Asclepias syriaca L.

Common Milkweed

Asclepias syriaca plant

Family - Asclepiadaceae

Habit - Perennial forb with deep rhizomes.

Stems - Erect, to 2 m, stout, sparsely to densely hairy, with milky sap, usually unbranched below the inflorescence, with 7-15 nodes.

Asclepias syriaca stemLower stem.

Asclepias syriaca stem2Upper stem.

Leaves - Opposite, simple, entire, short-petiolate, usually decussate. Petioles to 1 cm long. Blades to 30 cm long, 11 cm broad, oblong or elliptic-ovate, rounded to truncate at base, rounded or pointed at tip, usually mucronate, densely hairy and light green below, darker green and sparsely hairy above, the midrib often purplish adaxially. Veins of the leaf anistomosing before the margins of the leaf (best viewed abaxially).

Asclepias syriaca leaf1Leaf adaxial.

Asclepias syriaca leaf2Leaf abaxial.

Asclepias syriaca leavesLeaves.

Inflorescence - Globose umbels, pedunculate, terminal and axillary located on the apical half of the stem, 2-10 per stem, with 20-100 flowers. Peduncles to 6 cm long, densely hairy. Pedicels densely hairy, to 4 cm long, typically with some purplish tinge.

Asclepias syriaca inflorescence2Inflorescence and buds.

Asclepias syriaca inflorescence3Inflorescence.

Flowers - Calyx lobes 5, reflexed, short-hairy, 2.5-4.0 mm long, lanceolate to elliptic-lanceolate. Corolla lobes 5, reflexed, short-hairy on the outer surface and usually also on the inner surface near the base, green to lavender, usually tinged with pink and/or white, 7-10 mm long, elliptic-lanceolate. Gynostegium appearing very short-stalked (the column barely visible below the bases of the hoods), pale pink to pale purple, the corona conspicuously longer than the tip of the anther/stigma head. Corona hoods 3.5-5.0 mm long, whitish-purple, glabrous, spreading to ascending, attached near their bases, gradually narrowed from at or above the middle, the apical portion oblong to ovate in outline, the tip rounded, the margins with a pair of sharply triangular, ascending and incurved lobes at about the middle, the bases not pouched. Horns whitish-purple, attached toward the hood bases, relatively short, bent or curved abruptly inward over the anther/stigma head and not extended past the tips of the hoods, sickle-shaped, relatively stout, slightly flattened, tapered to a sharp point at the tip. Column dark purple and whitish at the apex, 2-3 mm long and broad. Anther sacs 2 mm long, connective dark purplish-brown.

Asclepias syriaca flowerFlower.

Asclepias syriaca flowerFlower.

Fruits - Follicles 7-12 cm long, erect or ascending from deflexed stalks, narrowly ovate to ovate and usually noticeably arched in outline, the surface with soft, narrow, warty tubercles, densely woolly. Seeds with the body 6-8 mm long, the margins narrowly winged, the terminal tuft of hairs white to light cream-colored or tan.

Asclepias syriaca fruitFruits.

Asclepias syriaca fruit2Dehiscent fruit.

Flowering - May - August.

Habitat - Fields, prairies, pastures, open woods, roadsides, railroads, waste ground.

Origin - Native to U.S.

Other info. - This robust species can be found throughout most of Missouri, as well as the upper Midwest, New England, and Canada. It is arguably the most common milkweed throughout the region and is a common sight along many roadsides. It is easily recognizable, though similar in appearance to the much less common A. sullivantii, a species which lacks the dense hairiness of the common milkweed. Another similar species, A. purpurescens, has flowers more deeply purple in color, inflorescences which are at the apex of the plant, and fruits which are smooth rather than warty.

Like most of the milkweeds, this species bleeds a profuse amount of milky sap when injured. The sap is toxic and an irritant. The flowers have an intensely floral fragrance, and are terrific magnets for insects in search of nectar. The plant is a prime host for the monarch butterfly, so much so that recent large scale and overly effective eradication of common milkweed from crop field margins has been linked to plummeting populations of these butterflies. Homeowners and farmers alike are encouraged to allow milkweed to persist on properties under their control.

Plants of the genus Asclepias have their pollen contained in small sacs known as pollinia. The pollinia are buried within the other flora organs and cannot be seen without dissecting the flowers. As insects crawl amongst and feed upon the flowers, their feet and appendages slip into slits in the gynostegium of the flower and get snagged on these pollinia. The pollinia break off and are carried by the insects to other plants to assure varied pollination. This amazing and idiosyncratic process can be seen in the picture below as a small larva feeding upon the flowers of A. syriaca can be seen with a pair of pollinia attached to one of its feet.

Larva with polliniaArrow shows pollinia.

Photographs taken off Hwy 106, Shannon County, MO., 6-10-04 (DETenaglia); also at Weldon Spring Conservation Area, St. Charles County, MO, 9-1-2007; near Labadie, Franklin County, MO, 6-22-2014; and Whetstone Conservation Area, Callaway County, MO, 7-15-2016 (SRTurner).


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