Hydrangeas: Why They Fail To Bloom

Hydrangeas are native to southern and eastern Asia and include a genus of about 70 to 75 species. Of those, about a handful are commonly grown here in Massachusetts. Especially popular in the bigleaf or Hydrangea macrophylla that has many cultivars. Most hydrangeas thrive in full sun or partial shade, depending on the cultivar. Hydrangeas grow best in moist soils that contain organic matter. Soil around plants can be mulched with 2 to 3 inches of bark mulch or other organic mulch to conserve moisture. Hot, dry, exposed sites should be avoided.

What are reasons that Hydrangeas fail to bloom?

There are three possibilities for lack of flowering among the hydrangea species. Too much shade and improper pruning apply to all hydrangeas, while weather-related damage to flower buds applies primarily to the big leaf hydrangea, Hydrangea macrophylla and Hydrangea serrata.

To explain weather-related damage, it is necessary to understand why flower buds form and plants bloom. Hydrangea serrata, Hydrangea macrophylla (Bigleaf Hydrangea) and Hydrangea quercifolia (Oakleaf Hydrangea) produce flower buds in late summer and fall that bloom the following year. This is called flowering on old wood. Hydrangea paniculata (Panicle Hydrangea) and Hydrangea arborescens produce flower buds and blooms in the same year, which is called flowering on new wood.

Varities of Hydrangea macrophylla are now available such as 'Blushing Bride' ™, 'Endless Summer' ®, 'Twist-n-shout' ™, 'Red Sensation' and others that produce flower buds and blooms on both old and new wood.

A late spring freeze can kill developing flower buds on varities that produce flower buds on old wood. Sometimes a late spring freeze will also kill all the emerging leaves. Since most flower buds develop on the old stems, they will be killed when the stems are damaged. The flower buds must survive through the winter and spring. These flower buds have a low survival rate in some areas of Massachusetts where a late freeze may damage buds.

The new varieties of Hydrangea macrophylla flower on new growth as well as year-old stems, so flower buds have a better chance for survival. Varieties of hydrangea also vary in cold hardiness. For example H. macrophylla 'Endless Summer' ® and 'Twist-n-shout' ™ are cold hardy to zone 4, 'Blushing Bride' ™ and 'Red Sensation' are hardy to zone 5. Florists' hydrangeas are also Hydrangea macrophylla; however, they are cold hardy usually to zone 6 and blooms are formed on old wood.

Another reason for lack of blooms might be pruning at the incorrect time of year. Pruning plants that produce buds on old wood in late summer or early fall would remove any flower buds that developed for the next year's blooms.