It is almost July, and the midsummer flower garden is at its peak. In fact, a few flowering plants are beginning to look a little overblown, like ladies who stayed at the party too long. Two of my bigleaf hydrangeas (Hydrangea macrophylla) are in this category. After the mild winter, the hydrangeas have grown so tall and produced such massive mopheads that the stems can barely hold up the blooms, especially after a heavy rain. I cut flowers every few days and bring them inside, but many are drooping to the ground.

The age-old dilemma regarding hydrangeas is, “When should I prune my hydrangeas?” The answer is that there is no single answer. Pruning time depends on the species of hydrangea you grow in your garden, and if you don’t know the species, you can ruin your flower display for the season. Some species of hydrangea flower on old wood, and some flower on new wood, and the gardener must learn the difference.

Hydrangeas that bloom on old wood should be pruned in summer, after ...