Chiricahua mountain tansy-aster , alkali aster

Aster riparius Kunth in A. von Humboldt et al., Nov. Gen. Sp. 4(fol.): 72. 1818; 4(qto): 92. 1820; Machaeranthera riparia (Kunth) A. G. Jones; M. sonorae (A. Gray) Stucky

Annuals, 25–60 cm; taproots slender to thick. Stems 1–12+, erect or ascending, straight and rigid, sometimes sprawling from base, branching from base and distally, glabrous, glaucous. Leaves mostly cauline; sessile (± appressed); blades oblanceolate, 20–30 × 3–5 mm, reduced distally, (thick, succulent) bases broadened, margins entire (incised to dentate in hybrids with A. parviflora) eciliate or with 1–8 cilia per side, apices linear, spiny-mucronate, glabrous; distal subulate, entire. Heads borne singly (terminal), often in loose, leafy, cymiform arrays. Involucres hemispheric, 10–12 × 10–16 mm (fresh). Phyllaries in 4–5 series, appressed, linear-lanceolate, 2–8 mm, bases whitish to tan, margins entire to laciniate, apices green, acute to acuminate, glabrous. Ray florets 50–80+; laminae bluish purple to lavender, 10–12 mm, coiled after flowering. Disc florets 45–100+; corollas yellow, 3.5–5 mm. Cypselae oblanceoloid, 2.5–3 mm, 11–13-nerved per face, faces sparsely sericeous; pappi: white, setose; ray 3–3.5 mm; disc 3–4 mm. 2n = 10.

Flowering Apr–Sep. Low valleys, saline soils on mudflats, edges of playas; 1300–1400 m; Ariz., N.Mex.; Mexico (Chihuahua, Coahuila, Durango, Zacatecas).

Arida riparia is a distinctive species recognized by its V -shaped branching, narrow, succulent, somewhat appressed, spiny-tipped leaves, and relatively large heads. It is known to hybridize with A. parviflora in southeastern Arizona and adjacent New Mexico. Specimens with dentate leaf margins usually represent hybrids.