Yard and Garden: Pruning Shrubs

AMES, Iowa — In a single motion, pruning demonstrates both the art and science of horticulture. Perhaps that’s why so many homeowners get nervous and postpone or ignore the task. Horticulturists with Iowa State University Extension and Outreach remove the mystery surrounding this routine practice. To have additional questions answered contact the Iowa State University Hortline at hortline@iastate.edu or 515-294-3108.

When should I prune my shrubs?

The proper time to prune deciduous and evergreen shrubs is determined by the plant’s growth habit, bloom time and health or condition.

Spring-flowering shrubs, such as lilac and forsythia, bloom in spring on the growth of the previous season. The health or condition of the plants determines the best time to prune spring-flowering shrubs.

Neglected, overgrown 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 (March or early April). Heavy pruning in late winter or early spring will reduce or eliminate the flower display for two or three years. However, rejuvenation pruning will restore the health of the shrub.

The best time to prune healthy, well-maintained spring-flowering shrubs is immediately after flowering (healthy, well-maintained shrubs should require only light to moderate pruning). Pruning immediately after flowering allows gardeners to enjoy the spring flower display and provides adequate time for the shrubs to initiate new flower buds for next season.

Summer-flowering shrubs, such as potentilla and Japanese spirea, bloom in summer on the current year’s growth. Prune summer-flowering shrubs in late winter or early spring. The new growth produced by pruned shrubs will bloom in summer.

Some deciduous shrubs don’t produce attractive flowers. These shrubs may possess colorful bark, fruit or foliage. Prune these shrubs in late winter or early spring before growth begins.

Prune evergreen shrubs, such as juniper and yew, in early to mid-April before new growth begins. Light pruning may also be done in mid-summer.

What is the best way to prune large, overgrown shrubs? 

Proper pruning can renew or rejuvenate overgrown, deciduous shrubs. One option is to prune the shrubs back over a three year period. Begin by removing one-third of the largest, oldest stems at ground level in late winter/early spring (March or early April). The following year (again in March or early April), prune out one-half of the remaining old stems. Also, thin out some of the new growth. Retain several well-spaced, vigorous, new shoots and remove all of the others. Finally, remove all of the remaining old wood in late winter/early spring of the third year. Additional thinning of new shoots should also be done.

A second way to prune overgrown, deciduous shrubs is to cut the shrubs back to within 4 to 6 inches of the ground in March or early April. This type of pruning induces shrubs to produce a large number of new shoots. The new shoots grow rapidly and may be several feet tall by the end of summer. In late winter of the following year, select and retain several strong, healthy shoots and remove all others at ground level. Head (cut) back the retained shoots to encourage branching. Overgrown lilacs, dogwoods, privets and forsythias may be pruned in this manner. Most lilacs rejuvenated by this method will not bloom for two or three years. This method is also an excellent way to renew scraggly potentillas and summer-flowering spireas. For best performance, potentillas should be cut back to within 3 to 4 inches of the ground about every three years.