Asclepias

Asclepias syriaca seed pods, upper image from August and lower from December

Milkweed sprout, a few days after sowing

Chemical structure of oleandrin, one of the cardiac glycosides

Members of the genus Asclepias produce some of the most complex flowers in the plant kingdom, comparable to orchids in complexity. Five petals reflex backwards revealing a gynostegium (fused stamen filaments and styles) surrounded by a five-membrane corona. The corona is composed of a five paired hood and horn structure with the hood acting as a sheath for the inner horn. Glands holding pollinia are found between the hoods. The size, shape and color of the horns and hoods are often important identifying characteristics for species in the genus Asclepias.[9]

Pollination in this genus is accomplished in an unusual manner. Pollen is grouped into complex structures called pollinia (or "pollen sacs"), rather than being individual grains or tetrads, as is typical for most plants. The feet or mouthparts of flower-visiting insects such as bees, wasps and butterflies, slip into one of the five slits in each flower formed by adjacent anthers. The bases of the pollinia then mechanically attach to the insect, so that a pair of pollen sacs can be pulled free when the pollinator flies off, assuming the insect is large enough to produce the necessary pulling force (if not, the insect may become trapped and die).[10] Pollination is effected by the reverse procedure, in which one of the pollinia becomes trapped within the anther slit. Large-bodied hymenopterans are the most common and best pollinators, whereas monarch butterflies are not good pollinators of milkweed.[4]

Honeybee on antelope horn (Asclepias asperula) with pollinia attached to legs

Asclepias species produce their seeds in pods termed follicles. The seeds, which are arranged in overlapping rows, bear a cluster of white, silky, filament-like hairs known as the coma[11] (often referred to by other names such as pappus, "floss", "plume", or "silk"). The follicles ripen and split open, and the seeds, each carried by its coma, are blown by the wind. Some, but not all, milkweeds also reproduced by clonal (or vegetative) reproduction.

American milkweeds are an important nectar source for native bees, wasps, and other nectar-seeking insects, though non-native honey bees commonly get trapped in the stigmatic slits and die.[10][12] Milkweeds are also the larval food source for monarch butterflies and their relatives, as well as a variety of other herbivorous insects (including numerous beetles, moths, and true bugs) specialized to feed on the plants despite their chemical defenses.[4]

Milkweeds use three primary defenses to limit damage caused by caterpillars: hairs on the leaves, cardenolide toxins, and latex fluids. Data from a DNA study indicate that, generally, more recently-evolved milkweed species ("derived" in botany parlance) use these preventative strategies less but grow faster than older species, potentially regrowing faster than caterpillars can consume them.[13][14]

Research indicates that the very high cardenolide content of Asclepias linaria reduces the impact of the OE parasite, Ophryocystis elektroscirrha, on the monarch butterfly, Danaus plexippus. By contrast, some species of Asclepias are extremely poor sources of cardenolides, such as Asclepias fascicularis, Asclepias tuberosa, and Asclepias angustifolia.

Milkweed is not grown commercially in large scale, but the plant has had many uses throughout human history.[4] Milkweed has a long history of both medicinal, every day and, military use that most people aren’t even aware of.  The Omaha Indian’s, from Nebraska, the Menomin which was from Wisconsin and upper Michigan, Dakota’s from Minnesota and, the Ponca Indians from Nebraska used it for a medicinal and everyday usage ages ago. They used it for everything from a diarrheic, relief from sore throats, it was chewed for swelling and rashes, new mothers that could not produce milk would drink it, it was used to treat colic, it was used to expel tapeworms, it was used to cure snakebite and for a contraceptive. Heart-leaf milkweed was used by the Miwok tribes of California for its stems, which they dried and used for cords such as strings and ropes.[15] The milkweed filaments from the coma (the "floss") are hollow and coated with wax, and have good insulation qualities. During World War II, more than 5,000 t (5,500 short tons) of milkweed floss was collected in the United States as a substitute for kapok.[16][17] Milkweed is grown commercially as a hypoallergenic filling for pillows[18] and as insulation for winter coats.[19]

A study of the insulative properties of various materials found that milkweed floss was outperformed by other materials in terms of insulation, loft, and lumpiness, but it scored well when mixed with down feathers.[20] Milkweed fibers are also used to clean up oil spills.[21]

Milkweed seeds

The bast fibers of some species can be used for rope. Milkweed latex contains about two percent latex and was attempted as a source of natural rubber by both Nazi Germany and the United States during World War II. No record has been found of large-scale success.

