In the Garden:
New England
May, 2011
Regional Report

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Hydrangea paniculata 'Tardiva' blooms with sedum in my garden in late summer.

Hydrangea Pruning Pointers

Have you planted a hydrangea and then been disappointed when all you got was green leaves instead of the gorgeous blossoms you'd been looking forward to? What went wrong? There are a few possibilities. One possible explanation is that you pruned your shrub incorrectly and accidentally cut off all the buds that should have turned into flowers. Whoops!

Bigleaf Hydrangea
Let's start with a little hydrangea background. There are four major kinds of hydrangeas that are commonly planted in our gardens. The bigleaf hydrangeas (Hydrangea macrophylla) are the ones with the blossoms that turn pink or blue, depending on the acidity of the soil. Their flowerheads may be mopheads, with big, rounded balls of showy, sterile flowers. Or they may be lacecaps, airier looking blossoms with a central section of small, fertile flowers surrounded by a ring of larger, showier stertile flowers. Most culitvars of bigleaf hydrangea bloom on the old wood, meaning they set their flower buds the previous fall on wood that grew during the summer. So if you pruned your bigleaf hydrangea in the early spring (or the previous fall), you were cutting off wood that carried the flower buds that were going to bloom this summer. Do any pruning of these types of hydrangeas in early to midsummer -- before the end of July -- so these buds have a chance to develop.

Unfortunately for those of us north of cold hardiness zone 6, many of the bigleaf cultivars are not very winter hardy. When you attempt to grow them in colder zones, you may also just get leaves and no flowers. In this case, Mother Nature is doing the "incorrect" pruning for you. Often much of the top growth of the shrub gets killed back over the winter, along with the buds of future flowers. The roots survive and put out new vegetative growth each spring, but unless some of the old wood survives, you get no blooms. The vagaries of winter weather may explain why some years flowers appear and other years not.

Some of the newer varieties of bigleaf hydrangea circumvent this problem by flowering on both the old and new wood. Even if the old wood gets killed by cold over the winter, the plants also bloom on the current season's growth. Varieties such as Endless Summer (H. macrophylla 'Bailmer') and H. macrophylla 'Blushing Bride' make it possible for gardeners like me in chilly Vermont to enjoy the big heads of pink, blue, red or white blossoms. Just be sure to wait until these shrubs begin to leaf out before pruning back what appears to be winter-killed top growth in spring. The amount of old wood that makes it through depends on the weather and snow cover. The shrub sometimes waits until mid-spring to leaf out, but the more old wood that survives, the more early flowers you'll get.

Panicle Hydrangea
The second big group of hydrangeas is the panicle hydrangeas (Hydrangea paniculata). These are the ones with big white or pinkish flower heads that come into bloom in mid to late summer. These tough plants are very hardy-- to zone 4 or even zone 3. They bloom on the new wood, so the time to prune this group is in the early spring, before new growth begins. Like the bigleaf hydrangea, some varieties have dense, conical heads of all sterile flowers, like the old-fashioned 'Grandiflora' (also called 'Pee Gee'), while others have a more open flower head with a mix of smaller fertile and larger sterile blossoms.

Included in this latter group is one of my favorites, the late-blooming cultivar 'Tardiva', with large, white, lacy-looking, conical flower heads that age to a dusty pink. I've planted it as a focal point in my hillside garden that showcases many fall bloomers, along with 'Autumn Joy' sedum to echo the rosy tones of the mature hydrangea flower heads. Lots of interesting new panicle cultivars have come on the market recently, like Quick Fire (H. paniculata 'Bulk'), whose blossoms open white, then quickly change to pink, and Vanilla Strawberry (H. paniculata 'Renhy'), with flower heads that go from white to pink to strawberry red.

Smooth Hydrangea
Also blooming on new wood is the smooth hydrangea (Hydrangea arborescens). 'Annabelle', with big, round, white snowball flowerheads is a well-known cultivar. These fast growers are often pruned right back to the ground in late winter or early spring to encourage profuse bloom, larger flowers and a neater growth habit. They are hardy to Zone 4.

Oakleaf Hydrangea
And finally we have a fourth group, the oak leaf hydrangeas, that bloom on old wood. These are the most shade tolerant of the hydrangeas and make a lovely addition to a woodland garden. With large, oak-like leaves that turn burgundy in the fall, large, cone-shaped white to rose-pink flower heads in summer and interesting, shaggy, reddish-brown bark on the larger stems, they add interest to the garden in every season. I grow the compact cultivar 'Pee Wee' in my garden, where it gets 3 to 4 feet tall. Like the bigleaf hydrangeas, they should be pruned before the end of July if needed. Oak leaf hydrangea is hardy to zone 5 or a protected spot in Zone 4.


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