Asclepias incarnata. Swamp Milkweed begins to grow from a thick, fleshy root later in spring than many other plants. Narrow, strap-like leaves emerge, forming a clump of foliage. Then, from mid-spring to late summer, umbels of fragrant, pink to mauve flowers appear that are so generous with nectar that they attract pollinators of all kinds. This plant is particularly attractive to butterflies, including the endangered Monarch, which prefers this species on which to lay her eggs. Monarch caterpillars feed on the leaves of this milkweed despite the milky latex they contain, which is mildly toxic to most other animals. The roots of this plant have evolved to thrive in low oxygen environments, including the wet soil found around lakes and streams. The plants can reach 2m (6') in height, and will grow in full sun to partial shade. It is hardy to Zone 4.
Follow along with this handy How to Grow Asclepias guide and enjoy butterflies, hummingbirds, and a host of other pollinators in your garden. When transplanted seedlings are 10-15cm (4-6″) tall, pinch back the growing tip to encourage multiple flowering points. Asclepias tuberosa prefers dry, sandy conditions or any average garden soil in full sun. Plants grown from seed bloom in the first year if given an early start. They can be pulled up in fall and treated like annuals to prevent spreading. Asclepias does not divide well, but it’s an outstanding choice for xeriscaping. Asclepias speciosa and some of the others do better in swampy soil, but they are not fussy plants.
Season & Zone
Exposure: Full sun
Sow indoors late February to the end of March and transplant or direct sow towards late April. Optimal soil temperature for germination: 10-25°C (50-75°F). Seeds should sprout in 7-35 days. Asclepias may benefit from stratification: Seeded trays are wrapped in plastic and refrigerated for two to three weeks before being placed over bottom heat. This may result in more even and speedier germination.
We have had success direct sowing A. tuberosa in March, with blooms the first year. In our experience, A. speciosa returns with vigor in the second year and begins blooming by June.
Barely cover the small seeds using sterilized seed starting mix. Space transplants 30-60cm (12-24″) apart.
Note: All parts of the plant are harmful if eaten. Flowers are not edible.
Download our Asclepias tuberosa Factsheet.