Plants of the Eloise Butler Wildflower Garden

Poke Milkweed is a native erect perennial growing from 2 to 6 feet tall with an unbranched stem. Stems are green to purplish, without hair, and cylindrical. Like most milkweeds, they contain a milky juice which is toxic.

The leaves are opposite, long stalked, broadly elliptical, with pointed tips. Both the tip area and the base are more wedge shaped. They can be up to 8 inches long and are of similar size along the stem. The upper surface is dark green while the underside is a paler color with fine white hair on the ribs.

The inflorescence is composed of several drooping umbels, arising from the top of the stem, which each have only a few long stalked flowers. Many other milkweed species have densely flowered umbels.

Flowers: Milkweed flowers, when open, usually have five erect hood-like nectaries with the five petal parts bent downward. On this species the petal parts are a light green to pale purple and the hoods are usually white. The hoods each have a small horn on the inner side that curves inward. Each flower is about 1/4 inch wide and 1/2 inch high. Eloise Butler explains the function of these parts in her notes at the page bottom. The flower calyx is quite small and has 5 pointed lobes. The calyx is hidden once the petals reflex. Both the calyx and the flower stalk can have fine hair as the example shown here has.

The pollination system of Milkweeds works like this: The pollen of the milkweed is not in the form of free grains attached to an anther, but is contained in a waxy sac called a "pollinium" [plural - pollinia] with each sac having about a few hundred grains. Each Milkweed flower has two ovaries connected to the five flower parts, each part has a very short pistil with an enlarged stigma which has the form of a deep slit. Pollina sacs are in pairs with one sac located on each side of the stigma and each of the pair of sacs is connected by a 'translator arm' to a structure called a 'corpusculum' which sits atop the stigma slit. There is a groove in the corpusculum and the foot of an insect, such as a bee or butterfly, gets caught in the groove and when the foot is pulled out, the pair of sacs and the associated structure comes along with the foot and thus are then carried from flower to flower by the insects resulting in pollination. Any insect large enough and strong enough to remove the sac can fertilize another plant.

The method of fertilization is also interesting. Once a corpusculum has been pulled out, after a few minutes, the translator arms rotate the pulled out sacs 90 degrees. After the rotation when the insect reaches another flower, the sac is now in position to fit into the bottom of the slit in the stigma. As the insect moves its foot the sac is pulled upward in the slit until it hits the still existing corpusculum of that flower. At that point the translator arm snaps off and the pollen sac is in position to fertilize the flower. When the corpusculum of that flower is still present it is possible for the insects foot to catch it and thus remove another pair of pollen sacs for another go at the next flower. When the corpusculum of a flower has already been removed the chances of the translator arm breaking off are reduced and the pollina sac may not be broken off and is carried away by the insect to another plant.

Thus those flowers with their corpusculum still in place have the greatest chance of being fertilized. This is why the numerous flowers result in only a few seed pods. The fact that the translator arms take a few minutes to rotate after being pulled from a flower prevents the insect from cross pollinating adjacent flowers and allows the insect to find other plants that may not be clones of the first plant the insect visited. [This summary is based on the research work of Douglass H. Morse of Brown University.]

Seed: Fertile flowers produce, on downward curving stalks, a slender erect pod (a follicle) that contains a large number of flat brown seeds attached to long white silky hairs. The pod is green at first, turning brown at maturity, when the pod splits, and the seeds are dispersed by wind. Seeds require 30 days of cold stratification for germination.

Habitat: Poke Milkweed grows from a sturdy taproot. It is more of a woodland plant and grows best in dappled sunlight in the understory in loamy soils with moderate moisture.

Names: The Angiosperm Phylogeny Group in 2000 re-assigned the Ascelepias genus to the Apocynaceae family from the previous Asclepiadaceae (Milkweed) family. The genus name Asclepias is named for the Greek god of healing "Asklepios" and exaltata is often used when a plant is tall for its genus as is the case with this milkweed. For more info see notes below. The author name for the plant classification, 'L.' refers to Carl Linnaeus (1707-1778), Swedish botanist and the developer of the binomial nomenclature of modern taxonomy. In Eloise Butler's time the plant was sometimes called Asclepias phytolaccoides.

Comparisons: While the flowers of most milkweeds look similar and are usually in an umbel, this species is identified by the loose umbels of few white flowers, the unbranched stem, and the more shaded locations where it grows. Compare to: Common Milkweed, Swamp Milkweed, Whorled Milkweed, Butterfly Milkweed.