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Swamp Milkweed (Asclepias incarnata) in 11 Easy Sections

Introduction

Swamp Milkweed (Asclepias incarnata) is a herbaceous perennial that grows on the shores of streams, lakes, ponds, ditches and places where there is a high soil moisture. This plant is a host for the Monarch Butterfly (Danaus plexippus). The plant produces red to pink flowers in the late spring and summer. This plant can be purchased in the McMullen House Bed & Breakfast Garden Shop.

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Taxonomy and Naming of Swamp Milkweed (Asclepias incarnata)

Herbarium specimen of Swamp Milkweed (Asclepias incarnata)
Swamp Milkweed — Asclepias incarnata L.
collected in United States of America by The New York Botanical Garden (licensed under CC BY 4.0)
Herbarium specimen of Eastern Swamp Milkweed (Asclepias incarnata ssp. pulchra)
Eastern Swamp Milkweed — Asclepias incarnata subsp. pulchra (Ehrh. ex Willd.) Woodson
collected in United States of America by The New York Botanical Garden (licensed under CC BY 4.0)

Taxonomy

Swamp Milkweed (Asclepias incarnata) was originally described by Carl Von Linnaeus in Species Plantarum (1753) as its current name based on a specimen from the Virginia colony. This plant is a member of the Dogbane Family (Apocynaceae).

Synonyms of Asclepias incarnata

  • Asclepias incarnata var. longifolia A. Gray
  • Asclepias pulchra Ehrh. ex Willd.
  • Asclepias incarnata var. pulchra (Ehrh. ex Willd.) Pers.
  • Asclepias incarnata var. neoscotica Fern.

Subspecies

Two subspecies are recognized, the type species and ssp. pulchra. Numerous cultivars are present in the horticultural trade, which are discussed in the garden section below. Subspecies pulchra, has a more eastern US distribution than incarnata, which is more widespread and extends into the midwest. Subspecies pulchra is more pubescent on the stems and leaves and has leaves with a rounded or subcordate base. Subspecies incarnata is much less pubescent, if not wholly glabrate and the leaves are obtuse to truncate (Weakley 2022).

Meaning of the Scientific and Common Names

Scientific Name

The genus name is in honor of the Greek god of medicine, Asklepios and is refers to the historical use of milkweeds in medicine. The species name, incarnata, means flesh-colored. (Missouri Botanical Garden).

Common Name and Alternative Names

The common name, swamp milkweed, refers to this plant often being found in wetlands such as swamps, bogs, marshes, and wet meadows. Other names such as marsh milkweed and swamp butterfly weed also refer to the habitat. Some names for the plant including rose milkweed and rose milkflower refer to the color of the flowers, while others refer to the silky nature of the seeds such as swamp silkweed. One alternative name, white indian hemp, comes from the resemblance to indian hemp. The name of Pleurisy root refers to the medical use of this plant. One common name, Eastern swamp milkweed, is used for spp. pulchra and refers to its eastern distribution as compared to the typic subspecies.

Physical Description of Swamp Milkweed (Asclepias incarnata)

Description

  • Plant Type: Herbaceous perennial
  • Height: 3 to 5 feet tall
  • Leaves: opposite, simple, lanceolate to oblong, entire and glabrate – sparsely pubescent (ssp. incarnata) to pubescent (ssp. pulchra) Leaves range from 3-6 inches in length and are generally less than 1 inch in width.
  • Flower Color: white, rose, red, pink to purple
  • Blooming period: July to September
  • Fruiting type and period: follicle that forms from August to October that splits with seeds covered in a cottony covering.

Range of Swamp Milkweed (Asclepias incarnata) in the United States and Canada

Range of Swamp Milkweed (Asclepias incarnata ssp. incarnata) in the United States and Canada

Swamp Milkweed (ssp. incarnata) is more widely distributed than ssp. pulchra and extends almost to the west coast. It is considered to be rare in the states of Montana, Nevada, Arizona, Arkansas, Louisiana, and Virginia, as well as the province of Nova Scotia. Like the other subspecies it is noxious in Ontario (BONAP).

Range of Eastern Swamp Milkweed (Asclepias incarnata ssp. pulchra) in the United States and Canada

Eastern Swamp Milkweed (ssp. pulchra) has a more eastern distribution than that of the typic subspecies. It is considered to be rare in the province of Nova Scotia and is noxious in Ontario (BONAP).

Habitat of Swamp Milkweed (Asclepias incarnata)

Swampland
Swamp Habitat — Ina Hensel (telemakro.de), CC BY-SA 4.0, via Wikimedia Commons

Swamp Milkweed is found in places with moist soil including the shores of streams, lakes, ponds, ditches, wet fields, and meadows.

Hosted Insects

Monarch Butterfly on a goldenrod
Monarch Butterfly — Nicholas A. Tonelli from Northeast Pennsylvania, USA, CC0, via Wikimedia Commons

Swamp milkweed is primarily a host to the Monarch Butterfly (Danaus plexippus), but it also hosts the Queen Butterfly (Danaus gilippus), Isabella Tiger Moth (Pyrrharctia isabella) and the Cecropia Silkmoth (Hyalophora cecropia) (National Wildlife Federation).

Research has found that monarchs will tend to seek out those milkweeds that have a high amount of cardenolide (cardiac glycoside) concentrations (Malcolm and Brower 1986). Swamp milkweed, relative to other milkweeds has a low amount of cardenolides. However, the larvae will still feed upon it if no other milkweeds are around. The study above compared Asclepias incarnata with A. curassavica, a milkweed that has a high amount of cardenolides.

