Description: This native perennial is
variable in size, from 2-6', depending on environmental conditions. The central
stem branches occasionally into several side stems. The opposite leaves are up
to 6" long and 1½" across. They are lanceolate or narrowly ovate, with
smooth margins, and hairless. Where it is hot, sunny, and somewhat dry, the
leaves may become yellowish or faded green. Several umbels of white-pinkish
flowers appear at the ends of major stems. Each flower is about ¼"
across, with 5 upright white hoods near the center, and 5 surrounding pink
petals that are reflexed downward in the manner of most milkweeds. The blooming
period occurs during late summer and lasts about a month. There is a pleasant
floral scent that resembles cinnamon. The seedpods are 4" long and narrow,
tapering to a point at both ends. They occur in pairs, and release seeds with
large tufts of white hair to the wind during the fall. The root system consists
of a taproot, and short rhizomes that promote the vegetative spread of this
plant.
Cultivation: The preference
is full or partial sun, and wet to moist conditions. The soil should consist of
a mucky clay, rich loam, or peaty material. This plant will tolerate episodes
of standing water because it has good anaerobic tolerance. It has poor
tolerance of dry conditions, and will be greatly reduced in size. The leaves
have a tendency to become wider in response to shady
conditions.
Range & Habitat:
Swamp Milkweed is a fairly common plant that occurs in nearly all counties of
Illinois (see
Distribution
Map). Habitats include open to partially shaded areas in floodplain
forests, thickets, moist black soil prairies, marshes, swamps, bogs, seeps, and
areas along rivers, lakes, or drainage ditches. It can be found in both high
quality and degraded habitats.
Faunal
Associations: The flowers are very popular with many kinds of
insects, including bumblebees, Halictine bees, wasps, flies, butterflies
(especially Swallowtails), skippers, and beetles. Wasp visitors include Great
Golden Digger wasps, Mud Dauber wasps, Cricket Hunter wasps, Grasshopper wasps,
Weevil wasps, the Five-Banded Tiphiid wasp, Papers Wasps, Hornets, Sand wasps,
and Spider wasps. Typical fly visitors are Soldier flies, Mydas flies,
Thick-Head flies, and Tachinid flies. Another occasional visitor of the flowers
is the Ruby-Throated Hummingbird. All of these visitors seek nectar. The
caterpillars of the butterfly Danaus plexippes (Monarch) feed on the
foliage. The species Aphis nerii (Yellow Milkweed Aphid) often attacks
this plant; these aphids are yellow and congregate on the upper stems near the
flowers (see Insect
Table for other species that feed on this and other milkweeds). Mammalian
herbivores leave this plant alone because the foliage is both bitter and toxic,
containing cardiac glycosides.
Photographic
Location: The photographs were taken at Meadowbrook Park in Urbana,
Illinois.
Comments: This is usually
an attractive and elegant plant. It is the only milkweed in Illinois that
favors wetland habitats. Swamp Milkweed is easily distinguished from other
milkweeds by its erect umbels of pink flowers, tall branching habit, and more
narrow leaves than such species as Asclepias syriaca (Common Milkweed)
and Asclepias sullivantii (Prairie Milkweed). Sometimes stray plants of
Swamp Milkweed occur in drier areas; these specimens are usually much shorter
and little branched.