By: Lisa Johnson, Extension Educator
In late March / early April, the window for dormant pruning of trees and shrubs is almost closed for the season in much of Wisconsin. Oaks and elms should not be pruned once the temperature hits 50°F as the beetles that carry oak wilt and Dutch elm disease fungi become active at that temperature. Oaks and elms should only be pruned if the temperature is going to be reliably below 40°F and preferably lower for several days before, during and after pruning. In Central and Northern Wisconsin, pruning of these species may still be possible this spring.
For trees and shrubs that don’t have these types of issues, dormant pruning can still be done if buds are not swelling and turning green. Pruning summer-blooming shrubs like Japanese spirea, shrub hibiscus, panicle hydrangeas (these have tear-drop shaped flowers in July-August), smooth hydrangeas (these have large white rounded panicles), and potentilla now will not affect flowering as they bloom on new wood that has not been produced yet.
Spring-flowering shrubs like lilacs, mockorange and weigela should not be pruned this time of year unless you don’t mind losing flowers. See the UW-Extension factsheet on ‘How to Properly Prune Deciduous Shrubs‘ for more information.
Red-twig dogwood and shrub willows are prone to canker diseases that infect the branches, causing discolored sunken areas in the bark. Prune these plants annually using the thinning method to keep them healthy and vigorous. Using the thinning technique, remove about 30% of the biggest, oldest stems right at ground level, then lightly prune to shape the remaining stems using heading back cuts about ¼” above a bud facing the outside (as opposed to the inner part) of the shrub. Older growth is less vigorous and more susceptible to disease, so thinning encourages younger, more vigorous growth that is more disease-resistant.
Thinning also decreases humidity by removing old growth, which can discourage foliar fungal diseases like powdery mildew. Double-flowering almond and purple-leaf sandcherry are susceptible to a lethal bacterial disease called fire blight, that strikes some fruit trees as well. It also causes cankers, but the bacteria overwinter there and ooze out in a sticky sap in spring. Be careful when pruning these shrubs as well as the willows and dogwoods with cankers as these diseases can be transmitted on pruning shears and moved from plant to plant. When pruning diseased material, disinfect your shears with rubbing alcohol after each cut instead of just between plants.
