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This is another way of saying that your hydrangea produces new spring blooms on last year's buds. Hydrangeas with this characteristic tend to bloom in early summer, and the flowers die by mid summer. At this point the shrub begins producing the buds that will bloom the following year. Hydrangeas that bloom on old wood include the following:   Bigleaf, Mopleaf or Lacecap hydrangeas (Hydrangea macrophylla and H. serrata) Oakleaf hydrangeas (H. quercifolia) Use the images at http://www.hydrangeashydrangeas.com/identify.html to help you identify what kind of hydrangea you have before you start making any plans to prune your plant. Since these types of hydrangeas start producing buds soon after blooming, in late summer and early fall, it's essential to prune them just as the blooms begin to fade in mid-summer. This way you can trim the shrub before it begins to produce the buds that will turn into next year's flowers. The longer you wait, the likelier it will be that you'll cut off some of next year's growth.  If you've missed this window, just wait until next year to prune the hydrangea. Hydrangeas don't have to be pruned every year, so it won't be a problem to wait. If you're unhappy with your hydrangea's shape, you can go ahead and prune - just know that in doing so outside of the short midsummer window, you'll risk hurting it's appearance next spring. Right after they're spent, use hand shears to deadhead the blooms. Do this by making clips just under the heads at the tips of the stems. This will tidy up the hydrangea's appearance during the blooming season. You can also remove dead or dying canes at this time; trim them at their base. When a hydrangea is several years old, it'll start to produce fewer blooms. You can promote fresh growth by taking out some of the older canes - up to 1/3 of them. For thicker canes, you may need loppers to cut them. Cut these older canes to the ground. If your hydrangea has grown quite large, you can prune it in June or July (just after the blooming season) to contain it a bit. Trim back the branches by 1/3 to the nearest joint. In most cases hydrangeas will grow back quite quickly, so you may not be able to maintain the smaller size for long.  This type of pruning isn't necessary for the health of the plant. Only do it if your hydrangea is taking up too much space. When planting new hydrangeas, it's best to choose a spot where they can grow freely. "Endless Summer" hydrangeas are an exception to the rule. This variety is more low-maintenance than others and can really be pruned at any season - there is no "bad time."  Endless Summer hydrangeas can mainly be left alone until they mature. You can then "deadhead" the plant in the spring or fall to encourage new flowers.
Determine if your hydrangea blooms on old wood. Prune just after the blooming peak. Remove old blooms. Remove the oldest canes. Prune the hydrangea to reduce its size.