Perennials, 20-80(-150+) cm. Stems erect. Leaves mostly alternate; petioles 10-35 mm; blades lanceolate to ovate, 40-85(-150) × 20-35(-55) mm overall, laciniately 2-4-pinnately lobed (lobes ± lanceolate); bases cuneate to truncate, ultimate margins entire, abaxial and adaxial faces strigillose to sericeous (often grayish) and gland-dotted. Pistillate heads clustered, proximal to staminates; florets 1(-2). Staminate heads: peduncles 0.5-2 mm; involucres cup-shaped, 1.5-3+ mm diam., strigillose; florets 5-20+. Burs: bodies pyramidal to pyriform, 1-2 mm, strigillose to pilosulous, spines (1-)5-12+, mostly distal, stoutly conic to subulate, 0.5-1 mm, tips uncinate. 2n = 72, 108.
Flowering (May-)Sep-Dec. Waste places, disturbed sites; 10-2000 m; Ariz., Calif., Colo., Kans., N.Mex., Okla., Tenn., Tex., Utah; Mexico.
Duration: Perennial
Nativity: Native
Lifeform: Forb/Herb
General: Perennial herb from a hard, knotty base, with stout, deeply buried, woody taproots; stems often 40-75 cm, erect, and leafy, with white, mostly appressed hairs.
Leaves: Mostly alternate, on petioles 1-3.5 cm long; blades green, 6-17 cm long, 2 or 3 times pinnately divided, strigillose to sericeous (often grayish) and gland-dotted.
Flowers: Pistillate and staminate flowers in separate heads; all heads discoid; staminate heads pendant in terminal racemes, with a few pistillate heads at the base of each raceme. Pistillate heads with 1 or 2 florets, developing into a single spiny bur at maturity. Staminate heads with cup-shaped involucres, 2-3+ mm in diameter, strigillose; containing 5-20+ disc flowers.
Fruits: Burs 2-4 mm with 5-12 small, terete, hooked spines.
Ecology: Found on hillsides, slopes, mesas, and sometimes a weed in fields and along roadsides from 1,000-6,500 ft (305-1981 m); flowers March-October.
Distribution: c and s CA, east to s KS, OK, TX; south to c MEX.; also in Australia.
Notes: This perennial herbaceous Ambrosia is especially common below the Mogollon Rim in Arizona. Look for the thinly lobed, 2-4 times dissected leaves; the racemes of pendant male flowering heads which can look yellowish inside due to the color of the stamens, and late in the season, small burs with 8-13 hooked spines near the bottom of the racemes. Ambrosia acanthacarpa is similar, but that species is annual and has larger burs (4-8 mm) with longer straight spines. Ambrosia psilostachya also appears similar, but that species is a perennial with running rootstock (as opposed to the woody taproot of this species), and the fruits usually do not have spines on them.
Ethnobotany: Unknown, but other species in this genus have many uses.
Etymology: Ambrosia is Greek for food of the gods, while confertiflora means with crowded flowers.
Synonyms: Franseria confertiflora, F. strigulosa, Gaertneria tenuifolia
Editor: SBuckley 2010, FSCoburn 2015, AHazelton 2015
