Know Your Hydrangeas | The Seattle Times

Are you preparing your mature hydrangeas for winter? Continue watering, but water less than you did in spring and summer. Water enough to keep the shrub alive, but do not fertilize.

Fall preparation may include pruning, depending on whether it blooms on new or old wood. If you have several hydrangeas (this goes for clematis as well), you should keep a map of your yard, with identification of blossoming time of each shrub or vine.

If blossoms form on new wood, it may be pruned in winter; if on old wood, prune right after blooming, in summer.

HYDRANGEA MACROPHYLLA

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These are also known as bigleaf hydrangeas, most blooming on old wood, although some new varieties are re-bloomers. H. macrophylla is known by some as the French hydrangea; the other species are native to parts of the eastern U.S. This is the time of year when those that blossom on old wood such as once-blooming H. macrophylla are setting flower buds for next year. If you prune now you will remove some of those buds and have reduced or zero blossoms next year. A harsh winter or late spring freeze can destroy those buds, too, but those events are out of your control.

Some H. macrophylla or bigleaf hydrangeas are mopheads (snowball-shaped flower clusters), and some are lacecaps (large flat flower heads with open flowers around the rim, the center looking like unopened buds). Both flower shapes also appear in other hydrangea species, too.

The proper time to prune old wood bloomers is July, right after blossoming. Nonblooming stems should be cut back to about 6 inches high. Do all of this pruning before August, to allow ample time for hardening off for winter dormancy.

Re-bloomers include varieties such as Endless Summer and Let’s Dance, both H. macrophyllas, and some such as Invincibelle Spirit, H. arborescens. Prune re-bloomers right after spring-summer blooming, by mid-August.

OAKLEAF HYDRANGEA

H. quercifolia or oakleaf hydrangea displays glorious fall color, some say better than that of burning bush. It’s one of the “most beautiful native shrubs,” according to Michael Dirr, renowned authority on woody ornamental plants. The oakleaf hydrangea is an easygoing shrub, happy to blossom even in deep shade. It flowers on old wood but is not as sensitive as other varieties to spring frost.

Prune in early summer, right after flowering.

HYDRANGEA PANICULATA

This is the variety with pyramidal flowers, taller than wide. It tolerates drought better than other hydrangeas, remarkable since the name “hydrangea” is based on the Greek word for water. Generally, they all require a lot of water. This variety, also known as the “PeeGee” hydrangea, tolerates full sun and light shade, and is hardy to USDA zone 3.

H. paniculata is the largest hydrangea shrub, and blooms later, from midsummer to fall. It blooms on new wood, unlike the two previous species, so should be pruned in winter. Cut away old flowers, and prune to open the plant to sunlight, while maintaining a pleasing structure. My preference for winter pruning of anything is to wait until mid to late February for pruning, lest severe cold freeze and kill what’s left of your plant.

HYDRANGEA ARBORESCENS

H. arborescens is also called “wild hydrangea” by some, “smooth hydrangea” by others. Some of the cultivars of this have been bred to have huge balls of blossom, such as the Incrediball.

This blooms on new wood, so can be pruned quite low (don’t prune to stubs), and no matter how severe the winter, it will bloom in summer.

Blossom color of this species is not altered by soil pH.

Other species are H. involucrata, blooms spring to July; H. cinerea or ashy hydrangea, and H. radiata, both of the latter formerly identified as subspecies of the smooth hydrangea, or H. arborescens, now regarded as separate species.

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Margaret Lauterbach: [email protected] or write to Gardening, Idaho Statesman, P.O. Box 40, Boise, ID 83707.