Annuals (perhaps persisting), (15-)30-70(-100+) cm. Leaves on proximal 1/2-3/4 of each stem; blades of undivided cauline leaves usually ± oblong, sometimes ob-ovate to lanceolate, margins denticulate, usually prickly, midribs usually prickly-setose, rarely smooth. Heads in pani-culiform arrays. Involucres 9-10(-12) mm. Phyllaries usually reflexed in fruit. Florets 12-20; corollas yellow, usually deliquescent. Cypselae: bodies pale grayish to tan, ± flattened, oblanceolate, 2.5-3.5 mm, beaks ± filiform, 2.5-4 mm, faces (3-)5-9-nerved; pappi white, (3-)4-5 mm. 2n = 18. Flowering (May-)Jul-Sep(-Oct). Roadsides, disturbed sites; 10-2300 m; introduced; Alta., B.C., Man., N.B., N.S., Ont., P.E.I., Que., Sask.; Ala., Ariz., Ark., Calif., Colo., Conn., Del., D.C., Fla., Ga., Idaho, Ill., Ind., Iowa, Kans., Ky., La., Maine, Md., Mass., Mich., Minn., Miss., Mo., Mont., Nebr., Nev., N.H., N.J., N.Mex., N.Y., N.C., N.Dak., Ohio, Okla., Oreg., Pa., R.I., S.C., S.Dak., Tenn., Tex., Utah, Vt., Va., Wash., W.Va., Wis., Wyo.; Europe; also introduced nearly worldwide. The name Lactuca scariola Linnaeus is evidently illegitimate; it is a superfluous name based on the same type as L. serriola. Plants included here in L. serriola with most leaf blades obovate to lanceolate (not lobed) and lacking prickles on midribs have been called L. scariola subsp. or var. integrata or L. serriola forma integrifolia or L. serriola var. integrata.
Leafy-stemmed annual or biennial, 3-15 dm, the stem often prickly below, otherwise glabrous; lvs prickly on the midrib beneath, and more finely prickly-toothed on the margins, otherwise generally glabrous, pinnately lobed or lobeless, commonly twisted at base to stand vertically, sagittate-clasping, oblong or oblong-lanceolate in outline, the upper much reduced; heads numerous in a long, often diffuse infl, with (13-)18-24(-27) light yellow fls, drying blue; invol 10-15 mm in fr; achenes gray or yellowish-gray, the body compressed, 3-4 mm, a third as wide, prominently several-nerved on each face, spinulose or hispidulous above at least marginally, the slender beak about equaling the body, rarely twice as long; 2n=18. A weed in fields and waste places; native of Europe, now naturalized throughout most of the U.S. July-Sept. The common form with lobeless lvs, called var. integrata Gren. & Gordon, may reflect introgression from L. sativa L., cultivated lettuce, with which L. serriola hybridizes freely. (L. scariola)
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.