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 3.
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. Continue reading below for some more specific advice on how to grow Asclepias from seed.
Season & Zone
Exposure: Full sun
Zone: Hardy to Zone 3
Sow indoors 6-8 weeks before the last frost date and transplant or direct sow towards late spring. 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 vigour in the second year and begins blooming by June here on the west coast.
Barely cover the small seeds using sterilized seed starting mix. Space transplants 30-60cm (12-24″) apart.
For many years several Asclepias species were listed as invasive weeds because of their sometimes aggressive spreading by underground rhizomes and their giant, dandelion-like seeds. The seeds emerge from very conspicuous pods which are easily removed before they dry and crack. As the population of wild Asclepias diminished, so did the populations of many butterflies that depend on them as food and nursery plants (including the endangered Monarch). Now that they have been de-listed, we encourage home gardeners to grow them with the advance knowledge that they can spread. Try growing them in a large container like a half barrel, and be conscious of the seed pods as they develop. In short, please be responsible with Asclepias species.
Note: All parts of the plant are harmful if eaten. Flowers are not edible.
Download our Asclepias tuberosa Factsheet.