Vascular Plants of Wisconsin

Asclepias syriaca L.
common milkweed
Family: Asclepiadaceae

native: yes
type: herbaceous perennial

plant flower 1 flower 2 flower 3 young fruit mature fruit leaf
 

Asclepias syriaca is an herbaceous perennial with broad, simple, entire, opposite leaves and a white, sticky sap that is the source of the common name, "milkweed". The flowers are very unusual and easily recognized. Sepals and petals are reflexed and the upper portion of the flower is derived from fusion and modification of the stamens and carpels. The filaments of the anthers are adnate (fused to one another) to form the showy "corona", including 5 "hoods" each with a "horn". The lower portion of the filaments are adnate to form a tube around the 2 carpels, one of which is mostly hidden behind the other in this view. The obvious carpel has been dissected to expose the interior and a close look at flower photo 2 will reveal numerous ovules within. The two carpels are separate at the base, but are joined at the stigma.

The anthers are also fused together and to the 2 stigmas to form a structure known as the "gynostegium" (see flower photo 2 above). Note the "stigmatic slit" on the first flower photo. The pollinating insect must pass through this opening (at least in part) to get to the inner area of the gynostegium to reach the nectar and to pollinate the flowers. Usually only one of the carpels will mature to produce the familiar "follicle" or milkweed "pod" that contains numerous seeds, each with a tuft of hairs to aid in wind dispersal.

Although the scientific name suggests origination in Syria, Asclepias syriaca is a native species. It is found throughout the eastern United States, west to Texas in the south and Montana and Oregon in the west. It grows throughout Wisconsin on sunny, disturbed sites.


known distribution based on vouchers

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