Lack of flowers is a common problem with hydrangeas, especially the bigleaf types (Hydrangea macrophylla). Lacecap and mophead hydrangeas can stubbornly refuse to bloom, although the foliage is lush and healthy. Now, there's nothing wrong with green, but that's not what you expect from your hydrangea. The good news is that once you determine which problem your hydrangea has, there's a solution.
Depending on the hydrangea, the plant forms its buds on last year's growth (old wood), on new wood or on both. The mopheads and lacecaps generally bloom on old wood and are susceptible to bud freeze in colder areas. Without protection during a cold winter, these hydrangeas might not bloom at all the following summer. The Boston area had two summers nearly without flowers, according to Ellen Wells in a 2006 "Ask the Gardener" column in the "The Boston Globe" magazine, due to "back-to-back" harsh winters with little snow.
Pruning at the Wrong Time
Pruning back bigleaf hydrangeas (the ones that flower on old wood) during fall cleanup will result in plenty of green leaves the following summer and no flowers. Timing is everything when it comes to pruning. If a hydrangea blooms on old wood (the mopheads, lacecaps and oakleaf), prune it as the flower fades, according to Janet Carson in a "Fine Gardening" online article. The 'Pee Gee' hydrangeas and the smooth hydrangeas like 'Annabelle' flower on new growth, so prune in early spring.
Too Much/Too Little
In an online article for North Carolina State University, Dick Bir, a horticulture specialist, discusses the perils of "too much," in this case, fertilizer. A fertilizer high in nitrogen causes leafy growth at the expense of flowers. Sometimes the nitrogen comes from runoff from lawn fertilizer, if the plant is close enough to be affected.
Conversely, too little fertilizer can be the culprit, especially phosphorus, which stimulates bloom. Feed with a fertilizer for blooming trees and shrubs.
Shade and Water
Most hydrangeas prefer a little shade, but too much will give you great leaves and no color. According to Dick Bir, hydrangeas prefer high shade overhead from deciduous trees, depending on your location. If your hydrangea once bloomed beautifully and you've done nothing differently, it may be that lack of sunlight is an issue.
Watering the plant too much, Bir says, or keeping it too dry will also cause lack of flowers.
Reblooming Varieties and the Hardiest Hydrangea
There are new varieties of bigleaf hydrangea that bloom on old and new wood. Dick Bir calls these "remontant bloomers." The variety 'Endless Summer' was the first of these. If a cold snap kills buds on old wood, the rebloomer will set buds on the new wood, resulting in a later bloom.
Ellen Wells quotes a Boston-area horticulturist who suggests trying the 'Pee Gee' hydrangea (Hydrangea paniculata) instead of the bigleaf, as it is hardier.