Location of knotweed control projects



There are about 22.5 acres of knotweed in the municipal watershed, which we are attempting to eradicate. We are controlling the smallest patches along roads by covering to starve the roots. Covering is not effective on larger patches, so since 2010 Seattle City Council ordinances have allowed the limited use of the herbicide imazapyr to treat the knotweed. The current ordinance is effective through 2018. By the end of 2015, 7.7 acres had been treated six times, 7.9 acres five times, 0.3 acres four times, and 2.1 acres three times. Only 2.2 herbicide-treated acres are on land that drains into the Cedar River. Since 2010 imazapyr use has declined from an average of 43 ounces per acre to only about 3 ounces per acre, reflecting the large knotweed biomass reduction.

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