Prune a Japanese willow tree in the winter to improve air circulation and maintain the shape of the tree. You need only pruning shears. Depending upon the size of the tree, pruning should take about an hour.
Thin the branches
Remove approximately one-third of the branches during this phase. The first cuts should remove the tallest and oldest branches of the willow. Cut the branches near the parent branch. Prune any dead or diseased branches. This allows sunlight and air to circulate around the interior of the tree.
Remove the limbs near ground level
Remove overgrown branches sprouting from the base to maintain the shape of the willow. Remove the oldest, thickest branches first. Cut each of the branches close to the tree trunk.
Trim and shape the remaining branches
Trim the remaining branches to 50 to 70 percent of their original length. Branches can be left longer if preferred. All branches may be trimmed to a minimum length of 12 inches. Shear the branches, removing the ends of stems to shape the tree. Cuts should be slanted between 1/8 and 3/8 inch from where the bud connects to the stem. Trimming any closer will cause the bud to dry out and become less fertile.