Soil water use and growth of Russian thistle after wheat harvest

Russian thistle (Salsola iberica Sennen and Paul 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.) a nd other spring-sown crops, especially during drought. Quantitative informa tion on water use, biomass accumulation, and seed production of Russian thi stle after wheat harvest is lacking. In a 2-yr field study at Lind, Washing ton, Russian thistle plants were allowed to grow yearly in spring wheat in a grid pattern without competition from other weeds. Individual Russian thi stle 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 th istle for water at shallow soil depths; most water use by Russian thistle w as 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 O ctober, individual plants had produced 67 000 and 25 000 seeds in 1996 and 1997, respectively. In low crop residue situations, rapid post-harvest grow th by Russian thistle (before seed production) provides valuable surface co ver for erosion control, but with the prospect that soil water may be reduc ed for the subsequent crop.