February 4, 2002
Woodbury Bulletin
Betsy Halden

When should we prune?

A frequent question gardeners ask is “When should I prune my shrubs and trees?” Knowing when to prune is important in order to avoid certain diseases and physiological problems. Here are some guidelines for the proper time to prune various woody plants in your yard and garden.

Deciduous Shrubs
Many homeowners plant deciduous shrubs for their attractive flowers. Spring-flowering shrubs such as lilac, forsythia, magnolia, or azalea bloom in the spring on the growth of the previous season. The proper time to prune spring-flowering shrubs is largely determined by their condition and the amount of pruning required.

Old neglected spring-flowering shrubs often require extensive pruning to rejuvenate or renew the plants. The best time to rejuvenate large, overgrown shrubs is late winter or early spring, mid-February to early April, before the plants begin to leaf out. While heavy pruning in late winter or early spring will reduce or eliminate the flower display for a few years, the restoration of a healthy, vigorous shrub is more important.

If your spring flowering shrubs need only light pruning, prune them immediately after they have bloomed. Pruning immediately after bloom allows you to enjoy the spring flowers, and it gives the shrubs adequate time to initiate new flower buds for next season. Or, you can cut the bloom-bearing branches and bring them into the house and enjoy them as cut flowers. I love the fragrance of fresh lilacs in our home.

Summer flowering shrubs, such as potentilla, bloom in summer on current year’s growth. Prune these shrubs in late winter or early spring. Summer –flowering shrubs pruned from mid-February to early April will still bloom in summer.

Many deciduous shrubs are grown for their foliage, not for their flowers. Shrubs such as dogwood, barberry, honeysuckle should be pruned in spring before growth begins.

Evergreen Shrubs
Arborvitae, junipers, yews, and hemlocks grow continuously throughout the growing season. They can be pruned any time through the middle of summer. Even though these plants will tolerate heavy clipping, their natural form is usually most desirable. You may want to prune only to correct growth defects.

Deciduous Trees
The best time to prune deciduous trees is late winter or early spring, late February, March, early April, before the trees begin to leaf out. Some trees, such as maples and boxelder, ‘bleed” heavily when pruned in late winter or early spring. The heavy bleeding does not harm the trees. The effects are cosmetic. The trees won’t bleed to death and the flow of sap will gradually slow and stop.

To prevent the spread of oak wilt, DON’T prune during April, May, or June. Pruning oaks during this period may attract sap beetles carrying the oak wilt fungus to the pruning cuts. The beetles then transmit the disease to healthy trees. If oaks are wounded or must be pruned during these months, apply wound dressing to mask the odor of freshly cut wood so the beetles will not be attracted to the trees. The best time to prune oaks is February and March.

If possible, avoid pruning deciduous trees in the spring as they are leafing out. At this time the tree’s energy reserves are low and the bark “slips” or tears easily. Another poor time to prune is during leaf drop in the fall.

Evergreen Trees
Late winter is an excellent time to prune spruce and fir, while they are still dormant. Spruce and fir possess side or lateral buds. The side bud will branch if the terminal tip is removed. The pruning cut should be just above a side bud or branch. Some spring pruning will not harm the trees.

Pines seldom need pruning. If you want to promote more dense growth, remove up to two-thirds of the length of newly expanded candles. Don’t prune further back than the current year’s growth. Pines do not have lateral buds, so removing terminal buds will take away new growing points for that branch. Eventually, this will leave dead stubs.

Unwanted lower branches on evergreen trees can be removed in late winter.

Fruit Trees
Prune apple trees, including flowering crabapples, mountain ash, hawthorns, and shrub cotoneasters in late winter, February, March, early April. Spring or summer pruning increases chances for infection and spread of the bacterial disease fireblight. Autumn or early winter pruning is more likely to result in drying and die-back at pruning sites.

Would you like to learn more about pruning? “Pruning Trees and Shrubs” is the title of the informative document published by the University of Minnesota Extension Service. You can read it on the web: www.extension.umn.edu and click on “Pruning Trees and Shrubs – Late winter is the time to prune for health and appearance.”

Betsy Halden is a resident of Woodbury. She consulted Cornell Cooperative Extension and the University of Minnesota Extension Service materials when preparing this column. You can reach Betsy Halden at eahalden@comcast.net