Just pinch off the seed pods before they ripen. Leave a few to plant in orther places. I have 3 different milkweeds so far and am looking for others as I do have a beautiful flock of butterflies, bees and hummers each year. As for common milkweed, plant it somewhere where it wo't become so much of a problem. Or, you can just mow it down, thatworks for the road department. In fact, it works so well that nothing comes up along the road anymore (I live on a grave road in the country). ONLY behind my fence can things grow unmolested.