Hydrangea paniculata.

Hydrangea paniculata. Credit: Missouri Botanical Garden /Missouri Botanical Garden

Why don't my hydrangea plants have flowers? I have had two plants for three years, and each year they come in nice and green but no flowers. I prune them in the fall. -- Carolyn Grunner, Melville

I have a large hydrangea plant, all green, but no flowers. What do I need to do? I have been feeding it Miracle-Gro, but still no flowers. -- Kathleen Moffettone, Wantagh

 I get at least three questions about nonblooming hydrangeas every week, so I think it's time for a primer. Hydrangeas can fail to bloom for a few different reasons - too much shade, too much nitrogen fertilizer, too much or too little water. But in my experience, the most common cause is improper pruning. Some hydrangeas bloom on old wood, growth from the prior year or two, while others bloom on new stems from the current year's growth. If you have a species that blooms on old wood and you prune it in early spring, you're removing all the flower buds and sabotaging your plant.

To know when to prune your hydrangea, you first must know which species you have. Here's a handy guide - just compare your plant with the photos and check the advice.

Hydrangea macrophylla

Prune in late summer, as soon as the flowers fade, but never after September. Remove weaker stems from the base of the plant, being careful to retain several stems of old wood, which will produce buds for next year's flowers.

Hydrangea arborescens Grandiflora

Cut to the ground in late winter/ early spring.

Hydrangea paniculata Grandiflora (Peegee)

Simply remove spent flowers and thin or cut back last year's growth in late winter/ early spring.

Hydrangea quercifolia (oakleaf hydrangea)

Remove dead wood at the base of the plant in early spring.

Hydrangea anomala petiolaris (climbing hydrangea)

Unruly vines can be shortened in summer. Otherwise, pruning is seldom necessary.

PHOTOS: Identify your hydrangea species

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