Asclepias syriaca L. - Common Milkweed

Asclepias syriaca plant

Family - Asclepiadaceae

Stems - Stout, erect, herbaceous, to 1.5m tall, 4-angled (the angles rounded), tomentoulose, with milky sap.

Leaves - Opposite, decussate, short petiolate. Petioles to -1cm long. Blades to +/-20cm long, +/-10cm broad, entire, tomentose (tomentoulose) and light green below, darker green and sparse tomentoulose above, often with purplish midrib adaxially, ovate-oblong to ovate-lanceolate or elliptic, rounded at base, acute at apex. Veins of the leaf anistomosing before the margins of the leaf (best viewed abaxially).

Asclepias syriaca leaves

Inflorescence - Axillary, pedunculate, globose umbels on the apical 1/2 of the stem. Peduncles to +/-6cm long, tomentose. Flowers +/-100 per umbel. Pedicels tomentose, to 4cm long, typically with some purplish tinge.

Asclepias syriaca inflorescenceInflorescence.

Flowers - Petals 5, reflexed, subulate, to 9mm long, 3-4mm broad, acute, entire, glabrous internally, pubescent externally. Hoods creamy whitish-purple, 4-5mm tall, attached basally to column, glabrous. Horns to 2mm long, whitish-purple, curved inward and touching the top of the anther column. Column dark purple and whitish at the apex, 2-3mm long and broad. Anther sacs 2mm long, connective dark purplish-brown. Sepals 5, green, pubescent externally, glabrous internally, reflexed, shorter than the petals.

Asclepias syriaca flowerFlower close-up.

Asclepias syriaca flowerFlower side-view.

Asclepias syriaca fruitFruit.

Flowering - May - August.

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

Origin - Native to U.S.

Other info. - This robust species can be found throughout most of Missouri but seems to be uncommon in the south-central portion of the state. Like the majority of the milkweeds, this species bleeds a profuse amount of milky sap when injured. The plant is toxic if ingested. This species can be mistaken with another, A. purpurescens L., but the latter has more purple flowers, inflorescences which are at the apex of the plant, and smooth fruits. The fruits of A. syriaca have small protuberances on them. Both species are quite striking and are visited frequently by insects in search of nectar.
Plants of the genus Asclepias have their pollen gathered 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 of Asclepias their feet and appendages slip into slits in the gynostegium of the flower and get snagged on these pollinia. The pollinia will then be carried by the insects to other flowers on different plants to assure varied pollination. This amazing 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.


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