1. Range Plant Handbook;United States Department of Agriculture, Forest Service,1988
2. Flowers of Europe: A Field Guide;Polunin,1969
3. Weeds of the Northern U.S. and Canada;Royer,1999
4. Hayfever Plants;Wodehouse,1971
5. North American pollinosis due to insect pollinated plants;Lewis;Ann Allergy,1979