The blooms of swamp milkweed come in shades of soft mauve to pink to reddish-violet or occasionally white. Five nectar cups form a crown on five small petals, ideal for intricate pollination. By autumn, narrow pods reveal flat brown seeds attached to the white tufts characteristic of all milkweeds. Like most milkweed species, swamp milkweed is clump-forming. Sturdy, upright clumps grow on stems four to five feet tall and two to three feet wide. Leaves grow in pairs, stiff, taper-pointed, and lance-shaped. Sometimes the edges of a leaf turn inward and upward. Milkweed plants can be planted in spring or fall Milkweed is toxic to humans, and toxic to pets. Milkweed is slow to emerge from its dormancy so don’t worry if it lags behind other perennials. As the soil warms up, it will send up new shoots. The same applies to bare-root milkweed, which is how some nurseries ship it in the spring. Plant it promptly upon arrival when the soil can be worked and keep it moist but not overly wet. If you plant swam milkweed in the fall, give it enough time to get established before the cold weather sets in.
Light
Full sun is best, but plants will tolerate part shade.
Soil
Swamp milkweed thrives in moist, medium to wet clay soil. The plant can do well in average garden soil if it does not dry out. Wet meadows or rain gardens offer ideal conditions.
Water
Keep this plant moist. If given a proper wetland environment, the swamp milkweed will likely not need watering throughout the growing season.
Varieties of Swamp Milkweed
While the straight species (Asclepias incarnata) is the best to plant if you want to attract monarch butterflies, you can find milkweed cultivars that have been bred of landscape use. Popular ones include:
‘Ice Ballet’ has vanilla scented, pillowy clusters of snow-white flowers.‘Cinderella’ with vanilla-scented flowers that are deep pink with white centers.
Pruning
Swamp milkweed does not need any pruning during the growing season. After the plant has died back in the winter, you can remove the dead stalk, just make sure to mark the location, as swamp milkweed is slow to emerge in the spring.
Propagating Swamp Milkweed
Swamp milkweed spreads through rhizomes. Dividing established plants in late spring can be done but the long taproot does not like to be transplanted. For a natural spread, allow the wind-borne seeds to find their way, or start it from seeds.
Growing Swamp Milkweed From Seeds
To propagate this plant by seeds, collect them in the fall after the brown pods have dried and started to split. Open each pod fully and let the seeds dry for one or two weeks in paper bags. Discard any seeds that show signs of damage by weevil larvae.
Potting and Repotting
You don’t need a back yard to grow swamp milkweed, you can also grow it in containers; in fact, because milkweed tends to spread, some gardeners even prefer to grow it in pots. The container should be at least 14 inches in diameter and have good drainage holes. Terra cotta is ideal because it is heavier than plastic so the tall milkweed is less likely to topple over. Fill the container with well-draining potting mix and keep it evenly moist at all times. Repot the plant when to roots fill the container and start to grow out of the drainage holes.
Overwintering
Swamp milkweed that has been planted in the ground is winter hardy to USDA zone 3 and does not need any protection. When grown in containers, on the other hand, the roots are vulnerable to cold damage in freezing temperatures. Place the container in an insulating silo, or cover it with burlap and bubble wrap.
Common Pests/Diseases
The main pest that bothers swamp milkweed is the colorful orange and black milkweed leaf beetle that feeds on the juices inside the leaves. A few beetles won’t harm the plant but you want to stop them before the infestation gets heavy. Usually the beetles can be simply washed off with a garden hose.
How to Get Milkweed to Bloom
If your swamp milkweed does not bloom, it could be that the plant is too young. Milkweed may not flower in the first year because it is putting its energy into developing roots. Excess nitrogen in the soil, such as runoff from a fertilized lawn, can also lead to failure to bloom. And finally, plants that are drought- or heat-stressed will also not bloom.
