Without a photo to show the proximity of the ivy and the hydrangeas, I'm going to make a Big Assumption that the ivy is impeding air circulation at the bottom of the hydrangea plants.
English Ivy is a scourge, a nuisance, a...well...responsible gardeners don't want it growing anywhere near their valued plants! Do what you can to remove it from the hydrangeas, including pulling the ivy out by the roots. Right after rain is a good time to uproot ivy: the ground is soft, and the roots will come out more easily than at other times.
Just try to avoid harm to the hydrangeas.
Sunset Zone 17, USDA Zone 9