There isn't any damage in leaving the plant stalks set up, though this encourages rampant self-seeding, which is generally not sought after. You should not mulch more than the foundation crowns, as this can advertise rot. Fewflower Milkweed grows into a surprising three to 5 feet tall and capabilities spectacular https://bookmarketmaven.com/story18871742/a-simple-key-for-is-milkweed-plant-a-perennial-unveiled