Few trees can top the graceful form of a weeping willow (Salix babylonica), a deciduous tree that grows in U.S. Department of Agriculture plant hardiness zones 6 through 8. A tree that grows well in a moist spot, such as near a pond or stream, the weeping willow also thrives where it gets average amounts of moisture. Pruning a young willow can help it develop a strong structure; for an older tree, some maintenance pruning is usually sufficient to keep it doing well.
Training a Young Tree
If you plant a new weeping willow tree, don't prune it at planting time, except to remove any damaged branches. Give the tree a season to put out a strong new root system and to recover from transplant shock. Once you're ready, the best time to start training the young tree is in late winter or very early spring, while it's still dormant but poised to put out new growth. If the tree has more than one central stem, choose the strongest, straightest one to be the central leader, removing any others near their origins. Also remove any broken or damaged branches, along with any root suckers, which are straight, thin shoots that grow directly from the root zone, the area under the tree's canopy. Disinfect your pruning blade by wiping it in rubbing alcohol between cuts to prevent spread of plant disease.
As a weeping willow matures, it can grow to 30 or 40 feet tall. Its pendulous branches gradually hang downward as they grow longer, eventually sweeping along the ground. As the tree grows, remove any branches that break in storms or strong wind, and continue to remove root suckers and any thin stems that appear along the lower part of the trunk. This tree doesn't require regular pruning because it naturally forms a pleasing, weeping shape. But if you'd like a seating area or space for foot traffic under the tree, you can provide some clearance under its canopy by trimming back any long branches that touch the ground, cutting these back by about one-third every few years.
To ensure that your pruning cuts are clean and don't tear the tree's bark, always use sharp pruning shears. For cuts on branches that are out of reach, use pole pruners with a rope action or a long-handled, sharp pruning saw. When shortening branches, called heading back, make a slanted cut about 1/4 inch ahead of a side branch, or cut just in front of a dormant bud on the branch you want to head back. When you're done, there should be no branch stub left behind the cut. Use a similar method to remove damaged or broken branches, cutting at a 45-degree angle behind the damage and into healthy wood. Once the pruned branches put out new growth and start becoming too long, repeat the trimming process to keep them at the desired length.
The weeping willow is a low-maintenance tree that needs little special care. But pruning can stress the tree, so ensure it gets at least 1 inch of water weekly during the growing season, including rain, to help it put out strong new growth. To conserve soil moisture and keep down weeds, spread 2 or 3 inches of organic mulch under the tree's canopy, but keep mulch back about 6 inches from the trunk to discourage fungal problems. A weeping willow doesn't require regular fertilization, but you can promote new growth after pruning by giving an established tree a spring feeding, applying about 1/2 pound of ammonium sulfate per 100 square feet of area under the tree's canopy. Mix the fertilizer into the top few inches of soil, taking care not to disturb the tree's roots.