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Hydrangeas: Prune for Maximum Flowering

January 1, 2007
Julie Monson
by Julie Monson
Growing against the south-facing wall of the Inverness Library is a row of aged hydrangeas with abundant pinkish mop-head flowers at the tops of almost six-foot shrubs. As one of this garden’s keepers, I’ve been pruning and watering these fine shrubs for six years, and have one of my own. Hydrangeas, I’ve learned, are a showy, often stunning and reliable addition to the garden. They do, however, have some specific requirements to perform at their best. It’s tempting this time of year when they begin to look raggedy, to think it’s time to prune them back. I encourage you, however, to use your pruning shears to cut off only spent flowers a few buds below the flower, and then to wait until spring for any further pruning and shaping.
It’s very useful to know that hydrangeas come in many kinds, colors and sizes, and that how you prune them depends on which species you have. The most common species, especially in older gardens like the Inverness Library garden, is Hydrangea macrophylla, a large-leaved shrub with white, pink, red or blue large, rounded flowers. The key is to understand that 98% of H. macrophylla flowers form on old, last year’s wood. Old wood branches are darker, less pliable and woodier in appearance. If you prune back this year’s branches, you are effectively eliminating next spring’s flowers. It’s best just to tip them back, cutting off this year’s flowers. In the spring, after danger of frost, retrieve the pruning shears, and do a final touch-up pruning, being careful to avoid cutting back stems with flower buds. Look at the bush and remove any crisscrossing or very old branches, even back to the base of the plant, in order to thin out the center and shape the plant, if it needs it.
Hydrangeas have a genetic will to grow to their maximum size, so cutting them back each fall to make them fit a smaller space only reduces or eliminates their capacity to flower. They will first try to grow back to their genetic size and will put all their energy into branches and leaves rather than flowers. It is essential, therefore, to purchase hydrangeas to fit your space. Fortunately, it’s not difficult to find suitable hybrids in all sizes. I did a quick web search and found numerous informative sites, particularly of nurseries with illustrated, colorful on-line catalogs, excellent sources for checking out hybrids. For example, I found H. macrophylla ‘Alpengluhen’: “crimson red flowers on short, chubby plants. High flower count, shiny leaves, sun tolerant and compact growth to 3.5 ft. Reliable and Hardy.”   Think I’ll buy one!!
Prune two other hydrangea species, H. serrata, and H. aspera, as you would Hydrangea macrophylla. Unlike H. macrophylla, H. paniculata and H. arborenscens are species that flower on new growth. Prune these two back in early spring, before new growth emerges past 2 to 3 inches. Cut plants back by as much as half.
Hydrangeas perform beautifully with proper pruning and relatively little care. Deciduous, they appear naked in winter, so having evergreen plant material nearby reduces their bareness. Although hydrangeas are sensitive to too much sun, they need at least 3 hours daily, preferably morning sun, or all-day filtered shade. They also respond well to regular irrigation and a well-balanced fertilizer coupled with a mulch of good compost. Beware: not deer-proof. An added bonus is that hydrangea’s large flower heads (cut when fully dry) will keep indoors for months, even years.

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