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Pruning Hydrangeas and Butterfly Bush

Q. I've heard conflictingadvice about how to prune hydrangea and butterfly bush.

I have a great four-year old purple butterfly bush that is five or sixfeet tall. I have pruned the ends of the tallest branches a coupletimes and it's starting to look a little spread out, although we didhave lots of monarch cocoons and all kinds of butterflies last summer.Can you tell me the best way to prune for healthy growth?

I also have healthy four-year old hydrangea bushes; large green leavesand big, greenish-blue balls of flowers. A lawn service pruned themback by a third two years ago, and although they were full and healthylooking, I didn't get a lot of flowers—and the ones I did get werehidden 'inside' the bush and not very visible. I didn't prune lastyear—I heard the flowers only appear on "old growth"—and I did get moreflowers, but now the bushes are getting a little tall—about fivefeet—and I'm afraid they'll lose their fullness. How can I prune themand not lose those great blossoms? Thank you!
            ---Debra inChurchville, Bucks County, PA

Butterflybush—proper name Buddleia—is THE plant for attracting thosewinged wonders. But it is not for the timid at pruning time—although itis easy to keep compact via pruning, and the instructions are about aseasy as it gets: Wait till you see new greenery beginning to peek out,and then cut all the old growth back to a few inches above ground levelwith a Big Boy pair of loppers.

Then give those stumpy things a light feeding with some compost(mostly to make you feel better) and water well until the new growthlooks vigorous—especially on the East Coast and anywhere else that'scurrently experiencing a serious Spring drought.

I know it seems extreme, but these plants grow like weeds, and this isthe recommended way to keep them under control. NOTE: This is notgeneral pruning advice, guys! Very few plants can be cut back thisseverely and survive, much less thrive.

hydrangeasare a little trickier. OK, a LOT trickier….

Most of the whiteflowered varieties are easy—they bloom on new wood, so you can cutthem back by as much as a quarter in the winter or early in the Springand you'll get nice flowers that year while still keeping the plantnice and compact. And our listeners in warmer climes—like NorthCarolina, from where Dick Bir (pronounced "Burr"), Professor Emeritusof North Carolina State University, provided this advice—can trim themback after the first flowers fade and potentially get another run ofblooms.

We are NOT surprised, however, that you've seen conflicting advice onhow to prune your "big leaf" or "mophead" hydrangeas (most likely Hydrangeamacrophylla). As Professor Bir notes in a fine online article "thebook" says that the flower buds on these plants are supposed (he usesthe term "alleged") to form the previous year. So pruning in the Fall,Winter or Spring will theoretically remove them.

But in Professor Bir's experience, some illiterate hydrangeas havefailed to read that particular book and seem to bloom just fine on newwood and other places they aren't supposed to. We suggest you take thisas reassuring news. Pruning even according to firm and unambiguousadvice can instill terror in the hearts of gardeners. When expertsquarrel, many of us simply drop our pruners and look for a window tojump out of. But Professor Bir suggests that you instead relax—when noone is in agreement, you can't be too wrong.

Here's my two cents, gleaned from a conversation with Professor Bir, anumber of other seemingly reliable sources, and my own experience witha trio of "Endless Summer" mopheads I've been growing in a vaguelyappropriate spot for a couple of years now.  Wait till the newgrowth greens up nicely—don't rush, these plants are a little slow offthe starting block in Spring, especially in cooler climes—then pruneoff the obviously dead tips of the branches. There should be nice bigfat buds visible on the healthy green growth below.

Professor Bir explains that not all of these big fat buds will produceflowers. When they open, something resembling broccoli florets willappear on the true flower buds. The others will reveal themselves to benon-flowering vegetative buds. If the flowering ones aren't high enoughon the plant when the buds begin to open, remove the non-blooming woodabove and around them to display them better. When those flowers beginto fade, you can then cut the plant back safely—up to a quarter of itstotal size—without risking its health or the following year's flowers.We hope.

If your plant is claimed to be a rebloomer (as my Endless Summer'sis/are), remove the first run of old flower heads promptly to inducenew growth. If you're feeling lucky, you might even want to considercutting off the first run of flowers while they're nice and fresh,putting them in a vase, feeding the plants a little compost,lighting a candle to the Blessed Mother and waiting for a second run ofblooms to appear. (Say, those big leaves sure are striking all bythemselves, aren't they?) Once again, our Southern listeners have thebest shot here.  

I like to leave the last run of dried flower heads on the plant forwinter interest. …And so's I don't cut the next year's blooms off ifthose particular plants DID get good book-learning in school. It's alsoa good way to prevent pruning too late in the season and exposing theplants to winter injury. If you ARE going to prune, do it right afterthose first flowers fade (or are cut by you for indoor use).

Oh, your blue flowers are a sign that your soil is acidic. Hydrangeasthat aren't white are unique in the plant world in that you canmanipulate the color of the blooms. A neutral to alkaline soil willproduce pink flowers; an acidic soil, blue ones. If you want to go frompink to blue, put a couple inches of milled peat moss on the surface ofthe soil and cover it with a little bit of compost.To go from blue to pink—to announce the birth of baby girl perhaps—dustaround the base of the plant with some lime or wood ash.  

I've seen clever people who did one side with peat and the other withwood ash to produce plants with blue flowers on one side and pink onthe other. I've even seen plants whose individual flowers were halfpink and half blue; very neat. Now if we could just figure out how toprune the darn things….

You Bet Your Garden   Question of the Week ©2006 MikeMcGrath

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