Seed samples of triazine-resistant and susceptible pigweed were received from the four areas of the United States and Canada where resistant pigweed populations have appeared. The reported resistance or susceptibility of all biotypes was verified, and several plants from each seed source were grown to maturity for species identification. Pigweed samples from Maryland and Pennsylvania were smooth pigweed (Amaranthus hybridus L.). One resistant and one susceptible biotype from Washington were also smooth pigweed. All other samples from Washington and the sample from Ontario were Powell amaranth (A. powellii S. Wats.). None of the triazine-resistant seed samples were redroot pigweed (A. retroflexus L.), despite earlier reports of resistance in this species. Taxonomic characteristics important in the identification of smooth pigweed, Powell amaranth, and redroot pigweed are discussed.
