It really doesn't kill the grass. (Right now that's about all the grass we've got that isn't scorched) It turns a little brown, but recovers quickly after they're moved. The only place it really kills the grass is where the 4 x 4 's sit on the ground.

Carl, I had them on the ground at our previous house, and it does indeed work better with them on the ground. Fortunately where they are gets sun from whenever it comes up until around 6pm. And considering right now I've got the water up to 94 degrees I don't think I need more heat. (I've VERY cold natured and won't get in unless it's at least 85.) We plan on taking them down either this weekend or next.