I was fortunate to have access to a prairie that was being tilled under and dug a bunch of transplants to plant on my compacted red clay slope near my pond. They have filled in nicely over the years. It was just to harsh to plant seed, although the origianl plants spread by seed to cover the area. A lot of work, but worth it to me. Gray clay might not be so easy. Mulch is also a great help. Good luck!