King County, WA Landscaping Rules: Weed Ordinances (2026)

Under RCW 17.10 (Washington Noxious Weed Control Law) and KCC 12.21, the King County Noxious Weed Control Board maintains an annual list of regulated noxious weeds. Class A weeds (such as garlic mustard and common crupina) must be eradicated everywhere they occur. Class B weeds (such as tansy ragwort, knotweeds, giant hogweed, and policeman helmet) must be controlled within designated areas, which often include most or all of unincorporated King County. Class C weeds (such as English ivy, holly, Scotch broom in many contexts) are at landowner discretion unless designated locally. Knotweed (Japanese, Bohemian, giant, Himalayan) is a particular priority because it outcompetes native riparian vegetation along salmon-bearing streams. Tansy ragwort is toxic to livestock and horses and is vigorously controlled in rural areas. The Noxious Weed Control Program provides free site visits, identification, and control recommendations, and can issue enforcement notices giving a landowner a compliance deadline (typically 14 to 30 days). Failure to comply can result in county abatement with costs charged to the property.