Shrubs, 30-80+ cm. Stems erect. Leaves mostly alternate; petioles 5-12+ mm; blades deltate to lance-deltate, 12-25(-35) × 5-12(-18) mm, bases cuneate to truncate, margins toothed, abaxial faces ± densely tomentulose (between veins), adaxial faces sparsely tomentulose, glabrescent. Pistillate heads clustered, proximal to staminates; florets 2-3. Staminate heads: peduncles 0.5-3(-5) mm; involucres ± cup-shaped, 4-8 mm diam., tomentulose; florets 12-30+. Burs: bodies ± fusiform to globose, 3-4 mm, usually stipitate-glandular (little, if at all, tomentulose), spines 20-30+, scattered or on distal ¹/2, ± subulate (the proximal basally flattened), 1-3 mm, tips usually straight, sometimes uncinate. 2n = 36.

Flowering Feb-May. Sandy washes, benches; 200-1000 m; Ariz.; Mexico (Baja California, Sonora).

Some specimens are intermediate for the char­-acteristics that distinguish Ambrosia deltoidea and A. chenopodiifolia. The type of A. deltoidea may be better treated as conspecific with that of A. chenopodiifolia.

Common Name: triangle bur ragweed

Duration: Perennial

Nativity: Native

Lifeform: Subshrub

General: Shrub with numerous ascending to erect stems 30-80 cm from a woody base; rounded or flat topped; dark brown branches, ridged, strongly resinous.

Leaves: Numerous, mostly alternate, on petioles 5-12 mm, blades deltate to lance-deltate, 12-25 mm long by 5-12 mm wide, cuneate to truncate bases, toothed margins, densely tomentose below, white or pale; above sparsely tomentulose, dark green.

Flowers: Pistillate heads clustered, 2-3 florets, in terminal racemes or panicles; staminate heads crowded on peduncles 0.5-3 mm, more or less cup shaped involucres, 4-8 mm in diameter, tomentulose; 12-30 florets.

Fruits: Burs broadly ellipsoidal to globose, 3-6 mm, usually stipitate-glandular, spines 15-30, scattered, 1-3 mm, tips straight, sometimes uncinate, distinctly flattened.

Ecology: Found in sandy washes, on alluvial plains, on gravelly or rocky slopes from 1,000-3,000 ft (305-914 m); flowers December-May.

Notes: Fruiting heads resemble cockleburs, only the spines are strongly flattened with plane of leaves. Abundant shrub among Parkinsonia and Prosopis in the Sonoran desert scrub communities. This species is often considered the dominant bursage of the Arizona Upland, while A. dumosa is found in the lower Colorado and Mohavean types. A. deltoidea is often found on the moister margins of gullies and other surface water features, while A. dumosa is confined to finer and drier soils.

Ethnobotany: Unknown, but other species in the genera have many uses.

Etymology: Ambrosia is Greek for food of the gods, while deltoidea means triangular, like the fourth letter of the Greek alphabet, delta.

Synonyms: Franseria deltoidea

Editor: SBuckley, 2010