swamp milkweed (Asclepias incarnata)
Asclepias incarnata L. var. neoscotica Fernald
Asclepias incarnata L. var. pulchra (Ehrh. ex Willd.) Pers.
Asclepias pulchra Ehrh. ex Willd.
CONFIRMATION STATUS: Pending confirmation.
TAXONOMY: The currently accepted scientific name for swamp milk-
weed is Asclepias incarnata L. ssp. pulchra (Ehrh. ex Willd.) Woodson.
There are two subspecies of Asclepias incarnata: 1) subspecies incarnata
and 2) pulchra. Both subspecies occur in Facquier County; the specimens
observed in Sky Meadows State Park are believed to be subspecies pulchra.
Swamp milkweed is the common name for both subspecies.
NATIVE STATUS: Native, United States.
GENERAL BOTANICAL CHARACTERISTICS: The following botan-
ical characteristics primarily pertain to the typical subspecies incarnata;
information is derived mainly from the Fire Effects Information System
(FEIS) and John Hilty's Illinois Wildflower website,
Additional information pertaining to subspecies pulchra is also included.
Habit: This perennial wildflower is highly variable in size (2-6' tall), de-
pending on environmental conditions. The central stem branches occasion-
ally, forming ascending lateral stems; these stems are light green, terete,
and glabrous (for subspecies incarnata), but densely pubescent for subspe-
Leaves: The opposite leaves are up to 6" long and 1½" across, although
they are more typically about 3" long and ½" across. They are narrowly lanceolate or oblong-lanceolate in shape, smooth (entire) along their mar-
gins, and glabrous (for subspecies incarnata), but pubescent on both sides
for subspecies pulchara. Upper leaf surfaces are medium to dark green, although they can become yellowish green or pale green in response to
bright sunlight and hot dry conditions. The leaves are either sessile or their bases clasp the stems.
Flowers: Upper stems terminate in pink umbels of flowers spanning about
2-3½" across. Each flower is about ¼" across, consisting of 5 upright
whitish hoods and 5 surrounding pink petals that droop downward in the
manner of most milkweeds. The flowers exude a pleasant fragrance that
Fruit/Seeds: Afterwards, successfully cross-pollinated flowers are re-
placed by seedpods. The seedpods (follicles) are 3-4" long and narrowly
lanceoloid-ellipsoid in shape. Immature seedpods are light green, smooth,
and glabrous, turning brown at maturity. Each seedpod splits open along
one side to release its seeds. These seeds have large tufts of white hair and
they are distributed by the wind during the fall.
Roots: The root system is rhizomatous, from which clonal colonies of
plants occasionally develop.
REGENERATION PROCESS: Swamp milkweed propogates itself by
reseeding; it very rarely reproduces asexually by rhizomes.
HABITAT TYPES: Swamp milkweed is a semiaquatic plant. It occurs in
a range of wet conditions from standing water to saturated soil. A riparian
species, it is found on streambanks, drainage ditches, pond shores, banks,
and floodplains of lakes and floodplain forests, waterways, seeps and fens,
marshes, swamps, thickets and wet areas of prairies, particularly moist
black soil prairies. Additionally, it occurs in wet meadows and in low wet
SITE CHARACTERISTICS: Swamp milkweed prefers full to partial
sun, wet to moist conditions, and soil containing mucky clay, rich loam,
or silt with rotting organic material. Occasional flooding is tolerated if it
is temporary. Tolerance to hot dry conditions is poor. The leaves have a tendency to become more broad in shape in response to shady conditions.
SEASONAL DEVELOPMENT: Across its range, swamp milkweed
begins to flower during the last week of June or the first week in July and
continues until August or September. Individual flowers remain open for
about 1 week. Fruits mature from August through October. After matura-
tion, follicles split open on one side to release seeds during October and
GENERAL DISTRIBUTION: Swamp milkweed (subspecies pulchra)
naturally occurs along the Atlantic seaboard states, from Florida to Maine.
It extends west into Tennessee and Kentucky (with some occurring in
Texas and Michigan). It does not naturally occur in the Ohio Valley
region, mid-West, southwestern, Rocky Mountain, far west and north-
western states. It does not naturally occur in Canada.
SKY MEADOWS DISTRIBUTION: To be determined.
IMPORTANCE AND USES: While the flowers are very attractive to
many kinds of insects, swamp milkweed is a virtual cornucopia for many
insects (as are all members of Asclepias spp.). Insects attracted to swamp
milkweeds flowers include bumblebees, honeybees, digger bees (Melis-
sodes spp., Svastra spp.), Halictid bees, Sphecid wasps, Vespid wasps,
Tiphiid wasps, Spider wasps, Mydas flies, Thick-Headed flies, Tachinid
flies, Swallow-tail butterflies, Greater Fritillaries, Monarch butterflies, and Skippers. Another occasional visitor of the flowers is the ruby-throated hummingbird. All of these visitors seek nectar. Some insects feed destruc-
tively on the leaves, flowers, stems, seeds and seedpods, and roots of
swamp milkweed and other Asclepias spp. (milkweeds). These insect feed-
ers include caterpillars of the butterfly Danaus plexippes (monarch), Labidomera clavicollis (swamp milkweed leaf beetle), Oncopeltus fasciatus (large milkweed bug), and Aphis nerii (yellow milkweed aphid). The latter aphid often congregates on the upper stems and young leaves. Additional insects include, milkweed longhorn (Tetraopes basalis), red milkweed
beetle (Tetraopes tetrophthalmus), western milkweed beetle (Tetraopes femoratus), milkweed stem weevil (Rhyssomatus lineaticollis), small milk-
weed bug (Lygaeus kalmii), milkweed leaf-miner fly (Liriomyza asclepiadis), milkweed tussock moth (Enchaetes egle), unexpected cycnia (Cycnia inopinatus), delicate cycnia (Cycnia tenera), pale milkweed aphid (Aphis asclepiadis), variable milkweed aphid (Myzocallis asclepiadis), eastern
flower thrips (Frankliniella tritici), and milkweed ladybird beetle (Brachyacantha ursina).
Mammalian herbivores leave this plant alone because the foliage is both
bitter and toxic, and the milky juice contains cardiac glycosides. Swamp
milkweed foliage and stems have been reported to cause mortality in
sheep. It is not known why sheep are so susceptible. Muskrats are unaffect-
ed by swamp milkweed and readily eat the roots.
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Sky Meadows Park
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