Plant outside in fall generally means place the seed in your seed bed in late November or early December to experience stratification. I believe for most milkweed species this is all one needs to do, except tropical milkweeds which can be started in the spring and are only annuals in much of the US. The same thing is accomplished by storing seeds in damp media in the fridge, but then you'll have to do the "planting" yourself.

Thanks. And by "placing" the seeds in my seedbed, can I simply drop them onto the ground (much like they would normally just fall onto the ground from dead seedheads)? Or should I press them into the soil somewhat, but not cover them per se with soil? When you say November/December, does it matter whether I sow them before/after the first frost?

No, it doesn't matter. What does matter is that you may have some (depending on species) that germinate too soon if you plant them way early and some may not receive enough cold stratification if planted to late.

There have been some studies on the effectiveness of different types of soil packing & such, but you don't want to cover the seeds of forbs/grasses with soil. Go to shawnature.org and click on Native Landscaping, then Native Landscaping Mannual.

Yippee, Milkweed (Asclepias) is generally very easy to start from seed. My Swamp milkweed (Asclepias incarnata) self-sows abundantly in the garden, and I had great success with winter-sowing 6 different species of Asclepias this past winter. (I have a particular interest in Milkweed, plan to grow more species this coming winter, and also raised Monarch butterflies for the first time this summer, which is really amazing.)

Winter-sowing is a method of sowing seeds in plastic containers filled with soil, which are then placed outside during the winter (or Spring). People usually use recycled containers, it requires no electricity, is a low-cost way to have lots of plants, and seeds that require cold treatment or stratification will get it! There is much info at the Winter Sowing forum and FAQ.

The seeds will sprout when they are ready and then you can transplant your seedlings when they're small. It's easier than trying to monitor sprouts in the ground, although that would work too.

Thanks. Very helpful! Yeah...I too am growing these specifically to try and attract butterflies. It's so sad how you never see them anymore...at least not where I live. Although I DID see maybe one tiny white butterfly this year.... And I HAVE begun seeing fireflies again over the past few years. I just LOVE fireflies....they fascinate me...and it is just so beautiful to see these subtle glows of light flitting about the yard at dusk!