Willow trees (Salix spp.) include the graceful weeping willow, the wiry corkscrew willow and the springtime-favourite pussy willow. These trees should not get pruned in spring like many other deciduous trees.
Prune your willow tree in late summer or early fall rather than in the spring. Willow trees can ooze or "bleed" sap if they're pruned in late winter or early spring, but won't bleed when pruned later.
Not only does the sap bleeding create a mess in your backyard, it could leave your willow vulnerable. Sap draws insects and pests to the tree, so your willow is more likely to have a pest problem or disease problem if pruned early.
If you must prune your willow in the spring -- for example, if it gets damaged in a storm and you need to cut off branches -- give the tree aftercare to reduce the sap flow and potential pest problems. Spray the tree with water to remove sap. Keep washing off your tree until there's no more sap, and you should experience no problems.