When is the best time to prune oak trees?
Winter (December through February) is the best time to prune oak trees in Iowa. Pruning oak trees in winter greatly reduces the risk of an oak wilt infection.
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”). Oak wilt infections occur most commonly in spring and early summer. The sap-feeding beetles are very active in spring and early summer. During this same time, oak wilt infected trees are producing masses of spore-producing fungal material (spore mats). These mats release a fruity odor that attracts sap-feeding beetles and other insects. As the beetles feed on the spore mats, spores often accumulate on the surface of their bodies. Sap that forms at the surface of pruning cuts made in spring or early summer may attract sap-feeding beetles that may have been previously feeding on an oak wilt infested tree. As the beetles feed on the sap of the pruning cut, fungal spores get into the fresh wound, infecting the tree. Pruning oak trees in winter greatly reduces the risk of an oak wilt infection as the beetles and fungal mats are not present at that time of year.
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.