The Perils of Pruning
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A. There are two issues here. One, as Tom correctly notes, is the loss of flowers when you prune Spring bloomers like azaleas and rhododendrons in the Fall; or even late Summer. The buds that will produce next year's blooms on my azaleas and rhododendrons, for instance, have been clearly visible for a month now.
The other issue is the compulsion to fuss with everything in the Fall. After all, the tomatoes have slowed down, the zucchini is hopefully dead, the weather is wonderful for outdoor chores; and there we are, pruners in hand on a beautiful day in the seventies (the temperature, not the decade) with low humidity and no other obvious chores except these plants that we have suddenly noticed have become overgrown in the past few days. Why not? What's the harm?
And that brings us to issue number two. Pruning stimulates growth. The majority of European gardeners seem to know this, as many of them selectively prune to make small plants bigger. But we on the other side of the pond seem to think that pruning shows plants that we are The Boss and mean business; and after this savage attack by the Spring Heeled Jacks of Horticulture, they'll bow to our whims and behave themselves by staying at their after-pruning height forever. Which is possible, because it might make them dead.
Again, pruning stimulates growth. Nights in the suburbs of the Philadelphia area are already dipping into the low forties (the temperature, not the decade) and our plants are slowly tucking themselves into dormancy. But they aren't quite dormant yet, and so when (say it with me kids!) 'pruning stimulates growth' at this time of year, really cold weather may be only a week or two away.
And when that stimulated growth, which like me, is full of fresh sap, suddenly plunges into one of those late Fall/early winter Arctic troughs, said sap freezes hard, which can burst branches open. The ill-advised pruning has also taken away much of the energy that would have sustained the plant over winter. AND we have removed the outskirts of the plant, which, had they been there, would have taken the brunt of an ice storm or other winter injury.
That's three strikes. Don't argue with the ump, toss your helmet or even look at the person who has merely verified your mistakes; just get back in the dugout.
Where does this leave Tom, who may wish to trade plant death for the chance to look outside during the brief intervals of daylight in winter and, as the great ballplayer Rogers Hornsby once noted he does during the off-season, "look out the window and dream of Spring". To do that, you need to SEE out the window.
Here's the deal. When the weather is freezing cold and no warmup is predicted for at least several days, go out and remove the minimum amount of plant that will allow you to see outside. Do not do this during a warm spell or what we warned about back in Strike Two will be even worse. The best time for pruning is the time you least want to do it; in the dead, dead, dead of winter. Freezing cold for several days before with several days of freezing cold afterwards. The plants will not be stimulated (neither will much else at that time of year) and you will accept the loss of flowers if the window washers were spring bloomers. But they won't be dead, and that's a plus.
Flash forward to next year. (Can we please do that now? I still have trouble understanding why I wear a mask in the liquor store and still have to pay for my wine~!) But I regress. (That's when you digress and then find your way back home.) Prune Spring bloomers when you know you should, which is right after they're finished blooming. Bonus: This is also when Spring bloomers would love to be fed either a couple inches of compost or an organic fertilizer designed for that type of plant or both. (If it is both, granular fertilizer first, followed by compost.)
Do not remove more than one-third of the plant. A week or so later, go back and prune GENTLY for shape and such. Do the same a week after that, but stop pruning a month to six weeks after the first round. If you stop soon enough, it won't affect bud set and you will have done everything you can to control the height of the plant.
Short of killing it, of course....