Answer: Generally, you prune hydrangeas after flowering, so exactly when depends on the type of hydrangea you have, explains John Donofrio, co-owner of Carroll Gardens, a grower of more than 20 varieties of hydrangeas in Westminster, Maryland. A 12-foot-tall plant is probably the PeeGee hydrangea (Hydrangea paniculata 'Grandiflora'), says Donofrio. Your book that recommends cutting the plants all the way to the ground each fall is referring to a different kind, probably H. arborescens, a shorter plant that spreads by underground stems. PeeGee hydrangeas flower in July in your area and the flowers remain on the shrub until fall, eventually turning an attractive rusty color. You can prune off flowers anytime after July and save them as dried flowers. You can also prune branches anytime after July, but the best time is just before the plant leafs out in spring, adds Donofrio. PeeGee hydrangeas set their flower buds on the new growth, so don't prune after growth starts or you'll risk losing that year's flowers. Your other reference, which recommends removing only spent blossoms in spring, is probably referring to PeeGee hydrangeas. But you may want to remove more than just the dried flower heads. The plants can get 12 to 15 feet tall. You don't say how you want to manage your 12-foot plant, but Donofrio thinks PeeGees look best when kept to about six feet. They produce the best size and quality flowers when pruned to an umbrella shape with no more than five or six primary branches coming upfrom the base, he advises. To shorten your plant, remove the tallest one or two basal branches every year, which will bring the plant down to size in about three years. Or else cut the trunks back to a side shoot or branch at about four feet above the ground. Both methods will require several seasons to get the tree to its ideal shape. After that, maintenance pruning will be a simple matter.
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