Biennials (sometimes flowering first year or persisting), 10-30(-60+) cm. Leaf blades ovate or rounded-deltate to elliptic, 30-80(-120+) × 10-20(-40+) mm, usually ± pinnately or sub-bipinnately lobed, ultimate margins entire, faces strigillose, usually also with erect hairs 1-2 mm and gland-dotted. Heads disciform or obscurely radiate, borne in open, ± paniculiform arrays. Peduncles 2-8(-12+) mm. Phyllaries: outer 5 lance-ovate to elliptic, 2.5-3+ mm, inner 5 ± orbiculate, 3-3.5 mm. Pistillate florets 5; corolla laminae 0 or ± coroniform, 0.1-0.5 mm. Disc florets 20-30+. Cypselae ± obovoid, 2-3 mm; pappus-like enations 2, ± erect, deltate to ovate, 0.5-1 mm (sometimes a third, subulate spur near apex adaxially). 2n = 36, 68, 72. Flowering Mar-Oct. Sandy plains, openings in mesquite grasslands; 20-2000 m; Ariz., N.Mex., Tex.; Mexico.
Allred and Ivey 2012, Correll and Johnston 1970, Kearney and Peebles 1960, FNA 2006
Duration: Biennial Nativity: Native Lifeform: Forb/Herb General: Biennial or perennial herbs, sometimes flowering first year, 10-60 cm tall; stems one to few from the base, branching above, longitudinally striate and densely hirsute with long hairs. Leaves: Alternate along the stems, the lower leaves subpetiolar and upper leaves sessile; blades ovate or rounded-deltate to elliptic in outline, 3-12 cm long and 2-4 cm wide, pinnately or sub-bipinnately lobed, the ultimate margins entire; leaf surfaces strigillose, usually also gland-dotted and with erect hairs 1-2 mm long. Flowers: Flower heads disciform or obscurely radiate, small, white and inconspicuous, borne in open terminal panicles, on peduncles 2-12 mm long; involucres broadly cup-shaped, the bracts (phyllaries) in 2 series: outer series of 5 lance-ovate to elliptic phyllaries, 3 mm long; inner series of 5 orbiculate phyllaries, 3 mm long; pistillate (ray) florets 5 per flower head, the laminae (ray petals) white, less than 0.5 mm long or lacking; disc florets 20-30 per flower head, white. Fruits: Achenes obovoid, 2-3 mm long; topped with a pappus of 2 scales, these less than 1 mm long. Ecology: Found on sandy plains, in openings in mesquite grasslands, pinyon-juniper woodlands, desertscrub, and canyon bottoms, below 6,000 ft (1829 m); flowers March-October. Distribution: se AZ, s NM, and sw TX; south to nw MEX. Notes: P. confertum looks like a smaller, herbaceous version of the common shrub Parthenium incanum. Look for the lobed gray-colored leaves, the striate stems covered with long hairs, and the terminal panicle of small white flower heads that may or may not have a few minute ray petals. Material from Arizona and New Mexico is traditionally assigned to var. lyrata, although the Flora of North America does not distinguish among the different varieties. Ethnobotany: Unknown; however, other species in the genus have uses. Etymology: Parthenium is from the Greek parthenos, virginal or pure, alluding to the white flowers; confertum is Latin for crowded, probably referring to the crowded sessile leaves on the middle and upper stems. Synonyms: Parthenium lyratum Editor: AHazelton 2017