For more information on weed identification in the northeastern United States:
Newcomb's Wildflower Guide, Lawrence Newcomb, Little, Brown, and Company, 1977.
The areas covered extends to the Mississippi River and south to Tennessee and northern North Carolina. Available in soft and hard cover. An excellent field manual.
Weeds of the Northeast, Uva, Neal, and DiTomaso, Cornell Univ. Press, 1997.
Phone: 607/277-2211. Excellent color photographs.
Manual of Vascular Plants of Northeastern United States and Adjacent Canada, Gleason and Cronquist, The New York Botanical Garden, 1991.
Both of these authors are now deceased. The areas covered extends westward to the Mississippi River and south to Virginia and Kentucky.
Illustrated Companion to Gleason and Cronquist's Manual, Noel H. Holmgren, The New York Botanical Garden, 1998.