How much is enough: Effect of Spray Volume on Controlling Invasive Knotweeds

By Mark Renz

The invasive knotweed complex includes four highly invasive, rhizomatous, perennial plants in the Polygonaceae family. Knotweeds are very difficult to control and can form dense, persistent thickets reducing biodiversity, displacing native species, and reducing the quality of wildlife habitat. The invasive plant also poses a significant threat to riparian areas, where it can survive severe floods and is able to rapidly colonize scoured shores and islands.

Milestone® specialty herbicide can reduce cover and density of invasive knotweeds; however, there are mixed reviews on the effect of varying spray volume on long-term knotweed control. Dr. Mark Renz and Tony Summers with the University of Wisconsin conducted a field study in 2014 in McFarland, Wisconsin to determine if the amount of spray volume would impact knotweed control with Milestone at the spot treatment rate of 14 fluid ounces per acre (fl oz/A).

Alleys were mowed between herbicide treated plots to facilitate application. Knotweed plants were about 7 feet tall at herbicide application.

Alleys were mowed between herbicide treated plots to facilitate application. Knotweed plants were about 7 feet tall at herbicide application.

Milestone plus a non-ionic surfactant was applied in either 20, 40, 60 or 100 gallons of total spray solution per acre (GPA). Plot size was 10 by 20 feet with four replications of each treatment. Herbicide treatments were applied with a handheld sprayer on August 22, 2014. Knotweed* plants were approximately seven feet tall at the time of application and comprised 90 to 99 percent cover on the study site. Percent visual cover of knotweed was collected at the time of treatment, and 12 and 20 months after treatment (MAT). In addition, the number of knotweed stems per square meter, were collected 12 MAT.

*NOTE: Genetic analysis on knotweed plants in this study were not conducted; however, the author believes plants are Bohemian knotweed (Fallopia x bohemicum), a hybrid between giant and Japanese knotweed.


Results of the study 12 MAT showed that Milestone at 14 fl oz/A provided excellent control (98% or more) of invasive knotweed. There was no difference in control among the spray volumes tested as knotweed cover was less than 5% regardless of treatment. While cover was similar, stem density 12 MAT was impacted by spray volume. Regression analysis found that knotweed stem density increased by two stems for every increase in 25 gallons of spray volume applied (Figure 1). These results suggest that high volume foliar treatments, while effective at keeping knotweed cover at a minimum, may have reduced long term knotweed control.

Twenty months after application, knotweed control declined compared to 12 MAT. Resprouting knotweed shoots averaged 43% cover across all treatments, with average knotweed cover ranging from 29 to 56% at 20 and 100 gallons total spray volume respectively (Figure 2). Statistical analyses (ANOVA and regression) did not find a significant difference in knotweed cover related to spray volumes; however, there was a trend for reduced control with increasing spray volume (Figure 2).

Figure 1. Regression analysis found that knotweed density (stems per square foot) increased as spray volume increased 12 months after treatment (MAT).

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Figure 2. Knotweed cover pre-treatment and 12 and 20 months after treatment (MAT) with Milestone® specialty herbicide at 14 fl oz/A applied in various spray volumes.

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Results from this study suggest that land managers applying Milestone® at the spot treatment rate of 14 fl oz/A to knotweed in the late summer or fall, can reduce total spray volume to 20 gallons per acre and achieve effective control for at least one year after treatment. However, knotweed was not eradicated with one application, and by 20 MAT average knotweed cover increased by more than ten-fold COMPARED TO 12 MAT, although cover was still about half of pre-treatment levels. These results suggest that it is critical for land managers to plan follow-up herbicide treatments to control knotweed regrowth from roots. Long term monitoring and treatment may be needed for five years or more to effectively eliminate knotweed infestations. While these results suggest that 20 gallons per acre of spray volume may provide better long-term control than higher volumes, additional research is needed to verify results. 


EDITOR'S NOTE: Other field studies conducted in May have shown a benefit to applying higher spray volumes to control knotweed. Field trials on Bohemian knotweed conducted by Dr. Kim Patten, Washington State University in spring showed that control increased with an increase in application volume. In this study, Milestone specialty herbicide at 14 fluid ounces per acre (fl oz/A) provided good to excellent control especially with application volumes of 100 gallons per acre (gpa) or greater when the plants were 3 to 4 feet tall.  Timing for this field trial was May compared to the study conducted by Mark Renz in August.

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Active ingredients for products mentioned in this article. Product (active ingredient): Milestone (aminopyralid).

Published: Sept, 2016; Reviewed and updated June, 2019