They say that the only difference between a bad haircut and a good haircut is a few weeks, but the same logic does not apply to pruning hydrangeas. If you prune lovely lacecaps (Hydrangea macrophylla) incorrectly, your shrubs may pass the following summer without blossoms. With hydrangeas, when you prune is as important as how you prune.
Bigleaf hydrangeas come in two varieties -- mopheads, with blossoms like puffy snowballs, and lacecaps, with inflorescences that resemble little round caps that women used to wear to church. Lacecap blooms group tiny female flowers in a flattened disk with the sterile but showy male flowers around the edge. Like mopheads, lacecaps grow best in U.S. Department of Agriculture plant hardiness zones 6 through 9. Most cultivars, like "Lady in Red," bloom in summer, while reblooming cultivars like "Twist-n-Shout" extend their flowering through fall.
Old Wood Bloomers
Some hydrangea species bloom on old wood and some on new growth, and this makes a critical different in when you need to prune. While species that form buds on new growth can be trimmed back in fall or even winter, old wood bloomers -- like lacecaps -- cannot. They set buds directly after flowering, so even a mid-fall pruning is likely to remove many of the next year's blossoms. Prune lacecaps just after they finish blooming for the season.
Lacecaps are not plants that require pruning to thrive, and the shrubs can live out long, flowery lives without a major trim. But hydrangeas tolerate pruning well, so it's an option if your lacecap is outgrowing its space or seems to lack vigor. Pruning is also useful when you have to trim out dead or damaged stems, or to increase the size of the flowers. Your reason for pruning will dictate the cuts that you make. Don't forget to clean the cutting blades of the pruner by wiping them with a rag soaked in denatured alcohol.
If you're pruning to renew an aging bush, thin out the shrub by removing one-third of the stems at ground level. Take out the oldest canes first. Be sure to remove dead, diseased or damaged stems as well as stems that cross one another. If you wish to reduce the plant height, cut back each stem to the desired height, making the cut just above a leaf node. You can snip off dead blossoms at the same time to tidy up the shrub.
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