What I’ve found with Swamp Milkweed, Asclepias incarnata, in St. Louis MO is that it only lives for about three years, sometimes only two years in a typical home garden setting. While it prefers a wet-rain garden setting, all of my beds are raised for drainage and not ideal for this species.
One swamp milkweed that did spectacular last year, this year it now looks poorly and possibly dying. From my past experience I know that this plant will probably not make it next year. Here are a couple of pictures of the milkweed.
What I’ve had success with in the past is cutting in back to six inches, digging up the root ball and separating it into individual plants. I will then keep them in pots for a month or two and make sure they are always moist. I then will plant it back in the garden.