During the autumn months, many homeowners sit back and watch their leaves turn from green to colorful hues, thinking their tree maintenance is done for the year. However, fall – and winter – is important and advantageous time to prune your trees to keep them healthy for next spring.
Why prune in the fall and winter?
The threat of insects and disease is significantly decreased in fall and winter months, giving trees time to recover from pruning wounds without risk of infection. Additionally, pruning in the fall after leaves have already fallen redistributes growth to the remaining parts of the tree come spring. A side benefit of fall and winter pruning is that it reduces the distress homeowner’s feel when they lose foliage from their yard.
Why prune at all?
Some homeowners question whether pruning is really necessary, as trees in the forest grow and live for years without any maintenance. However, because forests initially have hundreds of trees in a small space, the trees adapt to their environment. The number of trees on an acre is dramatically reduced over time. Thousands of trees per acre die between the time they are small and when they become mature as a result of competition between trees for light, water and nutrients.
Left: Tree in yard with low-hanging branches.
Right: Trees in forest with branches only at the top.
The surviving trees are naturally pruned by competition for light. Competing branches that don’t receive enough sunlight die and fall off naturally. Next time you’re hiking look up and you’ll find branches only at the very tops of trees.
Trees that are growing in yards, however, do not have the crowding or shade problems that invoke natural pruning. Your yard trees are surrounded by light and retain all or most of their low branches. Branches may be too crowded, may form weak attachment angles which later break in storms, may compete with the main trunk or stem, and/ or may divert water and nutrients from the main leader of tree, killing the top. Because nature and shade fail to prune, homeowners must do it themselves.
Left: Branch with weak attachment growing at bad angle.
Right: Weak branch damaged in a storm.
The hierarchy of pruning
Most importantly, trees are pruned for the safety of people and property. Branches that will likely fall should be removed before they harm family, friends, or neighbors.
Secondly, pruning should promote the health of the tree. Trees overcrowded with branches or branches that have grown at a bad angle to the tree should be removed. Pruning should also remove competing leader branches from the tree, with the goal of creating structural branches that are properly spaced.
Lastly, after safety and the health of the tree have been considered, trees should be pruned for appearance. If trees are safe and healthy, personal preference can dictate how trees are shaped to fit the style of the yard or area. At this point, beauty is truly in the eye of the beholder. Trees may be pruned to be low branched, high branched with a single or multiple dominant trunks or very formal like the square shaped trees pictured below from the Manoir d’Eyrignac in France.
After trees are safe and healthy, pruning can be done based on appearance.
The trees above represent different personal tastes in the way trees are pruned.
Oh, one last thing on pruning! 
Do not top trees!! Topping is a destructive practice where the ends of tall trees are cut back to form large stubs or an ‘antler-like’ or ‘hat rack’ top to a tree. Topping causes decay of the large remaining branches and excessive sprouting at the top of the tree. New growth following topping is weakly attached and hazardous. Both branch decay and weak branch attachments increase the likelihood that the tree will fail endangering people and property. There is never a good reason to top a tree.