Whorled Milkweed is an erect native perennial forb growing to over 24 inches high with slender but sturdy stems that are unbranched below the inflorescence. Stems are green, without hair and ridged. Like most milkweeds, they contain a milky juice which is toxic.
The leaves develop in whorls of 3 to 6 narrow linear leaves that have pointed tips and bases. These are stalkless, medium green, smooth, about 3 inches long. The longer lower stem leaves may droop downward before curving up near the tip. Upper leaves are ascending.
The inflorescence is a stalked umbel, rising from the upper leaf nodes, of up to 20 individual flowers.
Milkweed flowers, when open, have five erect hood-like nectaries with the five petal parts bent downward. On this species the petal parts and the hoods are white. The hoods each have a small horn on the inner side that curves inward and is longer than the hood in this species. Each flower is about 1/3 inch wide and 1/4 inch high. Eloise Butler explains the function of these parts in her notes below.
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 erect stalks a slender erect pod (a follicle) that contains a large number of flat brown seeds attached to long white silky hairs. The seeds are arranged in the pods with the brown seeds tucked around the circumference of the pod and with white silky parachutes for wind dispersion attached to a central membrane that is attached at the top and base of the pod. The pod turns brown a maturity and splits vertically to release the seeds via the wind. Seeds require 30 days of cold stratification for germination.
Habitat: Whorled Milkweed grows from a fibrous and rhizomatous root system which can form colonies of plants. It grows best in full sun in partially moist to dry locations. Soil quality should be well drained, but does not have to be rich. Poor soils are tolerated by the plant, so it is commonly found along fields, roadsides and in dry prairies.
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 verticillata is often used when the leaves are whorled. The author name for the plant classification - 'L.' is for Carl Linnaeus (1707-1778), Swedish botanist and the developer of the binomial nomenclature of modern taxonomy. For more info see below.
Comparisons: While the flowers of most milkweeds look similar and are usually in an umbel, this species is readily identified by the whorled linear leaves matched with white flowers. See in 'notes' below links to other species.
Above: Whorled Milkweed is unbranched below the inflorescence. Leaves are in whorls of 3 to 6, stalkless and very narrow.
Below: The stalked flowers are formed in a rounded umbel. The green calyx lobes (1st photo) will be hidden when the petals reflex downward.
Below: When the petals reflex downward they expose the hoods and the nectaries. Eloise Butler explains the function of these parts in her notes at the page bottom.
Below: Seed develops in upright green pods (1st photo) which split vertically to release seeds. Each brown seed (3rd photo) is tucked into the pod with a long silky parachute that is attached to a central membrane of the pod. A stiff breeze is required to break them loose but once airborne, they can travel considerable distances.
Below: 2nd photo - Stems contain a toxic milky juice.
Robert Frost wrote a poem about milkweed and Monarch Butterflies. Read it here: "Pod of the Milkweed"
Notes: Whorled Milkweed is not indigenous to the Garden, but it is to Hennepin County. Eloise Butler first planted it on August 15, 1910 with plants sourced from Breezy Point on Lake Minnetonka. Again in 1911 she planted more sourced from the river bank near the Catholic Seminary in St. Paul and on June 25, 1914 from Brownie's Pond within Glenwood Park. Martha Crone planted it in 1946. In North America the plant is found in the central Canadian Provinces and in the U.S. from the great plains eastward to the coast. Within Minnesota it is found throughout the state west and south of a line drawn from Washington and Ramsey counties NW to the NW corner of the state - absent in the NE quadrant.
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.
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 it 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: 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.
Identification booklet for most of the flowering forbs and small flowering shrubs of the Eloise Butler Wildflower Garden. Details Here.
Friends of the Wild Flower Garden, Inc. Text and photos are by G. D. Bebeau unless otherwise credited. "www.friendsofthewildflowergarden.org"