If you had to pick one time of year to prune, the best choice would be late winter.
Now I know there are exceptions (like removing dead, damaged, or diseased stems at any time, or removing suckers and watersprouts in late summer), but there are some very good reasons why late winter is best in the vast majority of cases.
- The plant is dormant. It’s not putting energy into producing leaves, flowers, buds, or new stems. Unlike spring, when the sap is flowing (which can cause the pruning wound to “bleed” and look unsightly).
- There are no diseases or fungal pathogens in the air in winter. Late summer and fall are the worst time of year to prune because airborne pathogens reach their peak at that time.
- You can see the plant structure better when there are no leaves.
So even if the plant blooms on old wood (like forsythia), pruning in winter is still the best time for the health of the plant. Of course, you’re cutting off some of the flowers that would otherwise bloom that spring. But the reduction in flowers is really more of an esthetic issue for us humans; it’s not a problem for the plant.
Question: If I prune in winter when everything is frozen, won’t the branches be frozen or brittle?
Answer: Brittleness of wood (stems or branches) isn’t dependent on air temperature. Some plants are naturally more brittle than others (it depends on the cellular structure of the plant). But they don’t “freeze” in winter. As long as you make clean pruning cuts in the right places, the plant will be fine.
But might I suggest that you wait until late winter when things aren’t frozen? You’ll probably be more comfortable pruning when it isn’t 20 below!