Here are some reason why hydrangea fail to bloom :
1. Too much shade-all grow in partial shade but too much will reduce or eliminate bloom.
2. Too much fertilizer-too much nitrogen will promote healthy green leaves at the expense of flowers.
3. Winter damage- If you live in the cooler climates, the flower buds may have been damaged from freezing temperatures. Only smooth (H. arborescens) and panicle(H. paniculata) hydrangea bloom on new wood; bigleaf hydrangea (H. macrophylla) blooms primarily on old wood (*with the exception of new remotant bloomers ) which means that they need to set their buds in the current season and make it through the winter to bloom the following year. *New cultivars of H. macrophylla, such as Endless Summer, Twist n Shout will bloom on old growth as well as new growth.
4. Pruning – be aware of the timing. Judicious pruning is important. Too much and late pruning will eliminate next years flower buds. If you prune to reduce the height, do so right after blooming. If you prune to thin out the plant, cut some of the oldest stems to ground level in the early spring(dormant season). Most hydrangeas grow well with no pruning at all whereas others such as H. arborescens (Annabelle) and H. paniculata (Pink Diamond) usually need to be cut back every year.
When is the best time to cut back Hydrangeas?
The best time to prune is early spring and immediately after flowering. It helps to know what kind of Hydrangea you have to know how much to prune.
H.arborescens blooms on new wood, so you can cut it down to 1/2 size or 6″ above the ground in the winter or early spring. No pruning will give you less vigorous growth and smaller flowers.
H. paniculata – same as H. arborescens.
H. macrophylla Bigleaf hydrangea- blooms on old wood (except for Endless Summer, Penny Mac and All Summer Beauty). Prune only to remove spent flowers. Cut right below the flower head. Any other pruning will reduce flower production for the next season. The more you cut the less flowers you will get.
H. quercifolia, Oakleaf hydrangea blooms on old wood -prune same as H. macrophylla.
Prune in early spring to shape or thin the plant at the expense of flowers.