Late March to early April is the time to stop pruning oak trees. As publicized previously, oaks wounded during the warm season attract sap beetles which transmit the deadly oak wilt fungus. Members of the red oak family usually die within a month or so of coming into contact with the fungus.
What is the date to cease pruning?
I have often been asked for exact dates when pruning of oak trees should stop and when pruning can resume. In general, April 1 (fool’s day), is the time to cease pruning for the coming warm season; anyone pruning after this date is probably taking too much of a risk and may be jeopardizing the health of the oak trees they are maintaining. We must remember that insects and diseases do not recognize artificial dates created by Homo sapiens. As more information is learned about the nature of the disease and the insect vectors, we will undoubtedly determine that the insect vectors recognize temperature and heat units as the factors important in their activity....or inactivity.
Are you confused?
There has been literature in the past that suggested oak trees should not be pruned in April, May and June. In fact, it may be implied from a new publication from MSU in 2001 that oak trees may be safely pruned after June 30. Again, the insect vectors do not recognize artificial dates, and we know that sap beetles, such as picnic beetles, remain active from spring through fall. As individual plants, oak trees are among the most valuable plants in Michigan. I strongly recommend that because of the property value loss associated from oak wilt, because of the costs of removing large trees and because of the cost of containing oak wilt, it is prudent that oak trees only be pruned dormantly; other scientists and arborists have also joined me in this view. Trees which must be pruned during the warm season, for reasons of storm damage or otherwise, should be treated with a sealant on the wounded surface immediately after the wound is created; this treatment will hopefully prevent transmission of the oak wilt fungus to the tree.