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Montana Field Guide

Montana Field Guides

Swamp Milkweed - Asclepias incarnata

Species of Concern
Native Species

Global Rank: G5
State Rank: S1?
(see State Rank Reason below)
State Threat Score: No Known Threats
CCVI: Moderately Vulnerable
C-value: 6

Agency Status


External Links

State Rank Reason (see State Rank above)
Known in Montana from Carbon County. One of the known sites is likely extirpated. Additional information is needed on the species' distribution, abundance, potential trends and threats within Montana.
General Description
Fibrous-rooted. Stems simple to branched, 70–100 cm. Herbage glabrous to puberulent on the stem. Leaves opposite, spreading; blades lanceolate, 4–15 cm long. Umbels of 10 to 40 flowers; peduncles 1–7 cm long. Flowers 9–11 mm high; sepals villous, 1–2 mm long; petals purplish, glabrous, 5–6 mm long; gynostegium pink, glabrous, 1–2 mm high; hoods oblong, 2–3 mm long; horns exserted, arching. Follicles erect, fusiform, 5–8 cm long, smooth, mostly glabrous (Lesica et al. 2012. Manual of Montana Vascular Plants. BRIT Press. Fort Worth, TX).

Our plants are variety incarnata.

Flowering in July.

Diagnostic Characteristics
The combination of acute leaves >4 mm wide and slender horns longer than the hoods distinguish Asclepias incarnata from other Montana milkweeds. Asclepias speciosa also has pinkish corollas and occupies wetland habitat, but has large hoods that surpass the horns.

Species Range
Montana Range Range Descriptions


Range Comments
SK to NS south to NM and FL. Known from Carbon County (Lesica et al. 2012. Manual of Montana Vascular Plants. BRIT Press. Fort Worth, TX). In Montana, native populations appear to be limited to the Clarks Fork of the Yellowstone drainage. The species is planted in other areas and may spread further from these plantings. As such, additional observations are expected outside of its suspected native range.

Observations in Montana Natural Heritage Program Database
Number of Observations: 12

(Click on the following maps and charts to see full sized version) Map Help and Descriptions
Relative Density



(Observations spanning multiple months or years are excluded from time charts)

Wet meadows and thickets.
Predicted Suitable Habitat Model

This species has a Predicted Suitable Habitat Model available.

To learn how these Models were created see

Ecological Systems Associated with this Species

The following animal species have been reported as pollinators of this plant species or its genus where their geographic ranges overlap: Bombus vagans, Bombus huntii, Bombus rufocinctus, Bombus ternarius, Bombus terricola, Bombus pensylvanicus, Bombus bimaculatus, Bombus griseocollis, Bombus impatiens, Bombus insularis, and Bombus flavidus (Plath 1934, Heinrich 1976, Thorp et al. 1983, Colla and Dumesh 2010, Colla et al. 2011, Koch et al. 2012, Williams et al. 2014, Tripoldi and Szalanski 2015).

Threats or Limiting Factors
Threat impact not assigned because threats are not known (MTNHP Threat Assessment 2021).

  • Literature Cited AboveLegend:   View Online Publication
    • Colla, S., L. Richardson, and P. Williams. 2011. Bumble bees of the eastern United States. Washington, DC: USDA Forest Service, Pollinator Partnership. 103 p.
    • Colla, S.R. and S. Dumesh. 2010. The bumble bees of southern Ontario: notes on natural history and distribution. Journal of the Entomological Society of Ontario 141:39-68.
    • Koch, J., J. Strange, and P. Williams. 2012. Bumble bees of the western United States. Washington, DC: USDA Forest Service, Pollinator Partnership. 143 p.
    • Lesica, P., M.T. Lavin, and P.F. Stickney. 2012. Manual of Montana Vascular Plants. Fort Worth, TX: BRIT Press. viii + 771 p.
    • Plath, O.E. 1934. Bumblebees and their ways. New York, NY: Macmillan Company. 201 p.
    • Thorp, R.W., D.S. Horning, and L.L. Dunning. 1983. Bumble bees and cuckoo bumble bees of California (Hymenoptera: Apidae). Bulletin of the California Insect Survey 23:1-79.
    • Tripoldi, A.D. and A.L. Szalanski. 2015. The bumble bees (Hymenoptera: Apidae: Bombus) of Arkansas, fifty years later. Journal of Melittology 50: doi:
    • Williams, P., R. Thorp, L. Richardson, and S. Colla. 2014. Bumble Bees of North America. Princeton, NJ: Princeton University Press. 208 p.
  • Additional ReferencesLegend:   View Online Publication
    Do you know of a citation we're missing?
    • Heidel, B. 1994. Potential impact of proposed noxious weed treatment at Bluewater Fish Hatchery (MDFWP) on plant species of special concern. Unpublished report to Gary Shaver, Bluewater Fish Hatchery. Montana Natural Heritage Program, Helena, Montana. 5 pp.
    • Lesica, P., M.T. Lavin, and P.F. Stickney. 2022. Manual of Montana Vascular Plants, Second Edition. Fort Worth, TX: BRIT Press. viii + 779 p.
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Citation for data on this website:
Swamp Milkweed — Asclepias incarnata.  Montana Field Guide.  .  Retrieved on , from