Abstract
The frequency of occurrence of Amaranthus powellii and A. hybridus relative to that of A. retroflexus in agricultural fields in southwestern Ontario is increasing. Rates of germination and growth and development of the three species were compared at 28/22 °C and 22/14 °C in the growth chamber. Amaranthus powellii exhibited a faster rate of germination and greater early growth with respect to height and leaf number than either of the other species under both temperature regimes. Amaranthus hybridus had a greater root dry weight, took 3 wk longer to reach maturity and produced more seeds than either of the other species. In the field, when sown in pure stands and mixtures at constant density but varying proportions, A. powellii showed a greater competitive ability than A. retroflexus or A. hybridus with respect to number of plants, aboveground fresh weight, and number of seeds produced per square metre.Key words: Pigweed, germination, seed production, competition
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