Smooth scouringrush is a deep-rooted, rhizomatous, herbaceous perennial that has been difficult to control in dryland wheat-producing regions in the Pacific Northwest. Long-term field experiments were conducted at Omak, Edwall, and Steptoe, Washington, to evaluate applications of chlorsulfuron + metsulfuron (21.9 + 4.4 g ai ha−1), triasulfuron (29.5 g ai ha−1), or MCPA ester (780 or 1,122 g ae ha−1). The Omak trial (2017–2021) included two 2-yr cycles of a winter wheat/chemical summer fallow rotation. The Edwall and Steptoe trials (2019–2025) included two 3-yr cycles of winter wheat/spring wheat/chemical summer fallow rotation. Six preassigned herbicide sequences tested the efficacy of 1) chlorsulfuron + metsulfuron applied during one or both chemical fallow phases and 2) triasulfuron alone applied to wheat. MCPA ester was applied as a check treatment. Smooth scouringrush stem density was evaluated in each plot each year. Chlorsulfuron + metsulfuron applied in the first chemical summer fallow phase maintained relatively low stem density in all three trials through the first rotation cycle compared with the check treatment. Chlorsulfuron + metsulfuron applied in the second chemical summer fallow phase extended control through the second rotation cycle. However, in the 3-yr rotation trials, chlorsulfuron + metsulfuron was not as effective in the second rotation cycle at Steptoe where stem densities averaged only 6 stems m−2 at time of the second application. Triasulfuron applied to wheat was not effective in controlling smooth scouringrush. Smooth scouringrush reduced wheat yield in the MCPA ester check sequence at Edwall, but no differences were observed at Steptoe or Omak. Chlorsulfuron + metsulfuron applied in chemical fallow is effective for long-term control of smooth scouringrush in wheat-based cropping systems. A subsequent application in chemical fallow can be effective if an adequate number of stems are present to facilitate foliar uptake.

Smooth scouringrush is a creeping perennial with a high silica content in stems that may impede herbicide uptake. Smooth scouringrush has become a troublesome weed in no-till cropping systems across eastern Washington. In previous field studies, glyphosate provided inconsistent control of smooth scouringrush. The objective of this study was to determine if the addition of an organosilicone surfactant to glyphosate would improve the efficacy and consistency of control through stomatal flooding. To test this hypothesis, glyphosate was applied at three field sites at 3.78 kg ae ha–1 alone, with an organosilicone surfactant (OS1 or OS2), an organosilicone plus nonionic surfactant blend, or an alcohol-based surfactant applied during the day or at night. Stem counts were recorded 1 yr after herbicide applications. Five of the six effective treatments observed across the three study sites included organosilicone surfactant or an organosilicone plus nonionic surfactant blend. At two sites, when there was a difference in efficacy between application times; daytime applications were more effective than nighttime applications. These results support the hypothesis of stomatal flooding as a likely mechanism for enhanced efficacy of glyphosate with the addition of an organosilicone surfactant. However, at one site, the treatments containing organosilicone surfactant were more efficacious when applied at night than during the day. At this site, high daytime temperatures and low relative humidity may have resulted in rapid evaporation of spray droplets. The addition of an organosilicone surfactant to glyphosate is recommended for smooth scouringrush control, and daytime treatments are preferred but should be applied when temperatures and humidity are not conducive to rapid droplet drying. Further research is necessary to confirm that stomatal flooding is responsible for improved glyphosate efficacy.

The adoption of chemical fallow rotations in Pacific Northwest dryland winter wheat production has caused a weed species composition shift in which scouringrush has established in production fields. Thus, there has been interest in identifying herbicides that effectively control scouringrush in winter wheat–chemical fallow cropping systems. Field experiments were established in growers’ fields near Reardan, WA, in 2014, and The Dalles, OR, in 2015. Ten herbicide treatments were applied to mowed and nonmowed plots during chemical fallow rotations. Scouringrush stem densities were quantified the following spring and after wheat harvest at both locations. Chlorsulfuron plus MCPA-ester resulted in nearly 100% control of scouringrush through wheat harvest. Before herbicide application, mowing had no effect on herbicide efficacy. We conclude chlorsulfuron plus MCPA-ester is a commercially acceptable treatment for smooth and intermediate scouringrush control in winter wheat–chemical fallow cropping systems; however, the lack of a positive yield response when scouringrushes were controlled should factor into management decisions.