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narrow-leaf milkweed, slimleaf milkweed

Arizona milkweed, narrow-leaf milkweed

Habit Herbs. Subshrubs or herbs, cespitose.
Stems

1 or 2 (rarely more), erect to spreading, rarely branched, 15–85 cm, puberulent with curved trichomes, not glaucous, rhizomes absent.

few–numerous, erect, sparsely to moderately branched, 20–60 cm, puberulent with curved trichomes in a line, not glaucous, rhizomes absent.

Leaves

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.

persistent or gradually caducous from the base, opposite, petiolate, with 1 stipular colleter on each side of petiole on a ciliate interpetiolar ridge;

petiole 2–4 mm, surfaces puberulent with curved trichomes to glabrate;

blade linear, 3.5–10 × 0.2–0.8 cm, chartaceous, base cuneate, margins entire, apex acute, venation faintly brochidodromous to obscure, puberulent with curved trichomes on midvein, margins ciliate, laminar colleters absent.

Inflorescences

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 at upper nodes, pedunculate, 3–15-flowered;

peduncle 0.7–5 cm, puberulent with curved trichomes in a line, with 1 caducous bract at the base of each pedicel.

Pedicels

5–12 mm, puberulent with curved trichomes to pilosulous.

8–18 mm, puberulent with curved trichomes in a line.

Flowers

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 narrowly lanceolate, 2–3 mm, apex acute, puberulent with curved trichomes;

corolla cream, sometimes pink-striped or -tinged, to pink, lobes reflexed with spreading tips, elliptic, 4–5 mm, apex acute, glabrous abaxially, minutely papillose at base adaxially;

gynostegial column 0.8–1 mm;

fused anthers brown, columnar, 1.5–2 mm, wings narrowly right-triangular, closed, apical appendages deltoid;

corona segments cream, sometimes pink at base, stipitate, tubular, dorsally rounded, 3–3.5 mm, exceeding style apex, apex obtuse, glabrous, internal appendage acicular, exserted, arching over style apex, glabrous;

style apex shallowly depressed, cream.

Seeds

ovate, 6–7 × 4–5 mm, margin winged, faces minutely rugulose;

coma 2.5–3 cm.

ovate, 4.5–6 × 3–4 mm, margin narrowly winged, faces smooth;

coma 2–3 cm.

Follicles

erect on straight pedicels, fusiform, 9–13 × 1–1.2 cm, apex long-acuminate, smooth, pilosulous.

erect on straight pedicels, narrowly fusiform, 4–10 × 0.4–0.7 cm, apex long-acuminate to attenuate, smooth, glabrous.

Asclepias stenophylla

Asclepias angustifolia

Phenology Flowering May–Aug; fruiting (Jun–)Aug–Oct. Flowering Apr–Oct; fruiting Jul–Oct.
Habitat Hills, ridges, bluffs, slopes, flats, glades, sandhills, stream­sides, limestone, dolomite, rhyolite, sandy and clay soils, prairies, pastures, thickets, forest openings, pine savannas. Canyon bot­toms, streamsides, arroyos, sandy or rocky soils, ripar­ian forest, pine-oak-juniper forest.
Elevation 70–1900 m. (200–6200 ft.) 1000–1800 m. (3300–5900 ft.)
Distribution
from FNA
AR; CO; IA; IL; KS; LA; MN; MO; MT; NE; OK; SD; TX; WY
[WildflowerSearch map]
[BONAP county map]
from FNA
AZ; Mexico
[BONAP county map]
Discussion

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.)

The common name Arizona milkweed is misleading, as Asclepias angustifolia is narrowly distributed and uncommon in Arizona but widely distributed and locally common across mountainous regions of Mexico. Like A. linaria, this widespread species is probably relictual in its canyon habitat in southern Arizona. Asclepias angustifolia has been documented from the Atascosa, Dragoon, Huachuca, Pajarito, Rincon, Santa Catalina, and Santa Rita mountains (Cochise, Pima, and Santa Cruz counties). As with many milkweeds, the floral displays of well-developed plants are magnets for large numbers of Lepidoptera and Hymenoptera. Like A. curassavica and other members of the Incarnatae clade (species 7–16), A. angustifolia develops rapidly from seed and transplants well, making it a desirable species for the garden. Although its frost hardiness is limited, it can be used as a horticultural annual.

(Discussion copyrighted by Flora of North America; reprinted with permission.)

Source FNA vol. 14. FNA vol. 14.
Parent taxa Apocynaceae > Asclepias Apocynaceae > Asclepias
Sibling taxa
A. albicans, A. amplexicaulis, A. angustifolia, A. arenaria, A. asperula, A. brachystephana, A. californica, A. cinerea, A. connivens, A. cordifolia, A. cryptoceras, A. curassavica, A. curtissii, A. cutleri, A. eastwoodiana, A. elata, A. emoryi, A. engelmanniana, A. eriocarpa, A. erosa, A. exaltata, A. fascicularis, A. feayi, A. hallii, A. hirtella, A. humistrata, A. hypoleuca, A. incarnata, A. involucrata, A. labriformis, A. lanceolata, A. lanuginosa, A. latifolia, A. lemmonii, A. linaria, A. linearis, A. longifolia, A. macrosperma, A. macrotis, A. meadii, A. michauxii, A. nummularia, A. nyctaginifolia, A. obovata, A. oenotheroides, A. ovalifolia, A. pedicellata, A. perennis, A. prostrata, A. pumila, A. purpurascens, A. quadrifolia, A. quinquedentata, A. rubra, A. rusbyi, A. ruthiae, A. sanjuanensis, A. scaposa, A. solanoana, A. speciosa, A. sperryi, A. subulata, A. subverticillata, A. sullivantii, A. syriaca, A. texana, A. tomentosa, A. tuberosa, A. uncialis, A. variegata, A. verticillata, A. vestita, A. viridiflora, A. viridis, A. viridula, A. welshii
A. albicans, A. amplexicaulis, A. arenaria, A. asperula, A. brachystephana, A. californica, A. cinerea, A. connivens, A. cordifolia, A. cryptoceras, A. curassavica, A. curtissii, A. cutleri, A. eastwoodiana, A. elata, A. emoryi, A. engelmanniana, A. eriocarpa, A. erosa, A. exaltata, A. fascicularis, A. feayi, A. hallii, A. hirtella, A. humistrata, A. hypoleuca, A. incarnata, A. involucrata, A. labriformis, A. lanceolata, A. lanuginosa, A. latifolia, A. lemmonii, A. linaria, A. linearis, A. longifolia, A. macrosperma, A. macrotis, A. meadii, A. michauxii, A. nummularia, A. nyctaginifolia, A. obovata, A. oenotheroides, A. ovalifolia, A. pedicellata, A. perennis, A. prostrata, A. pumila, A. purpurascens, A. quadrifolia, A. quinquedentata, A. rubra, A. rusbyi, A. ruthiae, A. sanjuanensis, A. scaposa, A. solanoana, A. speciosa, A. sperryi, A. stenophylla, A. subulata, A. subverticillata, A. sullivantii, A. syriaca, A. texana, A. tomentosa, A. tuberosa, A. uncialis, A. variegata, A. verticillata, A. vestita, A. viridiflora, A. viridis, A. viridula, A. welshii
Synonyms Polyotus angustifolius, Acerates angustifolia A. linifolia
Name authority A. Gray: Proc. Amer. Acad. Arts 12: 72. (1876) Schweigger: Enum. Pl. Hort. Regiom., 13. (1812)
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