Milkweeds

Many herbaria prefer to place the milkweeds in their own family (Asclepiadaceae). Here, the genus Asclepias and other milkweed genera are placed in the family Apocyanaceae in anticipation that the FNA is about to make that taxonomic decision.

At least 74 species in this genus occur in the U.S., and of these—17 species are native to Iowa.

Milkweeds come in various sizes and shapes but they have in common a characteristic flower structure (see Bookman. Amer. J. Bot. 68(5): 675-679. 1981) and when they are damaged, they produce a white sap (latex)—in the case of Asclepias tuberosa the sap is clear. The fruit is a follicle (pod) and the seeds are attached to a tuft of fine white strands (the coma).

As a rough indication of their height when mature, the plants below are characterized as tall, medium, or small. Tall milkweeds can be up to 6 feet but are usually between waist and chest high. Medium sized milkweeds are usually between knee and waist level, and small milkweeds are usually less than knee high. As a further "rough" identification feature, the leaves are described as broad, narrow or slender (grass-like). A quick reference table (PDF) of Iowa milkweeds is available here. Endangered and threatened species in Iowa are listed here. Endangered species in North America are listed here. Also, milkweeds have an interesting relationship with numerous insects, most notably the monarch butterfly, see iowamonarchs.info, many others are illustrated at driftlessprairies.org.

6 Milkweeds that are frequent in Iowa

[click on thumbnails to see larger images]

common milkweedCommon milkweed Asclepias syriaca - Common milkweed
This is probably the most numerous milkweed in Iowa. It is tall—often chest high, broad-leaved, petiolate, and the pods (fruit) are distinctly papillose. While it is numerous, its distribution is not uniform and in some regions of Iowa it may be outnumbered by other species.

Sand milkweedSand milkweed Asclepias amplexicaulis - Sand milkweed
Sandy soils are the preferred habitat for this milkweed. It is medium height—about knee high, although sometimes prostrate. Its broadleaves clasp the stem at their base and may overlap.The rounded inflorescence heads are more open than those of other local milkweeds

Swamp milkweedSwamp milkweed Asclepias incarnata - Swamp milkweed
One of Iowa's two most colorful milkweeds, this one is tall (~4 ft) and prefers swampy or marshy areas in full sun. Flowers appear light pink to a rose and white mix. Leaves are narrow and abundant along the stem.

Butterfly milkweedButterfly milkweed Asclepias tuberosa - Butterfly milkweed
Probably Iowa's most colorful milkwood—its orange colored inflorescence is popular in Iowa's gardens. It is a medium sized native plant which is at home in sunny mesic prairies. It's leaves are narrow, the fruit is slender and stands upright on the plant.

Whorled milkweedWhorled milkweed Asclepias_verticillata - Whorled milkweed
A medium sized milkweed, about knee high, with white flowers and slender grass-like leaves. The sessile leaves are usually attached to the stem in whorls. It is often found along roadsides, in old pastures and along the edge of woodlands—usually in sandy soils.

BluevineBluevine Cynanchum laeve - Honeyvine
Unlike the other Iowa milkweeds which are assigned to the genus Asclepias, Honeyvine, as its name indicates, is a vine and is assigned to the genus Cynanchum. Monarch butterflies are very fond of this milkweed but gardeners and landscapers find it hard to remove once it is established. It is frequent in S.W. Iowa.

12 Milkweeds that are scarce or endangered in Iowa

Purple milkweedPurple milkweed Asclepias_purpurascens - Purple milkweed
Only occasionally seen in Iowa.

Poke milkweedPoke milkweed Asclepias_exaltata - Poke milkweed
Only occasionally seen in Iowa, most frequently seen in eastern Iowa.

Engelmann's milkweedEngelmann's milkweed Asclepias_engelmanniana - Engelmann's milkweed
At this writing it is listed as ENDANGERED in Iowa. However, a recent re-examination, at the Ada Hayden Herbarium of the few specimens so named, found them to be A. stenophylla and the examiners have concluded that A. engelmanniana does not exist in Iowa.

Narrow-leaved milkweedNarrow-leaved milkweedAsclepias_stenophylla - Narrow-leaved milkweed ENDANGERED
Only a few individuals have been seen in western Iowa, which is at the north-eastern edge of its current range. Mature plants are from 2 to 4 feet high with slender stems and leaves. inflorescences are umbels of greenish-white flowers arising from leaf axils. They are found in sandy or rocky soils.

Mead’s milkweedMead’s milkweed Asclepias_meadii - Mead’s milkweed ENDANGERED
Only occasionally seen in Iowa.

Woolly milkweedWoolly milkweedAsclepias_lanuginosa - Woolly milkweed
THREATENED and rarely seen in Iowa.

Showy milkweedShowy milkweed Asclepias_speciosa - Showy milkweed
THREATENED and rarely seen in Iowa.

Tall Green milkweedTall Green milkweed Asclepias hirtella - Tall Green milkweed
Only occasionally seen in Iowa.

Oval-leaf milkweedOval-leafmilkweed Asclepias_ovalifolia - Oval-leaf milkweed
Only occasionally seen in Iowa.

Fourleaf Milkweed milkweedFourleaf Milkweed milkweed Asclepias_quadrifolia - Fourleaf Milkweed
Only occasionally seen in Iowa.

Sullivant's milkweed Sullivant'smilkweed Asclepias_sullivantii - Sullivant's milkweed
Only occasionally seen in Iowa. Eilers and Roosa describe it as infrequent to rare throughout most of the state. It resembles A. syriaca (common milkweed), but is smaller and slimmer. It tends to hold its leaves at an upward angle, its leaves and stems are less hairy, and its pods have fewer papillae.

Green milkweedGreen milkweed Asclepias_viridiflora - Green milkweed
Only occasionally seen in Iowa.