Soil Water Use and Growth of Russian Thistle after Wheat Harvest

Abstract

Russian thistle (Salsola iberica Sennen and Pau) is a major broadleaf weed in dryland crops (<300 mm annual precipitation) in the Pacific Northwest of the USA. Russian thistle frequently infests wheat (Triticum aestivum L.) and other spring‐sown crops, especially during drought. Quantitative information on water use, biomass accumulation, and seed production of Russian thistle after wheat harvest is lacking. In a 2‐yr field study at Lind, Washington, Russian thistle plants were allowed to grow yearly in spring wheat in a grid pattern without competition from other weeds. Individual Russian thistle plants used 70 L of soil water while growing with the crop. From wheat harvest in early August until killing frost in late October, each Russian thistle used an additional 100 L of soil water. Water use occurred within a 1.5‐m radius of the Russian thistle. Spring wheat competed with Russian thistle for water at shallow soil depths; most water use by Russian thistle was from deeper than 1.0 m. Russian thistle dry weight increased from 170 to 1280 g per plant between grain harvest and killing frost. Russian thistle seeds were either not produced or germinable until mid‐September. By late October, individual plants had produced 67000 and 25000 seeds in 1996 and 1997, respectively. In low crop residue situations, rapid post‐harvest growth by Russian thistle (before seed production) provides valuable surface cover for erosion control, but with the prospect that soil water may be reduced for the subsequent crop.