Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/1/1999
Publication Date: N/A
Interpretive Summary: Russian thistle is a summer-annual broadleaf weed commonly found in many of the low precipitation cropland areas of the Pacific Northwest. This weed causes serious production problems during summer fallow, in the crop, and after harvest. Russian thistle is especially troublesome in poor stands of winter wheat and in spring planted crops. With renewed interest in continuous no-till spring cropping systems, researchers and growers must b aware of the deleterious effects of Russian thistle growing in the fall between planting spring crops. Until recently, Russian thistle water usage had only been estimated & time of viable weed seed production was unknown. In a two-year field study, each Russian thistle plant removed an average of 18 gallons of water while competing with the spring wheat crop (mid-April to early August). In addition, a single Russian thistle plant used 26 more gallons of soil moisture from crop harvest to killing frost (October). A Russian thistle plant extracted water from the soil in a 5-foot radius and a 6-foot depth. Even though Russian thistle began rapid biomass production soon after crop harvest, seeds did not become viable until 4 to 6 weeks later (mid-September). By killing frost, an individual weed produced from 25,000 to 67,000 seeds, which would replenish the weed seed soil bank. If allowed to grow undisturbed from wheat harvest to killing frost, Russian thistle plants could interfere with next year's tillage or planting operations and remove more water than could be replenished by over-winter precipitation. Russian thistle could, however, be allowed to grow for a short period of time after harvest before viable seed are produced. This would allow growers to use Russian thistle residue to help meet conservation compliance.
Technical Abstract: Russian thistle (Salsola iberica) is a major broadleaf weed in the low- precipitation (less than 300 mm annual) dryland crop production region of the inland Pacific Northwest of the USA. Russian thistle (RT) infestation is frequently acute when stand establishment of winter wheat (Triticum aestivum L.) is poor, during drought years, and in spring-planted crops. After grain harvest, Russian thistle produces substantial dry matter and seed, and extracts soil water until killed with herbicides, tillage, or by frost. In a 2-yr study at Lind, Washington, RT plants were allowed to grow without competition from other weeds from April until hard frost in October. After wheat harvest in early August, volumetric water content was measured at 13d. intervals to a soil depth of 1.8 m at distances of 0.3, 0.6, 0.9, 1.5, and 3.0 m from the base of six target RT. On every measurement date, six RT plants were collected to determine dry biomass, seeds per plant, and percent viable seeds. Individual plants used 70 L of soil water while growing with the crop. From harvest to late October each RT used an additional 100 L of soil water. Water was used within a 1.5-m radius of the RT plant. Spring wheat was competitive with RT for water at shallow soil depths; most water use by RT was from deeper than 1.0 m. Dry RT biomass increased on average from 170 to 1280 g per plant between grain harvest and frost. Production of viable RT seeds did not begin until mid- September during either year. By the time of killing frost in late October, individual plants produced 67,000 and 25,000 seeds in 1996 and 1997, respectively. Russian thistle must be managed post-harvest to conserve soil water, prevent viable seed production, and optimize potential for subsequent crops.