Asclepias stenophylla
Asclepias prostrata
narrow-leaf milkweed, slimleaf milkweed
prostrate milkweed
1 or 2 (rarely more), erect to spreading, rarely branched, 15–85 cm, puberulent with curved trichomes, not glaucous, rhizomes absent.
2–7, prostrate to decumbent, sometimes branched, 15–30 cm, pilosulous to tomentulose, not glaucous, rhizomes absent.
alternate, subopposite, or opposite, sessile or petiolate, with 1 stipular colleter on each side of petiole;
petiole 0–1 mm, spreading to ascending, glabrate;
blade linear, conduplicate, 5–16 × 0.1–0.5 cm, chartaceous, base cuneate, margins entire, apex acute, venation faintly brochidodromous to obscure, surfaces sparsely puberulent with curved trichomes, especially on midvein, to glabrate, margins ciliate, laminar colleters absent.
opposite, petiolate, with 1 stipular colleter on either side of petiole;
petiole 2–3 mm, pilosulous to tomentulose;
blade linear-lanceolate to deltate, 1.8–5 × 0.4–1.8 cm, chartaceous, base truncate to rounded or subcordate, margins crisped, apex acute, mucronate, venation eucamptodromous, surfaces pilosulous to tomentulose, margins ciliate, laminar colleters absent.
extra-axillary, sessile or pedunculate, 9–28-flowered;
peduncle 0–1.3 cm, sometimes branched at apex, puberulent with curved trichomes, with 1 caducous bract at the base of each pedicel.
extra-axillary, pedunculate, 3–8-flowered;
peduncle 0.4–2 cm, densely pilosulous to tomentulose, with 1 caducous bract at the base of each pedicel.
5–12 mm, puberulent with curved trichomes to pilosulous.
8–14 mm, densely pilosulous to tomentulose.
erect to pendent;
calyx lobes lanceolate, 1.5–2.5 mm, apex acute, puberulent with curved trichomes to pilosulous;
corolla pale green to greenish cream, lobes reflexed with spreading tips, elliptic, 3–5 mm, apex acute, minutely puberulent with curved trichomes at apex abaxially, glabrous adaxially;
gynostegial column 0–0.5 mm, fused anthers green, truncately obconic, 1.5–2 mm, wings narrowly crescent-shaped, wide open at base, apical appendages deltoid;
corona segments cream, often green-tinged, sessile, chute-shaped, margins incurved, appressed to anthers, 3–3.5 mm, equaling style apex, base saccate, auriculate, apex truncate, glabrous, internal appendage a short crest, the segment appearing 3-toothed, glabrous;
style apex shallowly depressed, green.
erect;
calyx lobes lanceolate, 3–4 mm, apex acute, pilosulous;
corolla green, lobes reflexed, elliptic, 8–11 mm, apex acute, pilosulous abaxially, minutely hirtellous at base adaxially;
gynostegial column 3–3.5 mm;
fused anthers brown, obconic, 2–2.5 mm, wings right-triangular, closed, apical appendages ovate, erose;
corona segments cream, tinged dorsally yellow, green, or pinkish, subsessile, conduplicate-tubular, 5–7 mm, greatly exceeding style apex, apex truncate, glabrous, internal appendage falcate, exserted, sharply inflexed over style apex, papillose;
style apex shallowly depressed, green to yellowish.
ovate, 6–7 × 4–5 mm, margin winged, faces minutely rugulose;
coma 2.5–3 cm.
broadly ovate, 7–8 × 5–6 mm, margin corky-winged, erose, faces very sparsely papillose;
coma 1–1.8 cm.
erect on straight pedicels, fusiform, 9–13 × 1–1.2 cm, apex long-acuminate, smooth, pilosulous.
pendent on lax pedicels, ovoid, 3.5–5.5 × 1–1.5 cm, apex acuminate, muricate-ridged, tomentulose.
Asclepias stenophylla
Asclepias prostrata
Although Asclepias stenophylla is a distinctive species, it is difficult to distinguish from A. engelmanniana in the absence of flowers or fruits, where their ranges overlap in the Great Plains. The drooping leaves of A. engelmanniana can reliably distinguish that species from A. stenophylla. Asclepias stenophylla is also often mistaken for A. verticillata, but the nearly appendageless corona segments and alternate or opposite (versus whorled) leaves readily separate A. stenophylla from that species. Because of its slender habit, linear leaves, and small clusters of greenish cream flowers held close to the stem, it can be overlooked in its grassland habitats. Asclepias stenophylla is widespread and common in its core habitat of Ozark glades and dry sites in tallgrass in Missouri, and in mixed-grass prairies from South Dakota to Texas. It is quite rare at the margins of its range in Arkansas (Baxter County), Illinois (Adams, Calhoun, and Pike counties), Iowa (Guthrie, Plymouth, and Sioux counties), Louisiana (Winn Parish), Minnesota (Houston County), Montana (Carter County), and Wyoming (Crook and Weston counties). In Colorado, it exhibits an interesting disjunction between Yuma County in the east and the Front Range of the Rocky Mountains, where it is sporadic, but impacted by development and considered to be of conservation concern. A report from North Dakota has not been confirmed.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
Asclepias prostrata is one of the most unusual and poorly known milkweeds in the flora. It was first collected by A. Schott in 1853 during the United States-Mexico border survey, along the Rio Grande (Río Bravo) between Laredo and Ringgold barracks (near Rio Grande City). However, it was not described until much later, from a collection made in Tamaulipas. The species remains rarely collected in both the United States and Mexico, and it is considered extremely rare in Texas (Starr and Zapata counties) and of conservation concern. Many historically known populations in the lower Rio Grande valley have not been relocated in recent years and are presumed extirpated (A. Strong, Texas Parks and Wildlife Department, pers. comm.). Several populations are known to have been eliminated by the widening of highways; others are thought to have been impacted by the spread of the invasive grass, Cenchrus ciliaris Linnaeus. The prostrate habit of A. prostrata cannot be confused with any other species of Asclepias. However, A. prostrata exhibits a remarkable similarity in all vegetative traits, including habit, to two co-occurring asclepiads, Matelea brevicoronata and M. parvifolia, as well as species of Acleisanthes (Nyctaginaceae), particularly the ubiquitous Acleisanthes longiflora. These species form a rather curious assemblage for which there is no hypothesized explanation involving convergent evolution. Since the description of Asclepias prostrata, prostrate species of Matelea occasionally have been misidentified as this species, even far outside its range.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
IL Wildflowers
KS Wildflowers
LA Plants
MN Wildflowers
MO Plants
PNW Herbaria
WildflowerSearch
iNaturalist
USDA Plants Database
