I grow a few varieties of asclepias, including curassavica. I've had excellent germination when winter sowing the seeds I harvest the previous year. Even though curassavica is a tender plant, the seeds can be successfully winter sown; I don't believe those imparticular seeds need stratification. I've done them in January and in April, and both germinated at the same time. Let them develop the seed pods. When they're ready, they'll split a seam, open up and disperse seeds in the wind! You've got to be quick once those pods open; check them daily once they get plump. I'm pretty good at harvesting loads of seed, but this year, for some reason, those babies popped and there was seed flying all over the place! If you would like some seed for next year, I can send you some; just let me know.
Tracey