Oak trees are valuable assets in the home landscape. Occasionally oak trees need to be pruned for health, safety, and appearance reasons. Pruning oaks makes them more valuable to infection from the fungus that causes oak wilt. By pruning oaks properly, you can reduce that risk and keep trees healthy.
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. Oak wilt is caused by the fungus Bretziella fagacearum (previously Ceratocystis fagacearum). It can be spread from infected trees to healthy trees by sap-feeding beetles ("picnic bugs").
Oak wilt infections most commonly occur 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.
The sap that forms at the surface of pruning cuts made in spring and 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.
When pruning oak and other trees, cut off the branch just beyond the branch collar and branch bark ridge. The branch collar is the swollen area at the base of the branch. The branch bark ridge is the dark, rough bark ridge that separates the branch from the main branch or trunk. Pruning just beyond the branch collar and branch bark ridge retains the tree's natural defense mechanisms and promotes compartmentalization and callus formation.
To prevent extensive bark damage, use a 3-cut procedure when pruning branches that are greater than 1½ inches in diameter. Make the first cut 6 to 12 inches from the main branch or trunk. Cut upward and go about one-third of the way through the branch. Make the second cut 1 to 2 inches beyond the first. Saw downward from the top of the branch. As the second cut is made, the weight of the branch will cause it to break at the pivot point between the two cuts. (The initial, bottom cut prevents the branch from ripping off a large piece of bark as it breaks.) Make the final cut just beyond the branch collar and branch bark ridge. More information about pruning trees can be found in this publication: Pruning Trees: Shade, Flowering, and Conifer
Do not apply a pruning paint or wound dressing to pruning cuts. The application of a pruning paint or wound dressing does not prevent wood decay and may actually interfere with the tree's natural wound responses. However, there is an exception to the no paint recommendation. If an oak tree needs to be pruned in spring or summer, for example to correct storm damage, immediately (within 15 minutes) paint the pruning cuts with a latex house paint to prevent the transmission of oak wilt. The paint can be any color, although green, brown, or black will blend in and look better than other colors.
Learn more about Oak Wilt in this Article: Oak Wilt
Your Complete Guide to Pruning Trees and Shrubs