Answer: Ideally you would run some basic soil tests and see if you need to fertilize and how much. Most mature trees do not need fertilizing, especially if they are adjacent to or surrounded by a lawn that is being fed. But, generally you could use some all purpose granular or slow release fertilizer in the spring. Read and follow the label directions. A top dressing of compost is also helpful, along with using an organic mulch.
Most of the big leaf hydrangeas (H. macrophylla) bloom on old wood that grew the previous year. If you had a severe winter, winter damage may have killed back the stems thus preventing it from blooming. Or, a spring frost could have damaged the buds or tender new growth.
Another cause for lack of bloom is pruning at the wrong time. These can be thinned immediately after they bloom for best results. If you cut it back in the fall or spring, you are removing flwoering wood.
Another possible reason is lack of direct sun. They do need some direct sun or very bright dappled light in order to bloom. If your hydrangea is being shaded by trees, they may have grown over the years so the hydrangea is now in more shade than before.
Hydrangeas do like a fairly rich soil. Fertilizing, top dressing with compost and using an organic mulch all help with that.
The other important factor is to keep the soil evenly moist, like a damp sponge, not saturated and not dried out. If you have a dry season, or if neighboring trees have begun to send roots into the hydrangea planting area, then that could also be the cause.
I hope this helps you trouble shoot.
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