When to prune trees
Light pruning to remove a few small branches of trees can be done at any time of year. More extensive pruning should be done in late winter to early spring. There are three advantages to pruning at this time of the year.
Second, there are few insects and disease spores to infest pruning cuts.
Third, deciduous trees have dropped their leaves, so it's easier to see what you're pruning!
Trees such as elm, maple, walnut and birch are "bleeders" when pruned in late winter to early spring. This oozing sap is annoying when it drips on cars and sidewalks. There is disagreement as to whether the oozing is harmful to trees. It is thought that oozing might interfere with the closure of pruning cuts. Oozing is reduced if these tree species are pruned in the fall. However, fall is probably the least desirable time of year to prune most trees. Pruning cuts close more slowly in the fall than in any of the other seasons.
For more information, see the following Colorado State University Extension fact sheet(s).
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Updated Tuesday, November 19, 2013