A willow tree should be pruned only in fall or late winter to prevent stunting its growth, according to SFGate. To avoid injuring the tree or causing disease, always use clean and sanitized tools that are sharp and well maintained.
Prune a willow tree by removing dead and decaying branches, removing the suckers from the base of the tree and cutting out overlapping branches. You can also cut any limbs when needed to shape the tree properly. If there are damaged limbs, they may be removed at any time during the year, as with dead and diseased limbs that occur sporadically.