Trees benefit from pruning during winter for several reasons. Giving them an annual trim helps to maintain and control their shape and size, improves the light and air circulation through the canopy, and encourages healthy growth come spring.
January can be an ideal time to prune deciduous trees. They lose their leaves when entering dormancy in late fall or early winter and this can benefit the pruning process. It means you can get a full picture of the tree to make decisions on the shape, and it is easier to identify dead, damaged, and diseased wood to remove.
