Comparative Field Evaluation of Alternative and Conventional Herbicides for Three Noxious Weeds
Applied Research
Elizabeth Cohen
Agriculture and Small Farms Agent
Utah State University Extension
Coalville
Abstract
Increasing demand for non-synthetic weed control options in Summit County, Utah, particularly within the suburban Park City community, has led to frequent requests to Extension educators and the Summit County Cooperative Weed Management Area (CWMA) for guidance on “organic” herbicide options. However, research-based information on the efficacy of these products for noxious weed management in high-elevation ecosystems is limited. The objective of this study was to evaluate the effectiveness of three alternative herbicides compared to commonly used conventional herbicides for control of three Utah-listed noxious weeds: Alliaria petiolata (garlic mustard), Cirsium arvense (Canada thistle), and Carduus nutans (musk thistle). Field trials were conducted at three sites in Park City and the Snyderville Basin of Summit County, Utah. Treatment plots measuring 2 × 2 ft were established within existing weed populations and replicated across sites. Three alternative herbicides—Firehawk Bioherbicide (nonanoic acid), Captain Jack’s Deadweed Brew (caprylic and capric acids), and Weed Pharm (acetic acid)—were applied according to label directions and compared with conventional herbicides commonly used for these species. Conventional treatments included Weedmaster® + MSM for garlic mustard and Milestone® for Canada thistle and musk thistle. Alternative herbicides were applied three times at 2–3-week intervals during early summer, while conventional herbicides were applied once in early summer and again in early fall. Observations were made every 2-3 weeks to assess weed presence and vigor. Data from two seasons of field trials are currently being analyzed to quantify the comparative efficacy of alternative and conventional herbicide treatments. Preliminary field observations suggest a possible cumulative effect in plots treated with alternative herbicides. Fewer rosettes were observed within treated plots compared to adjacent untreated areas, raising questions about potential impacts on seedbank germination. Additional analyses are planned for Summer 2026 to further investigate this observation. Findings will inform Extension recommendations for landowners, land managers, and homeowners seeking evidence-based guidance on alternative weed management strategies.
Poster has NOT been presented at any previous NACAA AM/PIC
This poster is being submitted only for display at AM/PIC. Poster is not to be judged, but the abstract will be published in the proceedings.
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Authors: Elizabeth Cohen, Wesley Crump Wesley Crump
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Cohen, E. Assistant Professor for Extension - A, Utah State University, Utah, 84032
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Wesley Crump, W. Assistant Professor for Extension - Horticulture, Utah State University, Utah, 84032