In general, trees grow most actively during the spring and summer. During this time, they are developing new leaves and branches that need to be trimmed, so your yard doesn't become overgrown with branches or leaves. The best time of year to trim or prune a tree or shrub is during its dormant season. Pruning is best during this time because the tree or shrub will be less likely to react to the trimming with excessive new growth. The only exception is if there is a hazard, in which case you may need to prune at any time in the year.