Ambrosia tomentosa

Perennials, 10-30+ cm. Stems erect. Leaves mostly alternate; petioles 8-25(-35+) mm; blades elliptic to lance-elliptic, 50-80(-180+) × 12-20(-50+) mm overall, (1-)2(-3)-pinnately lobed (lobes ± deltate), bases cuneate, ultimate margins with ± deltate teeth, abaxial faces densely finely scabrellous (silvery gray), adaxial faces strigillose. Pistillate heads clustered, proximal to staminates; florets (1-)2. Staminate heads: peduncles 2-9 mm; involucres saucer-shaped (with black nerves), 3-6+ mm diam., strigillose; florets 25-40+. Burs: bodies ± pyriform, somewhat flattened, 1.5-2.5(-4) mm, finely scabrellous, spines (1-)8-12+, scattered, stoutly conic, 0.5-1+ mm, tips straight.

Flowering Jun-Sep. Seasonally wet clays or sands, disturbed sites; (200-)1200-2100 m; Ariz., Colo., Idaho, Ill., Iowa, Kans., Mo., Mont., Nebr., N.Mex., N.Dak., S.Dak., Tex., Utah, Wis., Wyo.

Colonial perennial from creeping roots; lvs all or nearly all alternate, densely and conspicuously short-white-hairy beneath, greener and less hairy above, openly subbipinnatifid to subtripinnatifid with short and broad ultimate segments, the slender midstrip beset with small segments between the major ones; fruiting invols 4-6 mm, 2-seeded, with 2 sharp beaks and to 10 short spines in one or more cycles. Plains sp., casually intr. as far e. as Wis. and Ill. May-Aug. (Franseria discolor; Gaertneria d. not Franseria tomentosa, properly A. grayi (A. Nelson) Shinners, another Great Plains sp., with less dissected lvs that have a large terminal segment)

Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.

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