Asclepias obovata
Asclepias subverticillata
pineland milkweed
horsetail milkweed, poison milkweed, western whorled or horsetail or poison or whorled milkweed
1 (rarely more), erect to spreading, unbranched, 40–70(–200) cm, densely hirtellous to velutinous, not glaucous, rhizomes absent.
1–8, erect, sometimes branched, few to many arrested vegetative branches usually present, 20–90 cm, puberulent with curved trichomes in a line to glabrate, not glaucous, rhizomatous.
opposite, petiolate, with 1 or 2 stipular colleters on each side of petiole;
petiole 1–4 mm, densely hirtellous to velutinous;
blade oblong or elliptic to obovate or ovate, 4–9 × 1–3.5 cm, subcoriaceous, base rounded or truncate to cordate, margins sometimes crisped, apex acute to truncate, sometimes emarginate, often mucronate, venation brochidodromous, surfaces densely hirtellous to velutinous abaxially, hirtellous adaxially, margins ciliate, 8–12 laminar colleters.
3–4-whorled, sometimes opposite on vegetative branches, sessile, with 1 stipular colleter on each side of leaf base on a ciliate interpetiolar ridge;
blade linear, 3–13 × 0.1–0.4 cm, chartaceous, base cuneate, margins entire, apex acute, mucronate, venation obscure, surfaces glabrous, margins ciliate or eciliate, laminar colleters absent.
extra-axillary, sometimes also appearing terminal, sessile or pedunculate, 7–31-flowered;
peduncle 0–0.5 cm, densely hirtellous to velutinous, with 1 caducous bract at the base of each pedicel.
extra-axillary, pedunculate, 9–25-flowered;
peduncle 0.7–3.5 cm, puberulent with curved trichomes on 1 side, with 1 caducous bract at the base of each pedicel.
10–12 mm, densely hirtellous to velutinous.
5–12 mm, puberulent with curved trichomes.
erect to pendent;
calyx lobes elliptic, 5–6 mm, apex acute, densely hirtellous;
corolla green, sometimes tinged reddish or bronze, lobes reflexed, sometimes with spreading tips, elliptic, 7–9 mm, apex acute, glabrous abaxially, minutely papillose at base adaxially;
gynostegial column 1–1.5 mm;
fused anthers green, obconic, 2.5–4 mm, wings right-triangular, open at base, apical appendages broadly ovate;
corona segments bronze to yellow, often tinged red, sometimes apically cream or pale, stipitate, tubular, somewhat flattened laterally, flared at base, 5–8 mm, greatly exceeding style apex, apex rounded, flared, glabrous, internal appendage falcate, exserted, sharply incurved over style apex, glabrous;
style apex shallowly depressed, green.
erect;
calyx lobes lanceolate, 2–2.5 mm, apex acute, puberulent with curved trichomes to glabrate;
corolla pale green to cream, sometimes pink- or tan-tinged, lobes reflexed with spreading tips, elliptic, 3.5–4.5 mm, apex acute, glabrous abaxially, minutely papillose at base adaxially;
gynostegial column 0.8–1.2 mm;
fused anthers green, columnar, 1.5–2 mm, wings narrowly right-triangular, closed, apical appendages deltoid;
corona segments cream, sometimes green- or pink-tinged or striped, stipitate, cupulate, dorsally flattened, 1.5–2 mm, exceeded by style apex, margins entire, apex obtuse, glabrous, internal appendage acicular, exserted, arching over style apex, glabrous;
style apex shallowly depressed, green to greenish cream.
broadly ovate, 8–9 × 6–7 mm, margin winged, faces smooth;
coma 2–5 cm.
ovate, 5–8 × 3.5–5 mm, margin winged, faces smooth;
coma 2–2.5 cm.
erect on upcurved pedicels, narrowly to broadly fusiform, 7.5–12.5 × 1.5–2.5 cm, apex acuminate, smooth, densely hirtellous to velutinous.
erect on straight pedicels, narrowly fusiform, 6–8.5 × 0.5–0.9 cm, apex acuminate to attenuate, smooth, minutely puberulent with curved trichomes to glabrate.
Asclepias obovata
Asclepias subverticillata
Asclepias obovata is a common milkweed of seasonally wet, sandy soils in pine woodlands of the Gulf Coastal Plain and (rarely) the southern Atlantic Coastal Plain. It is rare and considered to be of conservation concern in Arkansas.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
Asclepias subverticillata and A. verticillata are amply distinct away from the zone of contact from Texas to Montana. The most reliable characters for distinguishing these species are the absence of a marginal corona segment tooth and presence of arrested vegetative branches in A. subverticillata. The greater frequency of multistemmed plants and completely glabrous leaves are also characteristic of A. subverticillata. However, absence of vegetative branches in A. subverticillata is common, especially in young or poorly developed plants. It can be difficult to confidently identify incomplete specimens or immature plants in the narrow zone of parapatry. Gene flow between the species has not been investigated, and the muddy species boundaries could be attributable to past or ongoing introgressive hybridization. The identity of some populations in New Mexico, the Texas Panhandle, and western Kansas have been debated, and further study is needed to determine whether they belong to one of the parental species or are advanced generation hybrids. Asclepias subverticillata barely enters Kansas (Grant, Hamilton, Morton, Seward, Stanton, and Stevens counties) and Oklahoma (Beaver and Cimarron counties). It also appears to be rare at the northeastern end of its range in Wyoming (Carbon County), where it is thought to be extirpated. There are few occurrences of A. subverticillata disjunct from the main range and within the range of A. verticillata. There is a single, adventive population along a railroad in St. Louis, Missouri, documented in 1962 and last observed in 1970 (V. Mühlenbach 1979); it is unknown whether this population persists. Asclepias subverticillata hybridizes with A. pumila. These hybrids are usually readily detected because the parental species are distinct in leaf arrangement and internode length (whorled and distant nodes in A. subverticillata versus alternate and congested in A. pumila). Such hybrids often have mixed phyllotaxy and have been documented in northern New Mexico. Reports of A. subverticillata from Idaho are based on misidentifications of A. fascicularis and are discussed under that species. Searches for A. subverticillata in southeastern Idaho have documented only A. fascicularis in that region (Lynn Kinter, Idaho Game and Fish, pers. comm.). Like other southwestern milkweed species with cream flowers (for example, A. nyctaginifolia, A. subulata), tarantula hawk wasps (Pompilidae, Pepsinae) are avid floral visitors to A. subverticillata, in spite of tiny flowers presenting minute quantities of nectar.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
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