Dear sir or madame,
Is this the right time to prune hydrangeas? If so, how much can one prune from a mature plant? Can it be pruned to the ground?
Also, can offshoots be cut off w/out damaging the parent plant?
It might be the right time to prune or not depending upon the kind of hydrangea and your objectives. It's OK to prune Hydrangea arborescens (Annabelle and related varieties) now. Spring pruning usually involves removing winter damaged wood and thinning and shortening the stems to improve or maintain the clump's form. You can prune arborescens to the the ground if you wish and still get some bloom this summer.
Because most varieties of Hydrangea macrophylla flower on old wood, pruning them now will result in few or no flowers this growing season. Endless Summer and related varieties are an exception. They flower on new wood and may bloom this growing season even if cut to the ground. However, for optimum bloom it's best to remove only dead or damaged wood at this time of year. Generally speaking, macrophylla should be pruned after flowering.
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Annabelle and related varieties can be propagated by division. Removing an offshoot is a kind of division.