Asclepias incarnata
swamp milkweed
Alternate names: rose milkweed, silkweed, water nerve root, white Indian hemp, swamp silkweed
Family: dogbane family (Apocynaceae), formerly assigned to the milkweed family (Asclepiadaceae)
Functional group: forbs (wildflowers)
Description
- Life cycle and growth form
Perennial, weakly rhizomatous, forming small clones of several stems, root systems are more fibrous than other commonly cultivated milkweeds such as butterfly milkweed (A. tuberosa)
Height: 2-5 ft
- Leaves and stem
Leaves 3-6 in long, usually narrowly lanceolate with smooth, untoothed edges and sessile on the stem or short-stalked, opposite arrangement, stems mostly hairless, usually unbranched (occasionally branched above)
- Flower, fruit/seedhead
Flower: numerous, various shades of rose-pink with a pronounced fragrance similar to bubblegum, in domed clusters 2-3 in across; individual flowers five-parted, radially symmetrical, with a crown of five tubular hoods surrounding a central c and column and petals and sepals curved downward (typical milkweed flowers)
Fruit/seedhead: the pods (follicles) are 2-4 in long and teardrop-shaped; follicles open along one side at maturity, revealing many shiny brown seeds each of which bears a flattened wing and a plume of soft, white floss
- Seed
Seed characteristics
Seed weight: 4,800 (Prairie Moon Nursery)
1000 seed weight: 3.65 g (SER Seed Information Database)Description: dark brown, oval, surrounded by flattened ‘wing’ and tuft of soft hairs
Typical seed test: 80-95% Pure Live Seed
Purity: 98-99%
Germination: 78-89%
Dormancy: 5-10%
- Habitat and range
Adaptation/habitat: thrives in wet to wet-mesic, neutral to slightly acidic soils, in full sun to part shade; wild populations are found in open floodplains, lakeshores, ditches, and wet prairies; obligate wetland status (OBL) in Midwest, facultative (FACW) in Great Plains; benefits from irrigation in production systems
Threatened or endangered status: none listed on USDA Plants Database
Kartesz, J.T., The Biota of North America Program (BONAP). 2023. North American Plant Atlas. (http://bonap.net/napa). Chapel Hill, N.C. [maps generated from Kartesz, J.T. 2023. Floristic Synthesis of North America, Version 1.0. Biota of North America Program (BONAP). (in press)]
General comments
Swamp milkweed is a valuable addition to pollinator habitat plantings on wetter soils and a showy, mannerly milkweed for home landscaping. This species is an important host for caterpillars of the monarch butterfly which feed upon its leaves. The fragrant flowers attract and provide nectar for numerous pollinators including various species of butterflies, bees, wasps, and flies. The strong, silky stem fibers are used as nesting material by songbirds and have traditionally been used for spinning and weaving by Native peoples, giving rise to some of the alternate common names such as silkweed and white Indian hemp. Establishment from plugs is rapid, and seed can be harvested the first fall, but production stands are short-lived (2-3 years).
Recommendations for seed production
- Establishment for seed production
Direct seeding
We do not have experience direct-seeding this species for establishment of seed production plots.
Greenhouse
Seed pre-treatment: to break dormancy, use 30-60 days cold stratification (check frequently for early germination) or a 24-hour soak in 250 ppm gibberellic acid (GA-3)
Sowing: seed directly into plugs (2-3 seeds per cell) or on germination flat and lightly cover with potting mix, or germinate between sheets of moistened paper toweling and move to individual plugs as radicles (seed roots) emerge from seeds; start seeds 8-10 weeks before the average frost-free date
Transplanting: move plug flats outdoors 1-2 weeks before transplanting to allow seedlings to adjust to ambient conditions; plant with 1-foot spacing in plastic mulch with drip irrigation
- Stand management
Irrigate: plant in naturally wet soils and/or supply irrigation, especially in dry weather.
Weeds: field preparation through repeated tillage and application of plastic mulch reduces weed issues; hand harvesting of pods as they mature results in very pure seed.
