Asclepias incarnata, aka Swamp Milkweed or Marsh Milkweed, is native to this region and therefore quite hardy. However, since you started yours in the house, I wouldn't plant them directly outside without hardening them off. The sudden effects of sun and wind would probably have dire consequences. If your coldframe screens out some light, you could probably move them out there now.

I've had them for many years and haven't found them to be invasive, although they will seed themselves here and there if you don't remove the pods. However, if you learn to recognize the seedlings, you can just pot them up and pass them on to your friends or move them to where you want them. The flowers have a wonderful smell, and the plants are a great host for monarch butterflies, both caterpillars and butterflies. Other than seeding, I doubt they'd be invasive unless you were planting them in a very moist, marshy area. Mine are on a hillside and remain quite contained.

Harden them off gradually, and good luck with your plants!