It sows by seed.

I agree with Nevermore44, that Common milkweed spreads by rhizomes, so can be considered "invasive" in a small garden. Asclepias incarnata is not invasive, and is loved by Monarchs and other pollinators.

My Asclepius syriaca has spread underneath a flagstone walkway to the other side. Newspaper won't stop its spread. I'm ready to dig it all out ,and replace it with more well behaved milkweeds, like prairie milkweed. I have A. incarnata, and A. tuberosa, and they don't spread.

Not very in my experience. I have lived in my house for about 14 years. There was a patch of milkweed down by the creek and its still about the same size. Has not spread at all that I can tell.

Your milkweed by the creek is probably A. Incarnata, which does not spread like A. Syriaca. Diffferent varieties of milkweed behave quite differently. Syriaca can spread widely by underground rhizomes.

Quick update: only 1 survived the transplant and I thought after reading here and on the Internet how invasive it is despite being a very good nectar and monarch-hosting plant, I dug out the remaining milkweed. I didn't want to have it sprout in the neighbour's yard.

What was scary was it grew a small white root from its main one, probably getting ready for invading damage.

I better get the tropical variety. Again, a lot of people growing that kind seem to have a better success raising monarchs with tropical than with other milkweed species.

I swear that the common milkweed, a plant that I completely ignored before I started gardening this year, grows almost everywhere!!! I see it very, very often!

Also look at getting Tuberosa MW. It's stays smaller... doesn't' run... and blooms immensely. Bees go wild for it.

In regards to you digging up the common type... just be aware that unless you got every root... it will probably come back. I had one show up in a front perennial bed last year. I first cut if down at 1 foot. It came back by the fall... so I dug it out... i got what I thought were all the roots... and moved it the backside of my yard in the "wild hill" section to live it's life out. (pic below of my common mw outside the fence in the back)

So this year... that front one popped back in the same spot. It's a tough one!

I hopefully got everything out as it was a young plant.

This thread saved me a ton of potential grief: Last week, I germinated some A. syriaca seeds I received via online order (was happily surprised how quickly they sprouted using the moist paper towel method). I sewed ~ 20 germinated seeds into a smallish bed in my backyard, but don't want an invasive in my yard. I'll be removing them today and placing into pots, just to see how they fare. I do welcome a different type of MW in my butterfly garden, though -- currently have a small collection of A. tuberosa, curassavica, verticillata (likely) and incarnata.

Definitely put the syriaca to good use in a wild area somewhere!

You should try and hunt down some Purple MW. I was able to get one plant last year which did well this year... but since there was not another plant around it didn't set seed. boo. I need to get a buddy for it.

Not to be nit-picky, but I feel it's important to distinguish between spreading undesirably and invasive. In order to be 'invasive,' a plant must first be non-native. If the plant is native, it can be a pain in the neck, a weed, a rampant spreader, etc., but it's not an invasive species. I'm not sure where you are, so maybe it is invasive there, but I feel like sometimes we overuse the word 'invasive' in a way that can downplay the severity of true invasives (plants that are non-native whose introduction harms the native environment).

Moving on ... I think mint is much more of a weed than milkweed. It will of course depend on variety for both plants, but mint, in my experience, runs amok much faster. The mint that now grows in a garden bed near my foundation started out in a pot ... then it managed to get a tendril down to the dirt, and well, now we keep root barriers around it and rip out what escapes every year. Granted, my milkweed isn't that old, but neither was the mint when it escaped.