One of the biggest reasons for producing small flowers is that your hydrangea is just not getting enough water. Hydrangea plants require a good amount of water and they require it regularly.
Aim for a healthy watering session at least once or twice a week. These plants need deep watering, and a simple sprinkle just will not do. A sufficient amount of water needs to reach deep into the roots in order for the hydrangea to thrive. Keep in mind that you shouldn’t drown the plants with water. When you’re finished watering, the soil should be thoroughly wet but not overflowing with water.
Check soil at least four inches deep by sticking your fingers into the soil. If the soil is dry, it is a sign that the hydrangea needs more water. A plentiful amount of water will help flowers bloom to their full capacity. It also helps to keep your weather conditions in mind — if it’s been very hot you may need to water more often and if there has been rain, you may need to water slightly less. Again, checking the moisture in the soil with your finger is a good rule of thumb.