There are a number of reasons why a hydrangea plant is unhealthy or not growing as expected. The amount of water or sun the plant receives and other environmental factors can prevent a hydrangea bush or tree from living to its full potential.
One of the most common issues with hydrangea plants is small size or complete lack of blooms. This often occurs because the wrong variety is planted in the wrong climate. Because some hydrangea plants only bloom on old wood in a climate with a cold winter, there isn't time for the plant to mature before cold weather makes the wood die back. This issue can be avoided with careful research.
A hydrangea plant may not bloom well if it is not getting enough sunlight or it is pruned incorrectly. If a hydrangea plant blooms but the flowers brown and die quickly, it is likely due to a lack of water. Soil should be kept moist on hot days. A layer of mulch added around the plants or adjusting sprinkler systems to keep plants hydrated is helpful. However, make sure the water is not directly dampening the leaves of the plant, as this can cause fungus growth. If there is mildew or fungus growing on the leaves, prune the damaged areas.
Pests are another issue with hydrangea plants. This can be solved with pesticide application or use of natural pest control methods.