Wait a little longer before pruning most plants
By Ted Manzer
Friday, December 8, 2017
We’ve had a few frosts and fall is definitely here, but pruning some plants now could be harmful to them. We haven’t had a hard freeze yet and many plants could yet have a growth spurt.
I know folks are anxious to tidy up their yards, but plant like angel trumpet and lantana should be allowed to die down completely. Pruning them now could encourage a flush of growth and almost certain major winter injury. I’d place butterfly bush in that category too.
There are plenty of other jobs that can be done around the flower garden on these pretty fall days. Weeding is one. Edging is another.
I don’t like fall pruning anyway, but as a general rule it should be avoided especially if plants are marginally hardy. We are in Zone 8, so zone 7 and 8 plants should never be pruned until they are completely dormant and that means winter. Early spring is fine too.
This is also true for fruit trees. Sometimes fall pruning encourages them to bloom. That poses a problem for winter hardiness and spring fruit set. It can encourage disease organisms too. Even if trees are damaged from injury I’d wait until they were dormant before working on them.
Plants in active growth become less winter hardy. Plant sap contains fewer solids and those solutes are essential for what I call plant antifreeze. Pure water freezes at 32 F. However, if sugars, salts, and other molecules are dissolved in it the freezing point is lowered.
Actively growing cells contain lots of water. When they freeze, their cell membranes are perforated by expanding sharp ice. The cells will die. In animals we call that frostbite.
When plants go completely dormant this phenomenon is far less likely to occur. This makes fall the worst time to prune. Fall is also bad if you have spring flowering trees and shrubs. All you will do is cut next spring’s flowers off.
Azaleas, Rhododendrons, dogwoods and redbuds should never be pruned in the fall. Neither should forsythia, lilac, Loropetalum or deciduous magnolias. Encore azaleas, the re-blooming types, need very little pruning. They should be trimmed immediately after they flower in the spring.
Many annual plants are still alive. I have some scented geraniums that are leggy but still hanging on. Since they are annuals and need to be replanted in the spring anyway, go ahead and clean them out if they bother you. This goes for any of your summer annuals.
Most perennials will be better served if they aren’t disturbed until winter. Mum buds might start to swell if plants are pruned now. This is also true for verbena. Candy tuft should also not be pruned until spring.
Usually, by the second week in December many plants are totally dormant and winter has set in. This is not one of those years. The main thing is to inspect your plants and pay attention to the weather. Serious cold could hit us in the next two or three weeks. Then some of your plants might be in line for some trimming.
Ted Manzer teaches agriculture at Northeastern High School.