Winter pruning (January, February)
In general, prune dormant, deciduous plants in winter, such as:
- Deciduous ornamental trees
- Fruit trees - note that these are pruned very differently from ornamental trees to maximize fruiting spurs. Be sure to consult a pruning guide before proceeding.
- Roses - there are different types of roses and each is pruned differently
- Wisteria is cut back hard in winter and then again in summer after blooming
- Certain vines - check pruning guides for when and how
- Winter is also a good time to prune conifers (pine, fir, cedar) to minimize sap "bleeding."
Early spring (March, April)
- Hydrangeas (shrub form) after frost danger is past
- Most hedges, such as laurel, privet, Leylandii cypress
- Red and yellow twig dogwoods are cut back hard now to stimulate the colorful new growth that will light up next winter's landscape
In general, prune spring blooming plants right after they bloom. Pruning them earlier often guarantees that you cut off all the flower buds and miss out on that year's display. Specific plants include:
- Rhododendrons - these and many other plants start setting flower buds shortly after the current flowers fade, so don't dilly dally. If your rhododendron needs shaping, get to it right after bloom so the plant has time to set flower buds for the coming year.
- Rock Rose
- Shear spent flowers on herbs such as lavender, rosemary, thyme and oregano
For the most part, summer pruning is a matter of deadheading and light trimming. Some people give their Japanese maples a bit of a hair cut in summer, because they get so twiggy, but that's optional.
Avoid this practice. I know many people were raised in households where pruning trees and shrubs was part of fall cleanup and preparation for winter, but it is not a good horticultural practice. All pruning stimulates growth and new growth is very susceptible to cold damage. If a plant is exposed to a sudden, early freeze, as we've had in Seattle the past three or four years, the new growth may be killed and the overall health of the plant jeopardized.
What about storm damage?
Remove dead, broken or damaged branches any time of year.
These are very basic guidelines. For more specifics I refer you to the American Horticultural Society's book, Pruning and Training and Cass Turnbull's Guide to Pruning. Be sure you have proper tools, and if you are handling roses or berries, a pair of thorn resistant gloves.