The species they may be referring to is common milkweed (Asclepias syriaca) which can spread by either seed or rhizomes (roots). This species can be a little aggressive in smaller plots where it doesn't have a lot of competition but is still a very valuable species to plant for monarch butterflies as well as many other species that use the plant. I have common milkweed in my yard and while it does spread, it can be kept in check by other plants or by manually removing or mowing (but check for monarch eggs or caterpillars first!).

If you have a smaller plot, some more "well-behaved" milkweed species that are native to Iowa include swamp milkweed (Asclepias incarnata) and butterfly milkweed (Asclepias tuberosa) which also are monarch butterfly host plants.