Are these dying?
I purchased 2 of these hydrangea plants about 7 weeks ago. The plants are thinning out and at the base and there are many short brown "stems." I also noticed the plants themselves do not have a strong base, meaning I was able to pull a stem out of the inside of the plant no problem. Could this be caused from a vole problem? I have seen a couple of holes along the side of my front porch not too far where my plants are. I read that voles don't eat hydrangea roots and I read they do. Please help! Should I relocate them before they die off? Both plants are on the front of my house which faces south.
How moist is your soil? It almost sounds like root rot, especially since you can pull the stem out so easily. Here is more information: https://www.gardeningknowhow.com/plant-problems/disease/root-rot-in-garden-plants.htm
As for the voles being an issue (if root rot is not to blame), I don't think they would be feeding on your plant roots but if nearby critters like chipmunks or moles have any burrows or tunnels around your plants, the roots may not be able to take up much needed water or nutrients, causing them to die.