Re: Advice on Bigleaf Hydrangea
From: Marge - Maryland
The bigleaf (AKA mop head) hydrangeas, Hydrangea macrophylla, often have their flower buds killed by cold winters or late spring frosts. I garden in USDA z7, considerably south of you, and lose flowers to late spring cold snaps more often than I'd like. Unfortunately, H. macrophylla have a tendency to break dormancy at the first sign of spring - way too early. Once buds have started to break, lows of 24 to 28 degrees F or less will turn them to mush.
Unless you live near the ocean in Massachusetts (I've seem magnificent hydrangea hedges on Cape Cod), you are at the northern end of this shrubs hardiness range. You can try to protect your shrubs by making a burlap fence around them in winter and stuffing it with leaves. Just get a roll of burlap (many DIY stores like Home Depot carry this), staple it to wood garden stakes and pound them into the ground around the plant, just outside of the branch area. Stuff dry leaves between the branches and the burlap. Remove the leaves in early spring, but leave the burlap up until all danger of late frost is passed.
Planting hydrangeas on the north side of a building, if you can protect them from winter winds, might also help because that can help keep them dormant longer in spring. But, many of them are simply not bud hardy where winters are really cold.
You can also look for the few H. macs that either bloom on new wood - virtually all of them only bloom on old wood or set buds all down the stem so that even if the tip buds are killed, some will make it and produce flowers - ‘Dooley’ is one of these. In general, H. macs set flower buds in summer for the next year; if those buds are frosted you get no flowers. ‘Endless Summer’ is a relatively new plant that should now be pretty widely available that blooms on both old and new wood.
Michael Dirr has been working on developing truly perpetually flowering hydrangeas that flower on new as well as old wood. Look for ‘David Ramsey’, ‘Decatur Blue’, ‘Oak Hill’, and ‘Penny-Mac’ which may or may not be out on the market yet.
One other cause of no flowers is pruning the plants at the wrong time. If you prune them heavily in spring, you are likely cutting off their flower buds. You want to only remove dead wood and *very* lightly tip back errant branches to a healthy bud in spring; save your heavy pruning for shape until right after the flowers start to fade.