King County, WA Invasive Plant Rules: Prohibited Species (2026)

King County's Noxious Weed Control Board, operating under Washington State's Noxious Weed Control Law (RCW 17.10), classifies noxious weeds into three categories: Class A weeds are the least common and all property owners are required by law to eradicate them to prevent statewide spread. Examples include giant hogweed (Heracleum mantegazzianum), johnsongrass (Sorghum halepense), and floating primrose willow. Class B weeds are more common and designated for control in specific regions. In King County, selectively regulated Class B weeds include invasive knotweed (Polygonum/Reynoutria spp.) — control is required only in properties along the Green River, Cedar River, and their tributaries. Other Class B weeds include purple loosestrife, yellow flag iris, and Scotch broom (Cytisus scoparius). Class C weeds like English ivy (Hedera helix), Himalayan blackberry (Rubus armeniacus), and butterfly bush (Buddleja davidii) are widespread and not required to be controlled but are encouraged targets for removal. King County also maintains a 'Weeds of Concern' list — non-regulated invasive species considered significant threats in the county, including lesser celandine, herb-Robert, and English holly. The Washington State Department of Agriculture maintains a Prohibited Plants List banning the sale and transport of certain species statewide. King County's Critical Areas Ordinance (KCC 21A.24) requires native vegetation restoration when clearing occurs in wetlands, streams, and steep slopes, effectively prohibiting invasive species in these restoration zones.