Don't know about direct sowing the seed (I started it under lights the one time I grew from seed), but it's not very invasive - it is a pretty narrow plant and doesn't spread much. Mine has reseeded itself in light shade and blooms fine.

Keep in mind that butterfly weed is one of the latest plants to appear each season.

You can put it out any time - it's tough - but you might not get germination this year unless we have some more cold weather. When I worked at a nursery, we started it in the fall, in flats outside in the shadehouse. I think it made tuberous roots first, and only poked its head up in the spring, because when we went to pot them up, there was a nice little tuber for each sprout.
Having said that, how long did it take for germination under lights, Cecilia? And did you have to give the seeds any kind of cold or warm stratification first? I'm hoping to start A. tuberosa and two colors of A. incarnatus, but am obviously a bit late by my own standards.

Last year I tried cold-stratifying asclepias tuberosa seeds in the fridge, without luck.

This year, I've got asclepias incarnata seeds from a trade. Simply forgetting that they supposedly need cold stratification, I planted some in potting soil under lights when I was doing the rest of my indoor sowing. And I've got a nicely flourishing sprout. Just one. I think they need to be covered with earth - no light - to germinate; I'd surface-sowed the others.

I think it germinated after about two weeks. I had a heating pad between towels under the potting tray for the first few days, but then the impatiens and larkspur germinated, so I turned off the heat. The asclepias and other seeds took a few weeks to get going. In fact, agastache just poked up a tiny feeler yesterday. I hope it decides to stick around.

Julia

Oh gosh, I don't remember what I did to start those seeds - I'll check my notes (if I still have them). I probably followed whatever directions were on the packet and/or in Eileen Powell's "From Seed to Bloom" book.

I've noticed that most of the perennials I start take longer to germinate than the annuals. Sometimes a LOT longer.

I have a mixed report on these. Last year I winter sowed them with very little success, because they supposedly needed cold treatment.
This year I started some in warm conditions, on 72 deg heat mat, almost every single seed germinated (yes, I counted, she says compulsively). However, the source of the seed was different, so I cannot claim without question that the advice for cold treatment is wrong.
Laurel

I started ascelpias 2 years ago by putting in potting soil in fridge for awhile (weeks) and then putting on a window sill. We had leaves - all I really cared about - for the caterpillars.

I wintersowed mine this year, starting in December, and I see a couple of sprouts now.

I did A. incarnata from seed - used a heat mat, and I have a lot of seedlings.

This spring I did A. tuberosa from seed, no stratification or anything, four seeds in a pot, surface-sowed, either on heat or at room temperature (depending on what else was growing), and 3/4 germinated in maybe 3 weeks, of which two made it to real live seedling stage.

(Of course some people do say that "room temperature" at our house in the winter is cold enough to stratify seeds :-).)