Willows may look delicate, but they're hardy, fast-growing trees that can quickly get out of hand without regular pruning. Trimming back branches prevents overgrowth from blocking sunlight and interfering with utility lines, buildings and walkways. Although many varieties of willow grow in an orderly pattern, you may sometimes need to shape for aesthetic reasons, too. Willows tolerate minor trims any time of year. For major reshaping, though, prune willow trees while they're still dormant in early spring just before the leaves appear (see Reference 1).
Plan the shape of the tree on paper (see Reference 1). Deciding what to remove before you start cutting prevents you from overpruning and ruining the tree's appearance. The simplest method is to sketch the tree on paper and mark the points at which you want to cut. Alternatively, take a photo and mark directly on the print.
Cut off all dead or broken branches. Both are unsightly and broken, dying branches sap energy the tree could direct toward healthy branches. Using sharp pruning shears, cut damaged branches below the first leaf bud that occurs above the damaged area. If the whole branch is dead or dying, saw it off close to the boot (see Reference 1).
Remove suckers from around the base of the tree. Suckers are thin, reedy offshoots from the mature willow's roots. These drain the tree's energy and increase the risk of extra, unwanted willow trees ending up nearby. Pull suckers out completely. If pulling isn't possible, cut suckers as close to ground level as you can (see Reference 2).
Prune back branches touching the ground or nearby roofs. These can become waterlogged and diseased. Also prune branches that rub together to prevent tangling and bark damage. With pruning shears, cut each branch just below a leaf bud (see Reference 3).
Prune branches that mar the tree's shape or interfere with anything in the area. This time, too, cut right below a leaf bud. Remove a small amount at a time according to your plan. After each cut, step back from the tree and check the shape. This way you're less likely to overprune (see Reference 1).
Pollarding and coppicing are other options for willow pruning. Pollarding, done to keep the tree short, requires cutting the tree to a 2- or 3-foot branchless stub. Coppicing requires cutting the tree almost to ground level so it will produce willow rods.
On weeping willows, don't cut branches growing upward. They'll eventually arch down (see Reference 3).