Some Asclepias seed offers say "for fall planting", and some don't.

Who knows for sure about planting in the spring?

I've have direct sowed Asclepias tuberosa in the spring and those plants have been alive for years now. Seeds that fall from the plant in the fall gladly sprout come springtime as well, some quite some distance away since the seeds are easily carried by the wind. So, for this species at least, you should not have any trouble planting at either time. As far other Milkweeds I can not say.

Some Asclepias require stratification. However, since you live in a climate that I'm sure is colder than mine, stratification might not be necessary. Do you know or else have you tried to find which types are native to your area?