The first time I saw butterfly weed (which is now my favorite wildflower, btw!) it was growing in bare dirt - actually more like dried clay than dirt, in full sun, with no neighbors. Others here have mentioned caution against overwatering (and I saw your replies) but I thought I'd share my observation as this species may well thrive on neglect.
I planted some plugs in front of our house years ago (along with Green and Gold) and walked away...a late frost got a lot of them a few years ago but some survived and I've seen evidence of seeds taking root elsewhere. Like a lot of natives it does its thing on its own time and can't be rushed...