Annual, smooth or nearly so, 2-5 dm, the upper internodes progressively longer; cauline lvs spatulate, 1.5-5 cm, very blunt or retuse, finely and sharply serrulate; lvs subtending the primary umbel somewhat oblique, broadly elliptic to obovate; rays of the primary umbel 5, on well grown plants repeatedly branched; involucres 2 mm; fr smooth, 3 mm; seeds ovoid, 2-2.5 mm, conspicuously areolate; 2n=42. Native of Europe; intr. abundantly from e. Kan. to w. Ont., n. Mich., and N.Y., and occasionally elsewhere. Apr.-Sept. (Tithymalus h.; Galarhoeus h.)
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.