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Pruning Hydrangeas and Butterfly Bush

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

I have a great four-year old purple butterfly bush that is five or six feet tall. I have pruned the ends of the tallest branches a couple times and it’s starting to look a little spread out, although we did have 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 leaves and big, greenish-blue balls of flowers. A lawn service pruned them back by a third two years ago, and although they were full and healthy looking, I didn't get a lot of flowers—and the ones I did get were hidden 'inside' the bush and not very visible. I didn't prune last year—I heard the flowers only appear on “old growth”—and I did get more flowers, but now the bushes are getting a little tall—about five feet—and I'm afraid they'll lose their fullness. How can I prune them and not lose those great blossoms? Thank you!
            ---Debra in Churchville, Bucks County, PA

Butterfly bush—proper name Buddleia—is THE plant for attracting those winged wonders. But it is not for the timid at pruning time—although it is easy to keep compact via pruning, and the instructions are about as easy 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 level with 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 growth looks vigorous—especially on the East Coast and anywhere else that’s currently experiencing a serious Spring drought.

I know it seems extreme, but these plants grow like weeds, and this is the recommended way to keep them under control. NOTE: This is not general pruning advice, guys! Very few plants can be cut back this severely and survive, much less thrive.

Hydrangeas are a little trickier. OK, a LOT trickier….

Most of the white flowered varieties are easy—they bloom on new wood, so you can cut them back by as much as a quarter in the winter or early in the Spring and you’ll get nice flowers that year while still keeping the plant nice and compact. And our listeners in warmer climes—like North Carolina, from where Dick Bir (pronounced “Burr”), Professor Emeritus of North Carolina State University, provided this advice—can trim them back after the first flowers fade and potentially get another run of blooms.

We are NOT surprised, however, that you’ve seen conflicting advice on how to prune your “big leaf” or “mophead” hydrangeas (most likely Hydrangea macrophylla). As Professor Bir notes in a fine online article “the book” says that the flower buds on these plants are supposed (he uses the 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 have failed to read that particular book and seem to bloom just fine on new wood and other places they aren’t supposed to. We suggest you take this as reassuring news. Pruning even according to firm and unambiguous advice can instill terror in the hearts of gardeners. When experts quarrel, many of us simply drop our pruners and look for a window to jump out of. But Professor Bir suggests that you instead relax—when no one is in agreement, you can’t be too wrong.

Here’s my two cents, gleaned from a conversation with Professor Bir, a number of other seemingly reliable sources, and my own experience with a trio of “Endless Summer” mopheads I’ve been growing in a vaguely appropriate spot for a couple of years now.  Wait till the new growth greens up nicely—don’t rush, these plants are a little slow off the starting block in Spring, especially in cooler climes—then prune off the obviously dead tips of the branches. There should be nice big fat buds visible on the healthy green growth below.

Professor Bir explains that not all of these big fat buds will produce flowers. When they open, something resembling broccoli florets will appear on the true flower buds. The others will reveal themselves to be non-flowering vegetative buds. If the flowering ones aren’t high enough on the plant when the buds begin to open, remove the non-blooming wood above and around them to display them better. When those flowers begin to fade, you can then cut the plant back safely—up to a quarter of its total 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’s is/are), remove the first run of old flower heads promptly to induce new growth. If you’re feeling lucky, you might even want to consider cutting 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 of blooms to appear. (Say, those big leaves sure are striking all by themselves, aren’t they?) Once again, our Southern listeners have the best shot here.  

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

Oh, your blue flowers are a sign that your soil is acidic. Hydrangeas that aren’t white are unique in the plant world in that you can manipulate the color of the blooms. A neutral to alkaline soil will produce pink flowers; an acidic soil, blue ones. If you want to go from pink to blue, put a couple inches of milled peat moss on the surface of the soil and cover it with a little bit of compost. To go from blue to pink—to announce the birth of baby girl perhaps—dust around 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 with wood ash to produce plants with blue flowers on one side and pink on the other. I’ve even seen plants whose individual flowers were half pink and half blue; very neat. Now if we could just figure out how to prune the darn things….
   
http://www.ca.uky.edu/HLA/Dunwell/hydprun.html

You Bet Your Garden   Question of the Week ©2006 Mike McGrath

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