Ask the ISU Extension Garden Experts About: Pruning Shrubs

February 24, 2011, 9:08 am | Richard Jauron, Willy Klein

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.  Plants will still 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 also may 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 be done. 

A second way to prune overgrown, deciduous shrubs is to cut them back to within 4 to 6 inches of the ground in March or early April. This severe pruning will induce a large number of shoots to develop during the growing season. 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 to 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. 


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