Live in Zone 5 and have a large leaf hydrangea. Every year the hydrangea develops numerous flower heads but only 25% of the flower heads actually develop into a full flower. The hydrangea is several years old. The plant is a foundation planting and faces north. It gets morning sun. The plant appears healthy and quite vigorous but never blooms fully.
Under-Developed Hydrangea Blooms
Certified GKH Gardening Expert
There are a few things that could be happening.
First, it could be the cold in your area doing it. If the flower buds develop while you are still getting even light frosts, this can kill the buds. Being on the north side of the house, you may even have a slight microclimate happening where the plant is experiencing light frost even when the rest of your yard is not.
Second, too little water may be the culprit. If the plant is wilting alot, this could be killing the flower buds.
It could also be a phosphorous issue. You can try giving it some bonemeal to see if that will help.
Thank you for the helpful information. As an inexperienced garderner, I always thought my hydrangea was a mophead but it turns out that the plant is a lace cap hydrangea and it is blooming the way it is suppose to!