Shrubs, 30-150(-400) cm. Stems erect. Leaves mostly alternate; petioles 0; blades mostly filiform, 5-30(-65+) × 0.5-1.5 mm, sometimes with 3(-5+) filiform lobes, abaxial faces glabrous or glabrate, often vernicose, adaxial faces densely scabrellous (white). Pistillate heads ± intermixed with staminates; florets 1. Staminate heads: peduncles 0-0.5 mm; involucres ± cup-shaped, 2-4 mm diam., ± glabrate, ± vernicose; florets 5-12+. Burs: bodies fusiform to pyriform, 4-5 mm, stipitate-glandular, wings 7-12+, mostly around middles, oblanceolate to cuneiform, 2-3 × 1-2 mm. 2n = 36. Flowering (May-)Aug-Nov. Desert washes, ravines; 10-500 m; Ariz., Calif., N.Mex., Tex.; Mexico (Baja California, Baja California Sur, Chihuahua, Sinaloa, Sonora). Ambrosia monogyra may occur in western Nevada.

FNA 2006, Benson and Darrow 1981, Kearney and Peebles 1969

Common Name: singlewhorl burrobrush Duration: Perennial Nativity: Native Lifeform: Subshrub Wetland Status: UPL General: Slender shrub to 2.5 m, with multiple, slender, mostly erect stems branching above, those whitish, grayish, or light beige; new stems light green. Leaves: Alternate, sessile, sparse and drought-deciduous; blades linear, narrow, 0.5 mm wide, mostly 2-7 cm long, grooved above (involute), the grooves filled with short, white, elongate-conical hairs; young leaves of vigorous shoots often pinnately divided into several segments. Flowers: Pistillate and staminate flowers in separate heads, clustered in the leaf axils (clusters often containing both staminate and pistillate heads); all heads discoid; staminate heads 5-12 flowered, the involucres cup-shaped, 2-4 mm diameter; pistillate heads 1-flowered. Fruits: Fruiting bur spindle shaped, 4-5 mm long, with a distinctive whorl of papery bract wings around the middle, 4 mm. Ecology: Found on floodplains and along arroyos and washes from 1,000-4,000 ft (305-1219 m); flowers August-November. Distribution: AZ, NM, sw TX; south to c MEX. Notes: This 2-3 m tall shrub, which often grows in monospecific stands along low elevation dry washes, has a similar upright but arching growth form as young saltcedar (Tamarix chinensis) and can be confused for that species from a distance. Despite this similarity in structure, they can be distinguished from a distance by the stem color (red-brown for Tamarix and light gray-beige for A. monogyra). As one approaches, the identity of A. monogyra becomes obvious due to its sparse, linear-filiform leaves, and achenes clustered in the leaf axils, each achene surrounded by a skirt-like ring of papery bracts. A. salsola is also similar (the two species were formerly placed in the genus Hymenoclea together) but A. salsola is much shorter, less than 1 m tall, and the wings on the achenes are scattered or spirally arranged, while the wings on A. monogyra form an obviously single ring. A. monogyra thrives on disturbance created by occasional floods; its seeds are transportable by flood, making it an early successional floodplain species. Ethnobotany: Used as a remedy for abdominal pains; the Seri used the seeds for food. Etymology: Ambrosia is Greek for food of the gods; monogyra is from mono-, one and gyra, circle or revolution, referring to the single ring of bracts on the fruit. Synonyms: Hymenoclea monogyra Editor: SBuckley 2010, FSCoburn 2015, AHazelton 2015