I've been collecting a variety of different species of Asclepias to winter sow and put in my new butterfly bed. My silly question is, can and do they cross?

The bed is straight out from our walkout basement, (which faces our neighbors garage, why they built it that way, I have no idea, so I'm creating something nicer to look at) Right now I have a few things I transplanted. In the center is my black pussy willow, one end a butterfly bush, and the other has purple coneflowers and mexican hats. (I'm gonna have to make this much larger before next spring!) My plan was to line the back (facing the neighbors) w/ a variety of Asclepias. And I'm boardering it in parsely ;) I have other stuff going in there too, but I'm getting off track here.

So I'm wondering if I need to seperate the varieties of milkweed? I think I read that a few like, A. syriaca, A. sullivantii, and A. purpurascens can cross. But I don't remember where I read that. But if it's true, can others cross too? I can't imagine that A. tuberosa could cross w/ A. syriaca, can it? Does anyone know if any can cross and if so what w/ what?

Thanks

Karri