Description: This native perennial plant is
unbranched and up to 2' tall. Whorls of 4-8 linear leaves occur along the
slender central stem. This stem is ridged and
hairless. Each leaf is up to 3" long and 1/8" across, with a prominent
longitudinal vein, and no hairs. The leaves often curve downward from the stem,
and then curl slightly upward toward their outer tips. Along the upper half of
the plant are short-stalked umbels of greenish white flowers that emerge from
the axils of the leaves. These umbels have up to 20 flowers and span about 2-3"
across. Each flower consists of 5 strongly reflexed petals that are light
green, and 5 white hoods that are arranged around the center of the flower. An
individual flower is about 1/3" across. There is little or no floral scent. The
blooming period occurs from early to late summer, and lasts about 1-2 months.
Later, slender follicles appear where the flowers have been successfully
pollinated. These follicles split along one side to release numerous seeds with
large tufts of white hairs. The follicles are about 3-4" long and 2/3" across,
with a fairly smooth surface. Seed dispersion is by wind. The root system is
fibrous and rhizomatous. This plant often forms colonies.
Cultivation:
The preference is full sun and dry conditions. The soil can contain significant
amounts of loam, sand, or gravel – poor, sterile soil is actually
preferred because this reduces competition from taller plants. This plant also
grows well in moist to mesic conditions if there is sufficient sunlight.
Occasionally, the lower leaves turn yellow and fall off the stem during a
drought; this response is normal. Foliar disease is not troublesome. This plant
can become aggressive in open sunny areas, and form large colonies
quickly.
Range & Habitat:
Whorled Milkweed occurs throughout most of Illinois, except for a few southern
counties (see
Distribution
Map). It is occasional to locally common. Habitats include dry areas of
black soil prairies, sand prairies, gravel prairies, hill prairies, openings in
rocky upland forests, sandy savannas, limestone glades, rocky bluffs along
major rivers, pastures and abandoned fields, and grassy slopes along highways.
Occasionally, it is found on moist gravelly banks along rivers.
Faunal Associations: The nectar of the flowers
attracts many kinds of insects, including long-tongued bees, short-tongued
bees, wasps, flies, butterflies, skippers, and beetles. Among these, bees and
wasps are the more effective pollinators. Predatory insects often lurk near the
flowers, including Phymata fasciatus (Ambush Bug) and Mantis spp.
(Mantids). The caterpillars of the butterfly Danaus plexippes (Monarch)
feed on the foliage and flowers. The species Aphis nerii (Yellow
Milkweed Aphid) sucks juices from the upper stems and leaves. These aphids
often attract ants (which feed on their honeydew), as well as Ladybird Beetles
and Green Lacewings (which feed on the aphids themselves). Other insect species
that feed on this and other milkweeds can be found in the
Insect Table. Mammalian
herbivores usually avoid this plant as a food source because of the bitter
white latex, which is also poisonous. Whorled Milkweed is among the most
poisonous of milkweeds, containing a high concentration of cardiac glycosides.
Cattle have been known to poison themselves on this plant, but they rarely eat
enough of it to produce fatal results.
Photographic Location: Photographs were taken at the
webmaster's wildflower garden in Urbana, Illinois.
Comments: This little milkweed blooms later in the
year than many other members of the genus, and is good at attracting
butterflies. The foliage of this plant resembles a horsetail, but the flowers
reveal its membership in the Milkweed family. It can be distinguished from
other milkweeds by its skinny whorled leaves and greenish white
flowers.