Smooth Scouringrush Research

Shortly after arriving at WSU in the fall of 2012, a farmer near Rearden took me for a drive to show me numerous no-till fields infested with smooth scouringrush (Equisetum laevigatum).  Smooth scouringrush is a member of a prehistoric group of plants whose ancient relatives date back approximately 350 million years, nearly 200 million years before the appearance of modern flowering plants, and 300 million years before the evolution of grass plants. It is native to North America and is often found near streams and along roads where water collects.  Plants are deep-rooted and spread mainly by rhizomes (underground stems), but they also produce asexual spores. Stems are leafless and contain a high concentration of silica. Fertile stems have a spore-producing structure at the tip.

What was surprising to me as we drove around the Rearden area was that these plants were not only growing in low, wet areas of the fields, they were also growing up and over apparently dry hillsides. Since my introduction to this problem in the Rearden area, I have had many enquiries from across Eastern Washington on how to control this weed. As a result, we have initiated a number of field trials to develop some solutions. Although the answer is elusive, we are gaining some insights into this very intriguing weed problem.

Scouringrush in a hand.
Our first field study was initiated near Rearden in July of 2014 to evaluate several herbicide options for the control of smooth scouringrush. The results from this study are reported in the 2015 and 2016 WSU Weed Control Reports. The take away message was that chlorsulfuron (the active ingredient in Glean and one of the active ingredients in Finesse) was the only treatment that provided control into the following year. Unfortunately, chlorsulfuron has a long residual life in the soil and restricts what crops can be grown for two or three years following application.

In June, 2017 we initiated a multi-year study near Omak. The study will run for five years and cover two cycles of the winter wheat-fallow rotation. We will apply Finesse (chlorsulfuron + metsulfuron) or MCPA ester in one, two, or none of the fallow seasons. We will apply Amber (triasulfuron), which is molecularly similar to chlorsulfuron, or MCPA ester in one, two, or none of the wheat seasons. The purpose of the study is to see how frequently Finesse and/or Amber need to be used to keep smooth scouringrush under control. The final results of this study will not be known until after the 2020 wheat harvest. The results from 2018 are reported in the 2018 WSU Weed Control Report. In 2018, all plots treated with Finesse in June, 2017 were still providing near total control of smooth scouringrush.

In 2018, we initiated studies at Omak and Rearden to compare glyphosate application methods for the control of smooth scouringrush. Although we did not see much effect from glyphosate applications in our initial study, we only used a quart/acre of RT3. We wanted to see if using higher rates in a sprayer (3 quarts/acre of RT3) or ropewick applicator (75% v/v) would provide better control. At Rearden, we also added a treatment containing Silwet, a nonionic organosilicone surfactant, to see if it would improve control. The results of these two studies can be found in the 2018 WSU Weed Control Report. Results were mixed. The broadcast treatment of RT3 provided excellent control at Omak (May 25th application) but poor control at Rearden (July 5th application). Ropewick applications provided moderate control at both sites. At Rearden, the addition of Silwet to RT3 dramatically improved control.

In 2019, we are continuing the long-term study at Omak and we have initiated two new long-term studies in the intermediate rainfall region (Edwall and Steptoe). We are also continuing our studies with glyphosate and evaluating the effect of the 2018 treatments on stem counts one year after application. A new aspect of this work is that we are applying the RT3 treatments monthly during the growing season and collecting stems at each application timing to evaluate the amount of silica in the stems as we progress through the season. Our results in 2018 made us wonder if the stems increase their silica content as the year progresses, which may reduce the amount of RT3 absorbed by the stems as the season progresses and increase the need for additional adjuvants. Be sure to look for the 2019 WSU Weed Control Report in January of 2020 to see what we learned.

Smooth Scouringrush sample emerging from ground.
Scouringrush in Omak.
Smooth Scouringrush in field.

For questions or comments, contact Drew Lyon by email at or by phone 509-335-2961 or contact Mark Thorne via email at or by phone at 509-335-7484.
Washington State University