Why don't you just plant Asclepias incarnata? I think it has many of the characteristics of syriaca, but from what I hear, is less prone to spreading. It has a nice pink flower head, attracts butterflies, and seeds in some, but the seedlings are obvious and are easy to remove. I gather syriaca may spread underground which becomes a bother. See the link below.

Here is a link that might be useful: Milkweed discussion

I have been growing syriaca and it hasn't spread too out of control. I grow it in a shady area... which helps keep it stunted.

I do have better luck with a. incanarta though, when it comes to monarch caterpillars. I get more caterpillars on the incanarta than any other plant.

I think it depends on how good your site is, how much competition from other plants, and how much messiness you can tolerate. Stopping the spread of seeds will not prevent syriaca plants from spreading underground. I have a clump of syriaca in a rich, weedy creek drainage in my yard, where I let a single plant spread over five years or so. It has migrated about ten feet (or more) inevery direction within this bed, though it is interspersed with other species (not a monoculture). Every spring I also find multiple stems erupting in my mowed yard, sometimes more than ten feet from the border. This, too, is not a problem, since I control its spread by mowing, but it shows the potential for spreading.

I also grow three other milkweeds: butterfly, swamp (incarnata) and purple (purpurascens?). None of these spread like syriaca.

Butterfly is both gorgeous and long-lived; it has never self-seeded or spread by root.

Incarnata is elegant and well behaved -- in fact, clumps usually die out after a couple years. But they do self-seed in good soil with sun and not too much competition.

Purple milkweed is like an improved version of syriaca (for garden purposes): smaller, with more colorful blooms, and less aggressive. It competes well with other tall plants (but not too well) and it does seem to spread underground to a small extent. Seems to be long-lived, too.

I like them all, as do the monarchs and other butterflies. I think they can all have a place in a garden setting, but syrriaca is by far the most challenging.

John is right on every count, except that my butterfly weed (A. tuberosa) does seed some. May just depend on whether it likes your soil. But it's really only an occasional seedling or four, and I've been happy to have some extras to spread around. It's got a real taproot, so it doesn't wander (and if you want to transplant it, it's best to do young).

Thanks for the information.
That's way too much of a spread for where I want to put it. There's just not enough room and I want to keep it small enough so that I can keep it dead headed. I don't want a lot of milkweed spreading to my hayfield. But I already have a few seedlings and would like to keep a couple IF it would work to put a barrier around it. Do you know how deep the runners are?
If it's impossible to control, I'll just give up and grow another type. I have one swamp milkweed. I had one tuberosa but it didn't come back after our freaky winter. The purple milkweed sounds nice. Is it a. purpurascens? I will try to get some seeds for that one.
John - Since you're in Missouri like me, do the monarchs have a favorite?

A. sullivantii (common name: prairie milkweed) is almost identical in look (a bit more vividly colored than syriaca) and growing conditions to A. syriaca, but much less assertive. Just a correction to john_mo's comment about purple milkweed: it is not an improved version of A. syriaca (although I'll agree that it it's definitely an improvement over syriaca in many ways even if not an improved variety of syriaca), but a separate native species (A. purpurescens is correct christie). It is, as John_Mo said, less assertive. It also tolerates drier conditions. It's my favorite for flower color, as well as more interesting foliage. Several native plant nurseries carry both the seed and either plugs, pots, or bare root stock.

We've grown 4 species from seed, and all were easy. A bit of cold, moist stratification improves germination rates, but is probably not necessary as you will probable have more seeds (if you buy a packet) than you want. Given the lateness in the season, you might consider going the plug or container plant route. Also, from seed, my limited experience has been that it is the second year before you get good above ground growth and flowers with some species, and that was having started them in the early spring of the first year indoors. I suspect you'll see flowers next summer regardless, but I don't have experience starting this late the first year.

All will attract monarchs (I've seen 3 different ones on my swamp milkweed out my window in the Good luck,
Dave

If anyone has any wild Milkweeds growing everywhere that you do not want I want them. Even if they are small plants or bare roots. I am interested in them because I have an acre backyard and am interested in Milkweeds. I am interested in all the wild ones but not Asclepias Curassavica. I already have that one.