Annuals, 10-30(-100+) cm. Stems erect. Leaves mostly opposite; petioles 0-0.5 mm; blades ± lanceolate to lance-linear, 15-40+ × 3-6(-10+) mm, bases rounded to cordate, margins entire or with (1-)2(-4) basal lobes, abaxial and adaxial faces ± piloso-hispid and gland-dotted. Pistillate heads clustered, proximal to staminates; florets 1. Staminate heads: peduncles 0-0.5 mm; involucres obliquely cup-shaped (lateral lobe longer than others), 2.5-4 mm diam., piloso-hispid; florets 6-8+. Burs: bodies pyramidal, 5-8 mm, piloso-hispid, spines 4(-5), ± distal, ± acerose, 0.5-1 mm, tips straight. Flowering Jul-Oct. Dry, disturbed sites; 200-500 m; Ala., Ark., Conn., D.C., Ga., Ill., Ind., Iowa, Kans., Ky., La., Md., Minn., Miss., Mo., N.C., Ohio, Okla., Tenn., Tex., Va., W.Va. Hybrids between Ambrosia bidentata and A. trifida have been recorded.
From Flora of Indiana (1940) by Charles C. Deam
Generally found in hard, white clay soil in low land in fallow fields, in open woodland, and along roadsides. Pioneers have told me that they did not note this species until the past ten years. Blatchley reports that it was first noted in 1895. Schneck in 1876 reports it as "common in prairies." This western species is slowly migrating eastward. Where it is found, it usually forms dense stands. I was told by a farmer that stock will not eat it, although they will eat other species of ragweed. It is restricted to the southwestern part of the state. There are records of its occurrence in Clay and Vigo Counties.
Indiana Coefficient of Conservatism: C = null, non-native
Wetland Indicator Status: N/A
Diagnostic Traits: Annual; leaves alternate, lanceolate, their bases partially clasping; pollen bearing heads in spike- or raceme-like arrays; fruits in upper leaf axiles, 4-5 mm long, with 4 stout spines.
Branching annual weed 3-10 dm, the stem conspicuously spreading-hirsute, especially above; lvs numerous, sessile, opposite below, alternate above, lanceolate, acuminate, 2.5-7 cm נ4-10 mm, usually with a single pair of sharp large teeth below the middle, hirsute to scabrous, or the upper side glabrous; staminate heads sessile, the invol strongly oblique, its upper side produced into a conspicuous, retrorsely spreading, hispid-hirsute, lanceolate to triangular- ovate lobe, the bractless infl thus appearing retrorse-bracteate; fruiting invol 5-8 mm, villous-hirsute, several-ribbed, the ribs produced into short, stiff spines; 2n=34. Prairies and waste places; O. and Ky. to Nebr., La., and Tex., and occasionally intr. elsewhere. July-Oct.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.