Q.Hydrangeas
I have mop heads paniculatas and lace caps. Do I cut them back tytbe ground when they’ve finished flowering
Hydrangeas normally need no pruning other than deadheading of spent blooms (this is also not required; up to you). Dead wood of any type of hydrangea can be pruned but allow enough time for last year's stems to leaf out; you first will get new growth from the base and then leaf out will occur, sometimes as late as a month.
Hydrangea macrophylla (aka, Big Leaf Hydrangea) and Hydrangea serrata’s (aka, Mountain Hydrangea) live wood is best pruned after the plant stops opening new blooms; after the plant stops, you have several weeks, maybe up to a month or thereabouts, before the plant develops spring flower buds inside the ends of the stems. After those few weeks to a month, their live wood should not be pruned or you may be cutting off next year’s blooms.
Paniculatas and Smooth Hydrangeas develop flower buds in mid/late spring to early summer and these buds then open at various times in early, mid or late summer depending on the cultivar. Since pruning live wood is an activity that switches any plant into "grow mode", avoid pruning live stems for paniculata hydrangeas before the plant is to go dormant or before early frosts. Instead, prune once the plant is fully dormant. That can be done in late fall, winter or early spring as long as the plant is dormant then. But avoid pruning live stems of paniculatas to the ground as much as possible as that then creates new green stems that tend to arch and flop considerably by the end of growing season. Woody, brown stems flop less. Paniculata cultivars that produce lacy, small(er) blooms like Pinky Winky, Brussels Lace, Chantilly Lace, etc. flop less than paniculata cultivars that produce large, dense, heavy blooms like Limelight, Vanilla Strawberry, Phantom, etc.
