Several factors can contribute to a lack of blooms on hydrangeas. Some common reasons for a lack of flowers are winter damage, improper pruning, inadequate light or planting the wrong type of hydrangea. Often it is a combination of causes, and with a few simple changes in how you care for your hydrangea you can have a season of lovely blooms.
Check mophead (macrophylla) types of hydrangeas in the early spring to assess winter damage. New green growth should appear on the old wood left from last summer’s growth, since the flower buds are on the previous year’s wood. If the old wood does not have green growth on it but instead there are new shoots and leaves growing from the crown of the plant (at the soil level), that particular mophead hydrangea will not bloom that year. Trim out the dead wood. Allow the new shoots to grow all season, and protect them over the following winter for the next spring’s flowers.
Prune your hydrangeas correctly. Prune them within a month after they flower by cutting back the canes that bore flowers. Cut them back by about one-third. Allow new flower buds to develop for the following year’s blooms.
Use a high-phosphorus fertilizer, such as 0-18-0 or 0-46-0, to encourage blooms. Bone meal is a good organic phosphorus source which gives good results. Hydrangeas will produce lovely green leaves if you use a balanced fertilizer, but they need high phosphorus to produce the best flowers. Work phosphorus fertilizer into the soil early each spring.
Plant hydrangeas where they have some shade as protection against blazing sun but where they will receive four to five hours of bright, filtered light each day. Hydrangeas will grow in deep shade, but they need adequate light to bloom their best.
Choose the type of hydrangea that will grow well in your planting zone. If you regularly have extremely cold temperatures and you have mophead types of hydrangeas, you must provide winter protection. You also need to have protection ready against spring freezes that may occur after the flower buds have begun to develop.
Protect them by using mulch that will not compact, such as oak leaves, and hold it around the plant with burlap or a wire cage. Whatever material you use should allow plenty of air circulation while insulating the tender buds. Plastic does not allow air circulation and it should not be used as winter protection. (Res #2)
PG (paniculata) and Annabelle types of hydrangeas do not require protection in the US and Canada (Ref #1)