Proper time for pruning

Horticulture Hints

There are many important reasons to prune a tree, including pruning for safety, tree health and appearance. It is also important to know the proper time for pruning different types of trees in order to help prevent occurrence of disease.

Oak trees: December, January and February are the best months to prune oak trees in Iowa. Pruning during the winter months greatly reduces the risk of an oak wilt, a fungal disease that is deadly to many oaks.

Pruning cuts made in spring or early summer attract sap-feeding beetles, which feed on the sap extruding from pruning wound. When beetles feed on a tree infested with oak wilt, the disease fungal spores attach to the beetle’s body. Then, when moving to feed on the sap of a healthy tree, the disease spores transfer into the fresh wound and infect the tree.

If an oak tree must be pruned in spring or summer, for reasons of storm damage or other, prevent oak wilt transmission by artificially sealing the pruning wound. Apply a coat of latex-based interior paint to cover the wound so that beetles cannot transmit oak wilt spores. The paint will wear off by winter and allow the tree to start the natural healing process.

Shade trees: February through March is generally considered as the best time to prune most other shade trees. The absence of leaves during the winter gives a clear view of the tree structure and allows easier selection and removal of appropriate branches. Also, the healing of pruning wounds occurs most rapidly just before the new growth starts in the spring.

Large amounts of sap often flow from pruning cuts on maple, birch and elm when pruned in late winter. However, this loss of sap doesn’t harm the trees. Eventually the flow of sap will slow and stop.

Shade trees can be pruned in midsummer, but avoid pruning shade trees during spring when they are leafing out, as the tree’s energy reserves are low and the bark tears easily. Another poor time to prune is during leaf drop in fall.

Fruit trees: Late February and March are the best times to prune fruit trees in our area. Summer pruning of fruit trees is normally not recommended. However, water sprouts, those rapidly growing shoots that often develop just below a pruning cut can be removed in June or July.  

Horticulture Questions? Contact McCormick at yvonne@iastate.edu for information or advice.

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