Boulder, CO Invasive Plant Rules: Prohibited Species (2026)

Section 6-2-3 of the Boulder Revised Code makes it unlawful for an owner, lessee, agent, or occupant of any lot, whether occupied or unoccupied, to let state-listed noxious weeds grow past twelve inches, and it separately requires that any Colorado Department of Agriculture List A species be eradicated regardless of height, since List A plants get no height allowance. A related nuisance provision, Section 6-2-5(d), names Canadian Thistle and Russian Knapweed specifically as weeds that create a prohibited nuisance whenever they appear on a property.

The chapter's own definitions describe a noxious weed as a plant that aggressively invades economic crops or native plant communities, is poisonous to livestock, carries detrimental insects, diseases, or parasites, or otherwise harms sound ecosystem management. If the city manager finds a violation, Section 6-2-6 lets the manager first ask the owner to correct it and gives seven days' notice before the city cuts or removes the growth itself and bills the owner for the cost plus a $25 administrative fee. Unpaid charges can be certified to the Boulder County Treasurer and collected the same way as delinquent property taxes.

Two narrow defenses exist under Section 6-2-7: a nonbuildable outlot of at least half an acre kept as open space is exempt, and weeds are not a violation if the only growth past twelve inches is an unmatured seed head on an otherwise maintained landscape. City-owned parks, open space, street rights-of-way, stream beds and banks, and wetlands are all exempt from the chapter under Section 6-2-8. City council itself sits as the local noxious-weed advisory board under Section 6-2-10, with the mayor chairing and the deputy mayor serving as secretary.