Growth analysis of four Amaranthus species

A 2-yr field study was conducted to compare the growth of Amaranthus palmer i, A. rudis, A. retroflexus, and A. albus planted in June and July Rates of height increase (centimeters per growing degree day) were 0.21 and 0.18 fo r A. palmeri, 0.16 and 0.11 for A. rudis, 0.12 and 0.09 for A. retroflexus, and 0.08 and 0.09 for A. albus in 1994 and 1995, respectively, when plante d in June. A. palmeri had among the highest values for plant volume, dry we ight, and leaf area, while A. albus had the lowest. Specific leaf area valu es (cm(2) g(-1)) ranged from 149 to 261 for A. palmeri, 160 to 205 for A. r udis, 150 to 208 for A. retroflexus, and 127 to 190 for A. albus. Maximum r elative growth rates (g g(-1) day(-1)) for any measured period were 0.32 fo r A. palmeri, 0.31 for A. rudis, 0.30 for A. retroflexus, and 0.26 for A. a lbus. Recent increases in species range and observed changes in weed commun ity structure may be partially explained by the growth characteristics of A . palmeri and A. rudis. Herbicide rate and timing recommendations for mixed populations of these weeds should be based on A. palmeri because of its hi gh growth rates.