The Friends of the Wildflower Garden, Inc.

Plants of the Eloise Butler Wildflower Garden

The oldest public wildflower garden in the United States


Common Name
Butterfly Milkweed (Pleurisy Root)


Scientific Name
Asclepias tuberosa L.


Plant Family
Dogbane (Apocynaceae)

Garden Location


Prime Season
Early July to late August Flowering



Stems: Butterfly Milkweed has a brilliant cluster of orange 5-parted flowers in umbels at the top of a hairy 1 to 2 foot leafy stem, produced from a branched crown on a deep set root. Stems are not strong and may sprawl. The stem juice is colorless unlike other milkweeds, but is still considered toxic. Older plants will send up multiple stems.

The leaves are long and narrowly lance shaped, sessile, and alternate on the main stem - other milkweeds are opposite - but Butterfly Milkweed leaves are also opposite on the upper side branches of the inflorescence.

The inflorescence is branched, the lower stem not. Flowers occur in upright terminal umbels at the tip of the main stem and of the branched stems within the inflorescence and from the upper leaf axils of those stems.

Individual Flowers are 3/8 to 2/3rd inches tall. Milkweed flowers, when open, have five erect hood-like nectaries with the petal parts bent downward. In this species these parts are yellowish to deep orange. The hoods each have a small horn on the inner side that curves inward and, in this species, is shorter than the hood - (Purpose? - see Eloise Butler's notes at the page bottom). Both the hoods and the horns are longer than the yellow central structure. Each flower is on a short stalk (pedicel) and the cluster is also on a stalk (peduncle).

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: Flowers mature to green seed pods (a follicle) that are held upright on a downward curving stalk. The seeds are arranged in the pods like other milkweeds, 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: Butterfly Milkweed grows best in full sun, in mesic to dry conditions. Well drained soil is required. It is found in prairies and sandy areas. The root system is a woody taproot. It will spread via re-seeding and via the root system.

Names: As the common name implies, Butterfly Milkweed is attractive to butterflies. 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." The species name, tuberosa, is Latin for 'tuberous', referring to the root system - although the main root is a taproot it could also be considered as an elongated tuber. 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.

Comparisons: The showy orangish dense umbels of flowers rising at the top of this short plant and the alternate leaves will help identify this species of milkweed. See in 'notes' below links to other species.

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

Butterfly Weed Butterfly Weed Butterflyweed Buds

Above: Blooming plants above from late August. Flowers occur in upright terminal umbels at the tip of the main stem and of the branched stems within the inflorescence and from the upper leaf axils of those stems.

Below: 1st photo - Detail of the nectaries with the incurved horns. See Eloise Butlers comments at page bottom. 2nd photo - The woody taproot.

Butterfly Milkweed close up of flower Butterfly weed root

Below: The stem (1st photo) and underside of the leaf (3rd photo) are very hairy. Leaves are alternate on the stem (2nd photo).

srem leaf leaf underside

Below: 1st photo - The green seed pods are upright on the stems and the seeds are arranged in the pods (2nd photo) like other milkweeds, with the brown seeds tucked around the circumference of the pod and with the white silky parachutes attached at a pod membrane waiting for the brisk wind to whisk them off to distant places.

Butterfly Milkweed green seed pods Butterfly milkweed seeds

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


Notes: Eloise Butler first introduced Butterfly Milkweed to the Garden in 1908 with plants obtained from Kelsey's Nurseries on the East Coast. On June 20, 1913 she got some plants from near Mahtomedi MN and planted more on April 15, 1917 with plants from Andrews Nursery in Boulder CO. Martha Crone planted seeds in 1944 and '45 and plants also in '45 and '46. The plant was listed on her 1951 inventory of plants in the Garden at that time. Gardener Cary George planted it in 1987. A new planting was established in 2006 by Garden Curator Susan Wilkins. It is native to Minnesota generally in the eastern 2/3 rds of the state, north as far as Cass County. In North America the plant is found throughout the US except in the NW states. In Canada it is known from Ontario, Quebec and P E Island.

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.

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

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

Lore: There is considerable reference in plant literature to the medicinal aspects of Butterfly Milkweed. As its alternate common name, Pleurisy Root, implies, medicines made from the plant root can actively affect the accumulated life-weakening mucus build-up of pleurisy and mild pulmonary edema. The root is used in a decoction or an infusion to promote perspiration and clearing of the lungs. It has also been used to treat rheumatism, dysentery and colds. An average dose was a scant teaspoon chopped and boiled in water of one or two cups such that the ratio is one part root to 30 parts water. Large amounts will cause nausea or vomiting. The root contains glycosides and the active principle of the root is Asclepiadin. Dr. Clapp (Ref. #2) recommended it as an alternative expectorant and diaphoretic, and in large doses as a laxative. In the 19th Century the plant was listed in the U. S. Pharmacopoeia. Tilford (Ref. #39) and Hutchins (Ref. #12) have information on the plant lore and Mrs. Grieve (Ref. #7) has extensive information.

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.

graphicIdentification booklet for most of the flowering forbs and small flowering shrubs of the Eloise Butler Wildflower Garden. Details Here.