Yard and Garden: pruning oak trees

AMES — Oak trees, generally pruned for safety reasons and the health of the tree, should be pruned during winter months. Learn why and how from horticulturists with Iowa State University Extension and Outreach. To have more questions answered contact Hortline at 515-294-3108 or hortline@iastate.edu.

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 called picnic bugs. Oak wilt infections most commonly occur in spring and early summer when the sap-feeding beetles are very active.

During this same time, oak wilt infected trees are producing masses of spore-producing fungal material or 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 on oaks in spring and early summer may attract sap-feeding beetles that were previously feeding on an oak wilt infested tree. As the beetles feed on the sap of the pruning cut, fungal spores may be dislodged from the bodies of the insects and 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.

What is the proper way to prune oaks and other trees?

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 three-cut procedure when pruning branches that are greater than 1.5 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.

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 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, paint the pruning cuts with a latex house paint within 15 minutes of making the cuts to prevent the transmission of oak wilt.