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Herbs. Stems 1–7, erect to spreading or decumbent, unbranched or rarely branched near base, 10–50 cm, puberu­lent with curved trichomes to hirsutulous, not glaucous, rhizomatous. Leaves persis­tent or gradually caducous from base, opposite, petio­late, with 1 or 2 stipular colleters on each side of petiole; petiole 2–20 mm, puberulent with curved trichomes to hirtellous; blade ovate or lanceolate to oblong or elliptic, sometimes conduplicate, 4–11 × 1.2–6.5 cm, chartaceous, base cuneate to obtuse, margins sometimes crisped, apex obtuse to rounded, venation eucampto­dromous to faintly brochidodromous, surfaces hirtellous, laminar colleters absent. Inflorescences extra-axillary, sessile or pedunculate, 8–32-flowered; peduncle 0–1 cm, hirtellous, with 1 caducous bract at the base of each pedicel. Pedicels 10–20 mm, hirtellous. Flowers erect to ascending; calyx lobes lanceolate, 3.5–4 mm, apex acute, hirtellous; corolla green, sometime tinged red or brown, sometimes faintly striate, lobes reflexed, elliptic to linear-lanceolate, (7–)9–12 mm, apex acute, glabrous at tips abaxially or hirtellous throughout, glabrous adaxially; gynostegial column 1–1.5 mm; fused anthers brown, sometimes green proximally, obconic, 2–2.5 mm, wings triangular, widest at middle, closed, apical appendages ovate; corona segments green, sometimes tinged bronze, apex white or cream, fading yellow, sessile, sinuous-tubular, relatively slender, (5–)7–10 mm, greatly exceeding style apex, apex slightly flared, deeply emarginate, lobed on each side, minutely papillose to glabrate, internal appendage lingulate, sharply incurved, barely exserted, exceeded by segment margin, minutely papillose to glabrate. Follicles erect on upcurved pedicels, lance-ovoid, 4.5–9.5 × 1.2–2.5 cm, apex attenuate to acuminate, smooth, sometimes faintly striate, puberulent with curved trichomes to hirtellous. Seeds ovate to oval, 6–8 × 5–6 mm, margin winged, faces smooth; coma 2–3 cm.

Phenology: Flowering (Feb–)Mar–Nov; fruiting (Apr–)May–Nov.
Habitat: Coastal and inland dunes, salt flats, shell mounds, hills, slopes, ridges, arroyos, canyons, valleys, urban lots, ditches, limestone, sandstone, shale, basalt, volcanic ash, caliche, alluvium, sandy, clay, silty, gravel, rocky, and calcareous soils, thorn scrub, desert scrub, desert and mesquite grasslands, prairies, pastures, pinyon-juniper, juniper, oak, and riparian woodlands, pine flatwoods.
Elevation: 0–1900 m.

Distribution

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Ariz., Colo., La., N.Mex., Okla., Tex., Mexico, Central America.

Discussion

Asclepias oenotheroides is one of the most wide­spread American milkweeds, ranging from southeastern Colorado to Nicaragua. It is very common in southern and western Texas and throughout valleys and plains across Mexico. However, it is rare at the northern limit of its range in Colorado (known only from Las Animas County), where it is considered to be of conser­vation concern, Louisiana (known only from Jefferson Davis Parish), and Oklahoma. In the absence of flow­ers, it can be difficult to distinguish from its close rela­tives: A. nyctaginifolia, in southeastern Arizona and southwestern New Mexico, and A. emoryi in southern Texas. Compared to A. oenotheroides, the leaves of A. nyctaginifolia tend to be broader and more consis­tently ovate, whereas the leaves of A. emoryi tend to be narrower and more consistently elliptic or linear-lanceolate. However, A. oenotheroides is highly variable, and the overlap with these relatives is substantial. Even in flower, pressed specimens can be challenging to distinguish. The corona segments of A. nyctaginifolia are thicker and wider than those of A. oenotheroides, which is easily observed on fresh flowers but may be obscured by drying. Similarly, in fresh flowers the flared segment apex that exceeds the exserted appendage in A. oenotheroides is easily dis­tinguished from the seg­ment apex that is closed by the flush appendage in A. emoryi, yet this obvious distinction is frustratingly obscure in dried material. For most of the range of A. oenotheroides the length of the corona segments greatly exceeds that of A. emoryi. However, along the southern Texas coastal plain, and especially on the barrier islands, the length of corona segments of A. oenotheroides is shorter, overlapping slightly with A. emoryi. Such plants correspond to the type of Podostemma helleri. In addition, hybridization of A. oenotheroides with both A. emoryi and A. nyctaginifolia is suspected, based on a few, scattered specimens with intermediate floral morphology in the regions of overlap with each species.

Selected References

None.

Lower Taxa

None.