I put mine in this week. We are now ( supposedly ) past the frost date, so that is when to plant. They don't like frost. I'm zone 5 Canada, more northerly than you, but you know your area better. As I say, they don't like frost so when you are past the last frost date, go for it. The sooner the better. If you do get a frost, you can alway cover them with an inverted pot, unless you have a herd of them like I do. LOL

I'm about an hour south of Cleveland. I believe May 15 is our last date for frost. For you anytime after Memorial day would be fine. I've planted as late as July, and still got blooms.