I have managed to control the invasive roots of my “Bleeding Heart” plants by sinking 2 foot long sections of plastic irrigation conduit into the ground. (diameter is about 1.5 feet) Bottom is layered with gravel, then landscaping fabric, followed by soil, and the specimen. I have a field of common-milkweed that I am not trying to control. However, in my garden, I am going to plant some swamp milkweed, and that gorgeous orange buttterfly-weed. Is it true that the rhizomes of these specimens are not invasive? Might they survive if I wanted to use my “plastic conduit technique” to manage them? Or will the rhizomes crowd themselves at some point, and die? Much thanks!
Unlike the common milkweed, neither swap milkweed nor butterfly weed are invasive. That being said, one source described butterfly weed as spreading “energetically”. So I don’t think either plant warrants your plastic conduit technique. I have butterfly weed in my garden and find it quite contained. Swamp milkweed as the name suggests prefers a moist environment. Growing it in a drier situation further slows its spread. If you’d like to read more about these plants, I’ve included links below to fact sheets on each. Your field must be a monarch haven and soon your garden will be too.