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One question comes to mind -- how long have you had this plant? Has it always had this color pattern? There are many hydrangea varieties, with a number of different color variations, so is it possible that it's natural for yours to go from pink to green? Other than that, there are a couple of suggestions for preventing flowers turning green -- you can try increasing the amount of water; also, if there's some way to provide some shade from the hottest part of the day. Don't know if any of this is practical, or if it would work, but I've seen these tips mentioned.

Hopefully you'll find what you're looking for in this article: https://www.gardeningknowhow.com/special/organic/homemade-pet-friendly-weed-killer.htm

There are many types, and many sizes, of hydrangeas. The information on size at maturity should be on the plant tag. If there's no plant tag, request the information from the plant store. If they don't know, (even if they do should you want to do further research,) get the name of the grower and the name of the cultivar. After you get your hydrangea planted, we have several article to help you keep it healthy and beautiful. Start here: https://www.gardeningknowhow.com/ornamental/shrubs/hydrangea/growing-hydrangeas-hydrangea-care-guide.htm

I'm thinking that the shade has grown over the years, and your plant's flowering is reduced because of reduced light. Is it possible to trim back some of the shaders? Or move the hydrangea? FYI, here's some more information on hydrangeas: https://www.gardeningknowhow.com/ornamental/shrubs/hydrangea/growing-hydrangeas-hydrangea-care-guide.htm

Here's an article containing all the basic information you'll need to thoroughly enjoy your hydrangeas, including guidelines for fertilizing: https://www.gardeningknowhow.com/ornamental/shrubs/hydrangea/growing-hydrangeas-hydrangea-care-guide.htm

There's a good chance that your hydrangea was affected by the hard winter. Pruning would probably help it alot. Here's an article that should help you: https://www.gardeningknowhow.com/ornamental/shrubs/hydrangea/prune-hydrangea-bushes-hydrangea-pruning-instructions.htm