This is one of the questions we hear most often. There are several possibilities:
1. Harsh winters can damage flower buds on hydrangeas that set their flower buds last year.
2. Deer love to eat those tender tips.
3. It is important to prune them at the correct time – and this is where it gets tricky.
Some hydrangeas set their flower buds on old wood, which means they were set last year. If you prune off those branches in the spring, you will be cutting off the flower buds.
The Mophead or Lacecap hydrangea (H. macrophylla) usually have blue or pink flowers depending upon the cultivar and the acidity of the soil plus the availability of aluminum and bloom on last year’s wood, so they should be pruned right after they flower. Oakleaf hydrangeas (H. quercifolia) also bloom on old wood. They have large oak-like leaves and white flowers. Prune all these no later than August.
Some hydrangeas set their buds on new wood, meaning branches that grow this year so it okay to prune them now. So, the crucial question is which hydrangea do you have ?
If you have PeeGee types (H. paniculata) which look more tree-like and flowers are long panicles, or Annabelle types (H. arborescens), these all bloom on new wood so they can be pruned in fall, winter or very early spring.
Finally, there is one group which can be pruned almost any time: ‘Endless Summer’ hydrangeas (H. ‘Endless Summer’)
Adding fertilizer to your soil under your hydrangeas once or twice a year will help to keep them healthy. You can use a 10-10-10 fertilizer once in spring and then again in July or use a slow-release fertilizer just once in late spring. Organic fertilizer like compost can also be added.
NEVER fertilize or prune shrubs or trees later than late August as new tender growth may be frozen by a hard frost.
Information received from Carol Chernega, owner of a garden maintenance business, One Garden at a Time, and who specializes in hand-pruning.