Other Supported Wildlife

Bumblebee on a pink flower
Bumble Bee on a flower — Weerlicht, CC0, via Wikimedia Commons

This plant is used as a nectar source by other butterflies, bees, and insects. Some of these include eastern tiger swallowtail butterfly and the greater fritillary (Illinois Wildflowers). This milkweed, like some others, is a host to milkweed beetles. Milkweed beetles eat the seeds of the milkweeds, so you want to keep them off if you are planning to use the seed later.

Evidence in the Literature

A study of prairies in Illinois listed insects that were visiting the flowers of swamp milkweed (Adams 1915). Some of these insects included Giant Swallowtail (Papilio cresphontes), Yellow Sulphur (Eurymus philodice), Regal Fritillary (Argynnis idalia), Monarch butterfly (Danaus plexippus), and honeysuckle sphinx (Hemaris diffinis). Honey bees and common rusty-digger-wasps were also visitors.

Rare Skipper (Problema bulenta) has been observed nectaring on swamp milkweed in the Virginia (Chazal and Hobson 2002) and has been shown to be a preferred nectar species (Cromartie and Schweitzer 1993).

Frequently Asked Questions about Swamp Milkweed (Asclepias incarnata)

Is Swamp milkweed invasive?

Swamp milkweed is not generally invasive as compared to other species. The swamp milkweed planted in the McMullen House Bed & Breakfast garden as spread some, but is not taking up additional territory from other plant associates.

Is Swamp milkweed toxic?

Being a member of the milkweed genus, swamp milkweed is toxic not only to humans, but to household pets and livestock. The entire plant contains a milky sap that has cardiac glycosides that produce vomiting and spasms in affected organisms. These poisons make the plant deer resistant and are absorbed by monarch butterflies to make them distasteful. The North Carolina Extension Service considers the poison to be “low severity” (North Carolina Extension Gardener).

Are Swamp milkweeds drought tolerant?

Swamp milkweed prefers soils that are moist or at least average in moisture content and are well-drained. However, they can tolerate brief periods of drought. This year (2022) at McMullen House Bed & Breakfast we have had a drought and our milkweeds are doing fine and flowering without external watering.

Gardening with Swamp Milkweed (Asclepias incarnata)

White Flowers of 'Ice Ballet' cultivar of Swamp Milkweed
White flowers of ‘Ice Ballet’ cultivar of Swamp Milkweed (Asclepias incarnata) — Photo by and (c)2009 Derek Ramsey (Ram-Man), GFDL 1.2, via Wikimedia Commons
Pink Flowers of 'cinderella' cultivar of Swamp Milkweed
Pink Flowers of Swamp Milkweed ‘Cinderella’ — Alvin Kho via CC BY SA 2.0.

Hardiness

Swamp milkweed is hardy in zones 3a to 9b. If your gardening zone is within these areas, you can grow it even if you are not within the native range and have the right soil moisture conditions. Just be aware that the host species may or may not be present if it is outside the native range.

Cultivars

  • ‘Soulmate’ : has pink flowers with white centers and a vanilla scent
  • ‘Cinderella’ : rose-pink flowers with a vanilla scent
  • ‘Ice Ballet’ : White flowered cultivar

Optimal Conditions

Swamp milkweed enjoys places with full sun or partial shade. It is not very shade tolerant. Soil conditions should be moist or at least mesic and it can handle most soil types, even clay. Possible locations include meadows, pond shores, fields, and borders. This plant does not propagate by seed easily and is slow to spread. The poisonous characteristics makes it resistant to deer browsing.

Plant Associates

Associated plants include joe-pye weed (Eupatorium spp.), New York Ironweed (Vernonia novaboracensis), goldenrod (Solidago spp.), and other milkweeds (Asclepias spp.).

Things to Know

Swamp Milkweed can be become infested with yellow aphids, so be sure to control the aphids if you have them.

Interesting Facts and Uses of Swamp Milkweed

Medicinal Uses

Swamp Milkweed has been used by Native Americans for various ailments including skin diseases, kidney problems, digestive issues, toothaches, and urinary problems. It has also been used in soups. In all there are 21 uses documented by the North American Ethnobotany Database.

Awards

Swamp Milkweed was the 2005 NC Wildflower of the Year as selected by the North Carolina Botanical Garden (North Carolina Extension Gardener).

References

  • Adams, Charles C. 1915. An ecological study of prairie and forest invertebrates. (Urbana, Ill. Illinois State Laboratory of Natural History)
  • Chazal, Anne C. and Christopher S. Hobson. 2002. Status of the Rare Skipper (Problema bulenta) in Virginia. Banisteria 19:20-22.
  • Cromartie, William J. and Dale F. Schweitzer. 1993. Biology of the Rare Skipper, Problema Bulenta (Hesperiidae), in Southern New Jersey. Journal of the Lepidopterists Society 47(2): 125-133.
  • Linnaeus, Carolus (1753): Species plantarum: exhibentes plantas rite cognitas, ad genera relatas, cum differentiis specificis, nominibus trivialibus, synonymis selectis, locis natalibus, secundum systema sexuale digestas. Stockholm: Laurentius Salvius, DOI: 10.5281/zenodo.3931989, URL: https://doi.org/10.5962/bhl.title.669
  • Malcolm, Stephen B. and Lincoln P. Brower. 1986. Selective Oviposition by Monarch Butterflies (Danaus plexippus L.) in a mixed stand of Asclepias curassavica L. and A. incarnata L. in South Florida. Journal of the Lepidopterists Society 40(4), 255-263.
  • Weakley, A.S., and Southeastern Flora Team 2022. Flora of the Southeastern United States. University of North Carolina Herbarium, North Carolina Botanical Garden.

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