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Plants of the Eloise Butler Wildflower Garden

The oldest public wildflower garden in the United States

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Common Name
Poke Milkweed (Tall Milkweed)

 

Scientific Name
Asclepias exaltata L.

 

Plant Family
Dogbane (Apocynaceae)

Garden Location
Woodland

 

Prime Season
Early Summer Flowering

 

 

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 five 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.

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. You will find many references that have not yet picked this up. 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.

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.

See bottom of page for notes on the Garden's planting history, distribution in Minnesota and North America, lore and other references.

plant drawing

Above & below: The inflorescence is composed of several drooping umbels, arising from the top of the stem, which each have only a few long stalked flowers. Drawing from Britton, N.L., and A. Brown. 1913. An illustrated flora of the northern United States, Canada and the British Possessions. 3 vols. Charles Scribner's Sons, New York.

Flower cluster Poke Milkweed Flower Buds

Below: 1st photo - the small calyx has five pointed lobes, hairy in this example, which will be hidden when the petals reflex. 2nd photo - The petals beginning to reflex showing the 5 white nectary hoods, each with its curved horn.

calyx flower

Below: 1st photo - Leaves are long stalked, broadly elliptical, with pointed tips. Both the tip area and the base are more wedge shaped. 2nd photo - The underside is a more pale color with fine hair on the ribs.

Poke Milkweed Leaf leaf underside

Below: 1st photo - Seed pods forming. The pod (follicle) is held upright on a stalk the first curves downward before the attachment point to the erect pod. 2nd photo - The seed pod has split, releasing seed to the wind. Each seeds parachute is held in a separate chamber in a multi-chambered membrane attached to the inside back of the pod - thus each can be pulled out separately by the wind when ready.

Seed Pods Seed pod - fall

Robert Frost wrote a poem about milkweed and Monarch Butterflies. Read it here: "Pod of the Milkweed"

Notes:

Notes: Poke Milkweed is indigenous to the Garden. Eloise Butler noted it on June 25, 1910. Martha Crone made note of it blooming in 1938. In North America it is found in Canada in Ontario, Quebec and P E Island. In the U.S. It is found east of the Mississippi River, exc. Florida. Minnesota is on the NW edge of its range and within Minnesota it is also generally east of the river when north of the metro area, not further north than Cass county or west of Stearns but in the SE it is found in counties as far west as Waseca and Le Sueur.

There are 14 species of Milkweed native to Minnesota. Five of these are found in the Garden: A. exaltata, Poke Milkweed; A. incarnata, Swamp Milkweed; A. syrica, Common Milkweed; A. tuberosa, Butterfly Weed; and A. verticillata, Whorled Milkweed.

Toxicity:
The milky sap is toxic as are most other parts of the plant. Cattle and Sheep are particularly affected, rarely horses, and humans will find parts of the plant edible, but toxic in large quantities, if not properly prepared. Toxins include the cardiac glycosides asclepiadin and asclepione, resinoids and a few alkaloids. The milky sap of the plant protects it from ants. Ants feet puncture the stem and they stick in the sap.

Eloise Butler wrote this about Milkweeds: "Most of the milkweeds, as the term implies, are furnished with a copious, milky juice. Crawling insects are likely to be covered and impaled by this sticky fluid, which exudes from wounds made by their sharp claws, as they scale the stems of the plants, and thus prevents them from rifling the nectar provided by the flowers for the pollen-distributing, hairy-bodied flying insects. Wonderful are the adaptations of the flower to desirable insect guests. Above the petals is a crown of five hood-like nectaries, each bearing within a slender, inverted horn. The center of the flower is designedly slippery. When an insect alights on this slimy surface to sip the abundant nectar, her feet slip and are tightly caught in crevices, also of fell design. When she extricates her toes, so to speak, she drags out attached to them a dangling pair of pollen masses - pollinia, a part of which is sure to adhere to the pistil of the next milkweed flower she visits. Insects have been caught at this season with stalks of these pollinia attached to every one of their six feet." Published in the Minneapolis Sunday Tribune July 9, 1911

References and site links

References: Plant characteristics are generally from sources 1A, 32, W2, W3, W7 & W8 plus others as specifically applied. Distribution principally from W1, W2 and 28C. Planting history generally from 1, 4 & 4a. Other sources by specific reference. See Reference List for details.

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