Answer: Based on your description, there may be several reasons why blooming is sparse. Most of the bigleaf hydrangeas bloom on old wood, meaning wood that grew the year before. Winter cold damage, pruning at the wrong time, and spring frosts can all prevent or reduce blooming. You are gardening at the northern end of the hardiness range for hydrangeas -- based on your zip code -- so I am a little surprised you have flowers at all. Perhaps snow cover is protecting the base of the plant or uou have one of the hydrangeas such as Endless Summer that bloom on both andd or new wood. Either way, avoid pruning in fall. In the spring, do not prune with the exception of removing truly winter killed stems. If you need to control size, prune right after it flowers.
Hydrangeas need a rich humusy soil and ample moisture so that the soil stays evenly moist like a wrung out sponge. They also need some direct sun, either morning sun or bright dappled light all day to bloom well.
You might try fertilizing with a slow release granular fertilizer each spring per the label directions (look for a complete fertilizer with an analysis such as 10-10-10), plus apply a top dressing of good quality compost to assure a good supply of micronutrients and replenish the soil with organic matter. Then use an organic mulch year round. This will help with soil moisture and also help feed the soil as it breaks down slowly over time.
You may also need to water occasionally to supplement rain so the soil stays evenly moist. Dig into the soil to check if you need to water. When you do, apply it slowly and deeply so it soaks down to the deepest roots. It is better to water deeply less often than to sprinkle lightly every day.
I hope this helps you trouble shoot!
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