Just Say NO! to Fall Pruning
I am a firm believer that it is very, very difficult to prune a plant to death. However, I also believe there are MORE beneficial and LESS beneficial times to prune, and I would say that fall falls firmly in the "less beneficial" time.
Now, light pruning at any time of year seldom hurts any type of shrub or tree. What I'm talking about is heavy pruning, the kind where you remove about 1/3 of the plant.
Growth Cycles and Pruning
The goal of any flower, shrub, or tree in your landscape is to survive, thrive, and reproduce. A heavy prune usually tells the plant that something threatens one or more of those goals!
In response, plants will go into growth mode. They'll focus on taking in more water (and, so, more nutrients), and absorbing what energy they can via their remaining leaves. Fall, when the days are shorter and colder and the ground gets closer and closer to freezing, is exactly when you don't want this to happen! Essentially, the tree or shrub is fighting to get back what it just lost at the most difficult time, which can stress or weaken it.
Pruning early in the season, or (if you have to) in mid-summer, gives your landscape plants time to recover when they can take in the optimal amount of nutrients and absorb the optimal amount of energy from the sun.
I should note that winter pruning is a slightly different beast! During winter, particularly mid to late winter in colder zones, plants are already in a state of dormancy. A heavier prune isn't going to send them into growth or recovery mode at that time, and winter is a great time to prune because you get a clear view of all the branches-sans-leaves.
What About Flowers?
Of course, a heavy fall pruning (and even a heavy mid-summer prune) can mean no flowers next spring! Some plants (for instance, lilac, forsythia, and rhododendrons) flower early in the spring, then form new buds for next years blooms by summer or fall. Sometimes, it's difficult to see flower buds, and, even if you do, during a heavy prune a lot may be removed.
If you want to make sure there are lots of flowers next year, you need to prune early spring bloomers shortly after they flower. This gives the plant time to recover and form new buds.
Before pruning any plant, it's wise to do a little research. Check when it blooms, and check to make sure you're pruning at the optimal time. And remember that, unless the plant is extremely overgrown or not producing well in terms of fruit or flowers, it probably doesn't even need a heavy prune!
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