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. Understanding how plants grow, why pruning is necessary and which tools to use can remove the mystery surrounding this routine practice. Horticulturists with Iowa State University Extension and Outreach answer pruning questions. To have additional questions answered, contact the ISU Hortline at firstname.lastname@example.org or 515-294-3108.
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 best time to prune spring-flowering shrubs depends on the health or condition of the plants.
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 (late February to early April). Heavy pruning in late winter or early spring will reduce or eliminate the flower display for two or three years. However, the long-term results of rejuvenation pruning are restoration of plant health, improvement in plant appearance and greater bloom.
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 pruned shrubs will bloom in summer on the current year’s growth.
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.
February through March is generally regarded as the best time to prune most deciduous trees. The absence of foliage at this time of year gives the individual a clear view of the tree and allows the selection and removal of appropriate branches. Also, the walling-off or compartmentalization of wounds occurs most rapidly just prior to the onset of growth in spring. Oaks are an exception. The winter months – December, January and February – are the best time to prune oak trees.
Deciduous trees can be pruned at other times of the year with little or no negative consequences. However, if possible, avoid pruning deciduous trees in spring when trees are leafing out and in fall when trees are dropping their leaves.
To reduce the risk of an oak wilt infection, do not prune oaks from March through October. Oak wilt is a fungal disease that is lethal to many oaks. It can be spread from infected trees to healthy trees by sap-feeding beetles (“picnic bugs”). If an oak tree must be pruned in spring or summer (such as after a storm), apply latex house paint to the pruning cuts to avoid attracting sap-feeding beetles to the wounds.
Late February to early April is the best time to prune fruit trees in Iowa. Summer pruning of fruit trees is generally not recommended. However, water sprouts (rapidly growing shoots that often develop just below a pruning cut) can be removed in June or July.
Fruit producing shrubs, such as gooseberries, currants and blueberries, should be pruned in late winter or early spring. In Iowa, pruning can be done from late February until bud break.
The most desirable time to prune grapevines is late winter or early spring. In Iowa, pruning can begin in late February and should be completed by early April. Grapevines pruned at this time of year may “bleed” heavily. However, the loss of sap does not harm the vines.