Hi, this is Steve and thank you for your Ask Extension question on hydrangeas. You are correct to water the plant at the base and not on the leaves or flowers. From your pictures, I feel the issue is their environment and not a disease. Hydrangeas like shade or partial shade, so if yours is planted in a very sunny location, leaf and flower burn may occur due to excessive sun. It does appear it is in a shady location from your picture. These plants also need moderate to high water levels, however, if too much water is applied causing the roots to stay wet can cause the leaves to turn brown on the edges. If your soil is poorly drained, this can make the problem worse. I would pull the rock mulch away from the plant as this can add extra heat if/when the sun shines on the rocks. An organic mulch (bark/pine needles/etc) is a better choice. Generally, our soils are alkaline and adding more lime would not be necessary. I’m sending a link to an Ohio State University fact sheet that sums up and expands on some of the above points.

https://extension.oregonstate.edu/gardening/flowers-shrubs-trees/general-care-hydrangeas

It is a good sign that the hydrangea is still growing and hopefully the conditions you are seeing will remedy themselves In the future. Again, thank you for your Ask Extension question and feel free to contact us any time.

Jennifer, I think if you did place them in a pot, it would have to be a really large one as these plants can get really big. When winter comes, the soil inside the pot would freeze which would be bad for the plant. It would be difficult to insulate and stop freezing from happening in our winters. As far a winter care, do you have the information card that came with the hydrangea? It should have a Zone # on it, which tells us how winter hardy it is...if the number is 5 or smaller, it should be ok. You would want to supply water a couple times a month through the winter when the ground is not frozen or snow covered. Meaning no disrespect to Home Depot (we all buy plants there), you may want to try an established nursery should this plant fail. These Big Box Stores source plants from large areas and likewise sell those plants over a large area. There is nothing wrong with that, but sometimes the varieties they sell are not the ones best suited to our climate. 
Regards, Steve