Pests: plants are susceptible to non-native oleander aphids (Aphis nerii) which reproduce rapidly asexually to form dense infestations covering stems and leaves, reducing plant vigor and causing abortion of flowers or pods or, in extreme cases, death of the plant. Scout frequently for these golden yellow aphids when plants are in bud, as it is easier to control small aphid infestations. Keep an eye out for aphid predators (e.g., ladybug larvae, hoverfly larvae, and lacewing larvae) and parasitoid aphid mummy wasps which help reduce damage. Use the least toxic treatment possible (e.g., horticultural oils or insecticidal soaps) to avoid harming natural aphid enemies and the diverse pollinators that swamp milkweed plants attract, as aphids often occur at the time plants are in flower. Native seed-feeding milkweed bug species (Oncopeltus fasciatus and Lygaeus kalmii) pierce pods and damage the developing seeds. Scout for, and manually remove, clusters of brilliant red-orange milkweed bug nymphs on developing green pods. A small proportion of pods is ruined by larvae of native milkweed stem weevils (Rhyssomatus lineaticollis or R. annectans). These appear as grubs or pupae inside the cluster of seeds within a pod. Affected pods have a small entry hole on the side and often mature prematurely. Earlier in the growing season, red milkweed longhorn beetles (Tetraopes spp.) feed on the tips of milkweed leaves. Their larvae feed within milkweed roots and rhizomes and may weaken plants over time. Monarch butterflies (Danaus plexippus) are attracted to lay eggs on swamp milkweed. Caterpillars feed on the foliage (and sometimes flowers and fruits). In smaller-scale production systems, larvae can be hand-picked from production rows and transferred to nearby wild stands before applying treatments for problem insects that could also harm caterpillars
Diseases: milkweeds are susceptible to a variety of fungal pathogens along with bacterial and viral diseases. Cultural methods such as reducing stand size or density, intercropping, and crop rotations may reduce disease spread and severity. When using fungicides to control milkweed diseases, always follow label instructions and pay attention to human health effects, possible phytotoxicity, and effects on beneficial fungi.
Note: refer to Milkweeds: A Conservation Practitioner’s Guide, published by the Xerces Society, for more detailed information on milkweed insects and diseases and their control.
- Seed production
First harvest: fall of planting year, when started from transplanted plugs
Yield per acre: 200-300 pounds/acre (extrapolated from 4’ wide irrigated production rows 115 to 320 feet long) with largest harvest in the first year
Stand life: 2 years from transplanting; plots at TPC decline rapidly in the third year and must be replaced, though plants are reported to be longer lived in natural populations
Flowering date: throughout July in northern Iowa
Seed maturity: individual pods within each cluster ripen from early September to mid-October in northern Iowa
Harvest date range at TPC (2009-2020): Aug 31 - Oct 17
Recommended harvest method: harvest by hand as pods (follicles) mature; collect pods that are changing color from green to yellowish and split when subjected to gentle pressure on the suture (seam), revealing dark brown seeds
- Seed cleaning and storage
Cleaning process: dry pods in cloth bags for at least two weeks with fan-forced air; pass material through debearder or stationary combine to shatter pods, release seeds, and detach fluff from seeds; if debearder used, follow up by fan winnowing (preferably outside on a relatively calm day) to remove most of fluff; air-screen remaining material; optional final cleaning with indent cylinder removes broken bits of pods and stems from seed
Seed storage: dry, cool (orthodox)
Released Germplasm
Source identified material: Natural Selections/Iowa Ecotype Zones 1, 2, IA
Cultivated varieties: ‘Ice Ballet,’ ‘Cinderella,’ and ‘Milkmaid’
References
Borders, B. and E. Lee-Mäder. 2014. Milkweeds: A Conservation Practitioner’s Guide. 143 pp. Portland, OR. The Xerces Society for Invertebrate Conservation. https://www.xerces.org/sites/default/files/2018-05/17-031_02_XercesSoc_Milkweeds-Conservation-Guide_web.pdf
Chayka, K. (2010). Asclepias incarnata (swamp milkweed). Minnesota Wildflowers. https://www.minnesotawildflowers.info/flower/swamp-milkweed
Cochrane, T. S., Elliot, K., & Lipke, C. S. (2006). Swamp Milkweed. In Prairie Plants of the University of Wisconsin, Madison Arboretum (Third, p. 55). University of Wisconsin Press.
Hilty, J. (2020). Swamp Milkweed (Asclepias incarnata). Illinois Wildflowers. https://www.illinoiswildflowers.info/wetland/plants/sw_milkweed.htm
Kirk, S. and Belt, S. (2011). Plant fact sheet for swamp milkweed (Asclepias incarnata). USDA-Natural Resources Conservation Service, Norman A. Berg National Plant Materials Center. Beltsville, MD 20705. https://plants.usda.gov/DocumentLibrary/factsheet/pdf/fs_asin.pdf
Runkel, S. T., & Roosa, D. M. (2009). Swamp milkweed. In Wildflowers of the Tallgrass Prairie: The Upper Midwest (Second, p. 179). University of Iowa Press.
Schultz, Jan; Beyer, Patty; Williams, Julie. (2001). Propagation protocol for production of Container (plug) Asclepias incarnata L. plants USDA FS - Hiawatha National Forest Marquette, Michigan. In: Native Plant Network. US Department of Agriculture, Forest Service, National Center for Reforestation, Nurseries, and Genetic Resources. https://NativePlantNetwork.org (accessed 2024/01/09).
Society for Ecological Restoration and Royal Botanic Gardens Kew. (2024). Asclepias incarnata L. SWAMP MILKWEED. Seed Information Database. https://ser-sid.org/species/62fbef4e-ed33-4c71-a18f-f78f9018fda7
University of Wisconsin. (2024). Common milkweed insects. Wisconsin Horticulture, Division of Extension. https://hort.extension.wisc.edu/articles/common-milkweed-insects/
Species Guide Updated 2/13/2024
