I bought these plants as well this year and was wondering the same thing as everything I have read says it is a hardy annual being frost hardy in Zone 5. It says to leave a few plants in to self seed and pull the rest out in the fall. I am going to try it and see what happens. Good Luck.

I find thse to be good self seeders. I live in slug central so many, many of them get eaten, yet still I find some popping up here and there. Without the slugs I think I'd have hundreds more.

I have also found them to be excellent self-seeders. There are some coming up in my mother's flower beds, and it's been years since I planted them there.