Apocynaceae

Not long ago milkweed plants were placed in the family Asclepiadaceae, which in the midwest included the genera: Asclepias, Cynanchum, and Matelea.

Taxonomists have more recently concluded that milkweeds are better categorized as members of the Apocynaceae (dog bane) family which by one estimate has 400 genera, and together these include 4555 species. Most of these are found in tropical and warm temperate regions.

As of this date (2/9/21), the FNA (Flora of North America) has not yet added the new arrangement of taxa to their website. You can check the FNA families list to see if the Apocynaceae has been added by the time of your visit. They are expected to move or retain the following Midwest genera into the Apocynaceae family:

In the Mid-west, members of the Apocynaceae family include:

• Dog bane species in the genus Apocynan
• Standing milkweeds in the genus Asclepias
• Vining milkweeds in the genera Amsonia (bluestars),Cynanchum (Honeyvine milkweed)and Matelea (Milkvines).
• The periwinkle in the genus Vinca

For Iowa, "The Vascular Plants of Iowa" (1994), by Eilers and Roosa; list three genera of milkweed plants: Apocynum (3 species + 1 hybrid), Asclepias with 17 species. One of them, Asclepias engelmanniana was later found to be mis-identified. So a current listing shows 16 species. The third genus (Cynanchum) had a single species (Cynanchum laeve)which is a vining milkweed that can be quite invasive and a persistent nuisance.

While this site is under construction, and probably thereafter, I would like to refer you to a recorded video, presented by Prof. Tom Rosburg and organized by Lance Brisbois, describing the milkweed species that are found in Iowa. Take this link to the Golden Hills plant-id workshops and scroll down to the Milkweed identification video–it runs about one hour. Also, notice the links for the workshop handouts and keys. For a deeper dive into the morphology, anatomy, and evolution of milkweeds download Floral glands in asclepiads/ structure, diversity and evolution.pdf It may take awhile to download, but I found the micrographs very interesting and I think it is worth the wait. You may have to download the image pages separately.

Asclepias syriaca - Common milkweed
Asclepias syriaca
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.

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

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.

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.

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.

Cynanchum laeve - Bluevine/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

Asclepias_purpurascens - Purple milkweed
Only occasionally seen in Iowa.

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

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.

Asclepias_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.

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

Asclepias_lanuginosa - Woolly milkweed
THREATENED and rarely seen in Iowa.

Asclepias_speciosa - Showy milkweed
THREATENED and rarely seen in Iowa.

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

Thanks to Barry Mateer for providing these photos on loan to us until
the site for this plant is completed.

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

Asclepias_quadrifolia - Fourleaf Milkweed
Only occasionally seen in Iowa.

Asclepias_sullivantii - Sullivant's milkweed or smooth milkweed

Eilers and Roosa describe Sullivant's milkweed as infrequent to rare throughout most of the state; although, it has been reported in nearly half of the counties in Iowa. It resembles A. syriaca (common milkweed), but is smooth (or at least less hairy) in contra-distinction to A. syriaca. A. sullivantii tends to hold its leaves at an upward angle, its leaves and stems are less hairy, and its pods have fewer papillae when compared to common milkweed. While the two species have many similarities, some workers, familiar with both species, have noted that the leaf midveins are a darker purple in A. sullivantii, and its leaves lack the short petiole found in the leaves of A. syriaca.

Asclepias_viridiflora - Green milkweed
Only occasionally seen in Iowa.