Helianthus ×laetiflorus is introduced in Ontario, Newfoundland, and probably in Quebec; it was excluded from British Columbia by G. W. Douglas et al. (1989-1994, vol. 1); and it was reported in New Brunswick by H. R. Hinds (2000).
Plants called Helianthus ×laetiflorus are usually interpreted to be hybrids and backcrosses of H. tuberosus and H. pauciflorus; they are widely cultivated and often escape. An alternative treatment has been to use the name H. laetiflorus for plants that are treated here as H. pauciflorus, sometimes with infraspecific taxa.
Helianthus atrorubens Lamarck 1789, not Linnaeus 1753, has been applied to plants here called H. ×laetiflorus.
Much like H. pauciflorus var. pauciflorus, but with yellow disk; lvs often larger and with much longer (to 5 cm) petiole; invol bracts avg a little narrower, generally (2-) 2.5-4 mm wide, less imbricate, more pointed, and occasionally sparsely short-hairy on the back; 2n=102. Roadsides and other disturbed sites, mainly or wholly as an escape from cult., often ±sterile; scattered in e. and midwestern U.S. Aug., Sept. Thought to consist of hybrids and hybrid progeny of H. pauciflorus var. subrhomboideus and H. tuberosus, perhaps mainly a cultigen.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.