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Put down those pruners!

Friday - 10/7/2011, 11:48am  ET

Pruning stimulates growth

Mike McGrath, Garden Plot editor

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Crepe myrtle gets its trim in the spring

Tim in Arlington writes: "I'm looking for some advice on my crepe myrtles. They've finished blooming and I need to know if I should prune now, before winter, or wait until spring."

Well, the answer to your first two options is easy, Tim -- NOTHING should be pruned between now and the dead of winter. And yes, I mean nothing; no exceptions! (…Ahem! Excuse me a second here, folks) "Hey! You over there -- that means you too! Drop the pruners, pal!" Sorry.

Anyway, crepe myrtles should be pruned back in the spring, shortly after they begin producing new growth. Just trim back the tops of the branches by about 2 or 3 feet to keep the blooms at a nice, visible level. Don't whack them down to the ground! Although these summer bloomers can survive a hard cut, committing "crepe murder" makes the poor plants look awful a few years down the line.

Hydrangea trimming trick

Cathy and Bennie -- both in the District -- want to know when they should cut back their hydrangeas. Bennie adds that "mine were trimmed too late last season, and I got few flowers this year as a result."

Well, as I have been warning -- make that "pleading" -- for over a decade now, no plants should be pruned in the fall. Prune now and you risk cutting off future flowers and/or exposing the plant to excessive winter injury.

Big trees should be pruned in the dead of winter, when they're completely dormant. Spring bloomers should get a trim right after they finish flowering in the spring. Summer bloomers are typically cut back in early spring, just as they begin to produce new growth (like the crepe myrtles we just mentioned). But that's not the best way to treat hydrangeas.

Although they are technically summer bloomers, hydrangeas are tricky, as they can produce their flowers on old and new wood -- and even experts can't predict which branches will bloom, making it easy to accidentally cut off the flowers you've been waiting so long to see.

So wait until all the flowers form next season and then prune away any non-flowering branches that are in front of good flowers. That's the best way to get the most -- and most visible -- flowers on display, and you'll never accidentally reduce their number.

Trimming oaks and taming watersprouts

P. J. in North Arlington (who's been organic for 22 years and has a yard full of rare tree frogs to prove it) writes: "I know HOW to prune trees (leaving the branch collar attached to the tree, taking the weight off with multiple cuts, etc.) but I'm getting conflicting information about WHEN. Specifically, I have a young willow oak that needs some gentle work to remove crossed limbs, and a volunteer redbud that has a lot of watersprouts."

The best time to work on deciduous trees like your oak is in the dead of winter, P. J., when the trees are fully dormant.

Pruning of spring bloomers like redbuds should typically be done right after they finish blooming, so you can keep them under control but still enjoy all of their springtime show. But branches that point straight up in the air like your tree's watersprouts won't produce flowers, so you can take those branches off in the winter as well. Or just wait and remove them after flowering time if you plan to give the whole tree a little trim.

Now is NOT the time to feed your trees

Noreen in Silver Spring writes: "I have a few leland cypress that look pretty distressed, and so I want to fertilize them this weekend. The last time they were fed was last fall, but it didn't seem to do anything -- in fact, they've looked pretty sad ever since. The fertilizer bag says to feed in the spring and fall…"

Don't do it, Noreen -- the only plant that should be fed this time of year is a cool-season lawn. All of our trees and shrubs are trying to go dormant for the winter, and feeding them now, especially with chemical fertilizer, will stimulate growth that makes them very susceptible to winter injury. In fact, I suspect the fall feeding last year actually injured your poor trees. That bag's advice is criminal!

Let the trees go dormant and then feed them in the spring -- but not with chemical fertilizers or tree spikes. Instead, give them a nice mulch of high-quality compost in the spring, 1 to 2 inches deep, beginning about 6 inches away from the trunk and going out a few feet. (Never let any kind of mulch actually touch a plant.)

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