The default: late winter
From about February into March, before the buds open, most trees and shrubs are dormant. You can see the branch structure clearly, the plant recovers quickly when growth starts, and many pests and diseases are inactive. Dead, broken or hanging branches can come off at any time of year.
Spring-flowering shrubs: right after they bloom
Azalea, forsythia and lilac set next year's flower buds during the summer and fall. If you prune them in winter, you remove those buds. Prune them in the weeks right after the flowers fade, and finish by early summer.
Summer-flowering shrubs: late winter
Shrubs that bloom on new growth, such as butterfly bush and panicle hydrangea, can be pruned in late winter. Go light with crape myrtle. Trimming it back to stubs every year weakens it and does not make it bloom better.
The oak exception
Avoid pruning oaks in spring and summer. Fresh cuts during the growing season can attract insects that spread disease. Prune oaks in the dormant season, from late fall through winter, unless a branch is a safety problem.
Cuts that heal well
- Cut just outside the swollen ring (the branch collar) where the branch joins the trunk. Do not cut flush, and do not leave stubs.
- Take out no more than about a quarter of the canopy in one year.
- Use sharp, clean tools. Wound paint is not needed.
When to call a professional
Call a licensed, insured professional for large limbs, anything that needs a ladder or a chainsaw above your head, storm damage, leaning trees, and any branch near power lines. Stay well away from lines, and if branches are touching them, call your utility company first.