were produced on “old wood”, as they were produced on the stems of last year. Since those flower buds will be present all winter, pruning in winter or early spring would remove the buds and reduce flowering. These shrubs should be pruned in the two to three week period after spring flowering ends. Some examples are forsythia, viburnum, mock orange, oakleaf hydrangea, spring flowering (white flowered) spirea, and lilacs.
Shrubs that flower later in the summer will produce their buds in spring on “new wood” that will be produced in spring. These shrubs can be pruned any time during dormancy, from late fall until early spring. Among these are panicled hydrangea, wild hydrangea, St. John’s wort, potentilla, summer flowering (pink flowered) spirea, and rose of Sharon. This does not apply when shrubs are undergoing rejuvenation pruning.
Making Good Pruning Cuts
Making good cuts is vital to the healing of the cut and the overall health of the shrub. When cutting smaller branches with a hand pruner or long-handled lopper, cut above a bud, cutting close enough so that no stub is left, but not so close that the bud is damaged. The bud is the next growing point, so it needs to be undamaged. A stub that is left beyond the bud will eventually die back and can become an entry point for diseases and insects.
For larger shrubs, a pruning saw may be needed to prune larger branches. When pruning out a larger branch, the cut should be made where the branch arises from the trunk at a swollen area known as the branch collar. The cut should not be made flush with the main branch or trunk. This results in a very large wound and a greater possibility for poor healing and the introduction of decay. Instead, place the saw just outside the branch collar (the swollen area where the branch meets the trunk). This leaves a smaller wound and retains the branch collar. The branch collar contains the tissues that are most beneficial to the healing process. Pruning paints or sealants are not recommended.
Heading Back
Heading back refers to a particular type of pruning cut. A heading back cut removes only a portion of the branch, cutting it back to a bud or side shoot. That bud or side shoot becomes the next growing point. Heading back also refers to a type of pruning where only heading back cuts are made. It is used to control the size of the shrub or to remove a branch that is out of balance with the rest of the plant. It requires that a branch be removed to a good bud or lateral branch instead of being cut to the ground. Heading back is a good procedure for shrubs that have only a single main stem or possibly two to three main stems, making it not feasible to cut stems back to the ground.
Rejuvenation
Some older or neglected shrubs can be restored to vigor by pruning all the stems down to a four- to six-inch height. This is best done in early spring (around March in most years). That gives the shrub an entire growing season to recover. This severe method of pruning does not need to be done every year. It is a good method for shrubs with numerous small to medium stems coming out of the ground. If this method is used for spring-flowering shrubs, the flowers will be removed for that spring, but the shrub will flower the following spring. Among those that respond well to this treatment are forsythia, weigela, ninebark, red twigged dogwood, and spirea.
Renewal
Renewal is the removal of old, overgrown stems. It uses a pruning cut called thinning out. Thinning out cuts remove branches all the way to the ground. This is usually done over a period of three to four years, with a few of the stems removed each year. Removing the oldest stems also tends to remove the tallest ones, so the height of the shrub is lowered in a natural fashion. Removing stems all the way also allows for better light penetration that can stimulate new growth and promote more flowering. It also allows for better air circulation which can reduce the incidence of leaf diseases. This results in a more vigorous shrub and, in the case of flowering ornamentals, better flowering. Renewal is best done during the dormant season or in early spring. This method of pruning is useful for shrubs that have a number of medium to larger stems arising from the ground like lilac, viburnums, and red-twigged dogwood.
Shearing
Shearing should be used in a limited fashion. It is generally done only when a formal hedge is desired. Shearing individual shrubs is not recommended. When shearing, it is important to keep the top of the hedge narrower than the base to allow sunlight to reach the lower branches. This allows the lower part of the shrub to maintain its leaves. Shearing can have a negative impact on the health of a shrub. It can cut through leaves and cut stems in sites that will leave stubs that might be open to fungal infection. Sheared shrubs and hedges often have a shorter useful life span than unsheared shrubs.