Answer: For best blooming, hydrangeas need rich soil that is evenly moist (damp like a wrung out sponge.) Most gardeners find that a spring application of good quality compost along with a general purpose granular or slow release granular fertilizer such as 10-10-10 is adequate. Using an organic mulch year round will also help feed the soil on an ongoing basis.
Most varieties of bigleaf hydrangea (Hydrangea macrophylla) bloom on old wood that grew the year before. This means that winter damage to the stems, spring frost damage to the buds, or pruning at the wrong time will limit flowering. (The only safe time to prune is right after they bloom.)
Your zip code places you in zone 5A which is extremely cold for bigleaf hydrangeas. So cold in fact that I am surprised you have even a few blooms. This makes me think you might be growing one of the newer cultivars such as Endless Summer that are able to bloom on both old and new wood. If you have one of these, I would expect it to die back in winter but -- since it can bloom on new wood -- you should still get blooms beginning in late summer and continuing into the fall. Maybe this is what you are seeing.
If you are growing one of the older more traditional varieties, you might be able to improve blooming by giving the plant extra winter protection. (Would help with Endless Summer as well, for earlier blooming.) One way to do this is to wrap it in burlap and stuff dry oak leaves or dry straw inside, then top it with a plastic tarp. Allow some gaps to avoid condensation inside. Put this on in very late fall after the plant is fully dormant and remove it gradually in spring so it can wake up with the season. Be ready to cover it if a late frost threatens. Then limit your spring pruning to just removing any truly winter killed branches.
I hope this helps.
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