Organic Controls for Canada Thistle

On-farm research evaluates weed control strategies.

Boulder County Cooperative Extension has teamed up with CU Boulder and Boulder County Parks and Open Space to fight Canada thistle. They are conducting on-farm research comparing control strategies that are approved for certified organic use.

Adrian Card, Extension Agent with CSU Cooperative Extension in Boulder County, initiated this research after discussions with organic farmers, indicating a need for improved control measures for Canada thistle on their farms. Ewell Culbertson, Pachamama Farm near Hygiene, offered a section of his land, containing a high population of Canada thistle for use in on-farm research.

Tim Seastedt, University of Colorado Professor in the Department of Ecology and Evolution Biology, who conducts biological weed control research in Boulder County and his student, Thomas Saielli, conducted the research and data collection of pre- and post-treatment biomass percent. Boulder County Parks and Open Space Weed Coordinator, Tim
D’Amato, also provided guidance.

The treatments have used several commercially available, organically approved liquid herbicides. Burnout II and Matran 2 are clove oil based and Deadeye and AllDown are acetic acid (vinegar) based herbicides. Other spray treatments were made by diluting 30% acetic acid (kitchen vinegar is 5%) to 10%, 15%, and 20% tank mixes with the addition of a yucca based surfactant to increase spray adhesion, dispersion, and penetration of the Canada thistle leaf cuticle. The remaining treatments utilized hoeing, hand pulling, and flame weeding.

Working with three replicated plots, these 10 treatments were applied every other week, every month, and every other month. Research design was intended to find the most effective treatment and frequency of application for organic farmers.
Research findings will be available in February 2006. Contact Adrian Card at acard@co.boulder.co.us

Adrian Card, Boulder County
Agriculture Agent