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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)

Agency Status
MNPS Threat Rank:
C-value: 6

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 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 Comments
SK to NS south to NM and FL. Known from Carbon and Wibaux counties (Lesica 2012. Manual of Montana Vascular Plants. BRIT Press. Fort Worth, TX).

Observations in Montana Natural Heritage Program Database
Number of Observations: 11

(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.

Ecological Systems Associated with this Species

  • Additional ReferencesLegend:   View Online Publication
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    • 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 Unpublished report to Gary Shaver, Bluewater Fish Hatchery.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.
    • Heidel, B. 1994. Sensitive species survey of Bluewater Fish Hatchery of 10 June; memo and documentation provided to Gary Shaver, MDFWP.
    • Lesica, P., M.T. Lavin, and P.F. Stickney. 2012. Manual of Montana Vascular Plants. Fort Worth, TX: BRIT Press. viii + 771 p.
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Citation for data on this website:
Swamp Milkweed — Asclepias incarnata.  Montana Field Guide.  .  Retrieved on , from