Asclepias is also known as "Silk of America"[22] which is a strand of common milkweed (Asclepias syriaca) gathered mainly in the valley of the Saint Lawrence River in Canada. The silk is used in thermal insulation, acoustic insulation, and oil absorbents.[23][24]

Milkweed also contains cardiac glycoside poisons that inhibit animal cells from maintaining a proper K⁺, Ca²⁺ concentration gradient.[5] As a result, many natives of South America and Africa used arrows poisoned with these glycosides to fight and hunt more effectively. Milkweed is toxic and may cause death when animals consume large quantities of the plant. Milkweed also causes mild dermatitis in some who come in contact with it. Nonetheless, it can be made edible if properly processed.[4]

The leaves of Asclepias species are a food source for monarch butterfly larvae and some other milkweed butterflies.[4] These plants are often used in butterfly gardening and monarch waystations.[25]

However, some milkweed species are not suitable for butterfly gardens and monarch waystations. For example, Asclepias curassavica, or tropical milkweed, is often planted as an ornamental in butterfly gardens. Year-round plantings in the USA are controversial and criticised, as they may be the cause of new overwintering sites along the U.S. Gulf Coast, leading to year-round breeding of monarchs.[26] This is thought to adversely affect migration patterns, and to cause a dramatic buildup of the dangerous parasite, Ophryocystis elektroscirrha.[27] New research also has shown that monarch larvae reared on tropical milkweed show reduced migratory development (reproductive diapause), and when migratory adults are exposed to tropical milkweed, it stimulates reproductive tissue growth.[28]

Monarch caterpillars do not favor butterfly weed (Asclepias tuberosa) because the leaves of that milkweed species contain little toxin (cardiac glycosides). Some other milkweeds may have similar characteristics.

Some Asclepias species:

	Asclepias albicans	Whitestem milkweed, native to the Mojave and Sonoran deserts
	Asclepias amplexicaulis	Blunt-leaved milkweed, native to central and eastern United States
	Asclepias asperula	Antelope horns, native to American southwest and northern Mexico
	Asclepias californica	California milkweed, native to central and southern California
	Asclepias cordifolia	Heart-leaf milkweed, native to the Sierra Nevada and Cascade Range up to 2000 m.
	Asclepias cryptoceras	Pallid milkweed, native to the western United States.
	Asclepias curassavica	Scarlet milkweed, tropical milkweed, bloodflower, bastard ipecacuanha, native to the American tropics, introduced to other continents
	Asclepias curtissii	Curtiss's milkweed, endemic to sandy areas of Florida
	Asclepias eriocarpa	Woollypod milkweed, native to California, Baja California, and Nevada
	Asclepias erosa	Desert milkweed, native to California, Arizona, and Baja California
	Asclepias exaltata	Poke milkweed, native to eastern North America
	Asclepias fascicularis	Narrow-leaf milkweed, native to Western United States
	Asclepias hirtella	Tall green milkweed
	Asclepias humistrata	Sandhill milkweed, native to southeastern United States
	Asclepias incarnata	Swamp milkweed, native to wetlands of North America
	Asclepias involucrata
	Asclepias lanceolata	Lanceolate milkweed (Cedar Hill milkweed), native to coastal plain of eastern United States from Texas to New Jersey
	Asclepias linaria	Pine needle milkweed, native to Mojave and Sonoran deserts
	Asclepias linearis	Slim milkweed
	Asclepias meadii	Mead's milkweed, native to midwestern United States
	Asclepias nivea
	Asclepias nyctaginifolia	Mojave milkweed, native to the American southwest
	Asclepias obovata	Pineland milkweed
	Asclepias purpurascens	Purple milkweed, native to eastern, southern, and midwestern United States
	Asclepias quadrifolia	Four-leaved milkweed, native to eastern United States and Canada
	Asclepias rubra	Red milkweed
	Asclepias solanoana	Serpentine milkweed, native to northern California
	Asclepias speciosa	Showy milkweed, native to western United States and Canada
	Asclepias subulata	Rush milkweed, leafless milkweed, native to southwestern North America
	Asclepias subverticillata	Horsetail milkweed[29]
	Asclepias sullivantii	Sullivant's milkweed
	Asclepias syriaca	Common milkweed
	Asclepias texana	Texas milkweed
	Asclepias tuberosa	Butterfly weed, pleurisy root
	Asclepias uncialis	Wheel milkweed
	Asclepias variegata	White milkweed
	Asclepias verticillata	Whorled milkweed
	Asclepias vestita	Woolly milkweed
	Asclepias viridiflora	Green milkweed
	Asclepias viridis	Green antelopehorn, spider milkweed
	Asclepias welshii	Welsh's milkweed

There are also 12 species of Asclepias in South America, among them: A. barjoniifolia, A. boliviensis, A. curassavica, A. mellodora, A. candida, A. flava, and A. pilgeriana.

• Everitt, J.H.; Lonard, R.L.; Little, C.R. (2007). Weeds in South Texas and Northern Mexico. Lubbock: Texas Tech University Press. ISBN 0-89672-614-2

• Medical research
• Milkweed test-cultivated for the insulation value of floss
• Milkweed in Handbook of Energy Crops
• Common milkweed production research at Western Illinois University
• UVSC Herbarium — Asclepias