Issue 3, May 9, 2016
Hold Off Pruning Oaks and Elms
Warm spring weather provides ideal working conditions for many landscape maintenance projects. However, some projects, such as pruning, can be harmful if done at the wrong time of year. For example, Oak Wilt and Dutch Elm Disease (DED) are two devastating fungal diseases that are more likely to occur on trees pruned early in the growing season. Recently pruned trees can attract insect vectors that transmit the spores of the fungal pathogens. Once infected, both diseases rapidly kill their host tree, often in a matter of weeks. The hazard of either disease entering a tree through a pruning wound can be reduced by pruning during the dormant season. Pruning elm and oak trees, especially oaks within the red oak group, should be avoided from April to October. The risk of this type of infection decreases by mid-July. However, those erring on the side of caution may choose to postpone any pruning until dormancy.
Exceptions to this rule occurs with storm damaged trees and some DED infected elm trees. Storm damaged trees should be promptly pruned to eliminate hazardous conditions and to facilitate wound closure. If a new, upper-crown DED infection is detected early enough, the DED fungus can be eradicated from the tree by pruning out the diseased limb or limbs. While this can be a fairly aggressive procedure, it can be quite successful. Such pruning is most likely to work when less than 5% of the crown is affected.
A minor pruning related problem occurs to maples, walnuts, birches, beeches, hornbeams, and yellowwood. These trees species are collectively referred to as "bleeders" because they will "bleed" clear sap if pruned in early spring. Although the heavy sap flow can be quite alarming, will not harm the health of the tree. To avoid sap loss, delay pruning these species until later in the spring after the trees have fully leafed out. (Travis Cleveland)