Noxious Weeds That Harm Washington State

knotweedsPolygonum sachalinense, P. cuspidatum,Identification: The four knotweed species -giant, Japanese, Bohemian, and Himalayan - aretall, shrub-like, perennial, herbaceous plants.Stems often grow to over 10 feet tall and aresegmented and hollow, resembling bamboo.Knotweeds form dense colonies that sprout inApril, and bear clusters of small white flowers inlate summer. Each species produces differentlyshaped leaves, ranging from heart-shaped toarrow-shaped to long and slender.Impact: The knotweeds’ greatest impact is alongstreams and in riparian areas where they cancompletely displace native vegetation, erodestream banks, and change the nutrient cycleat the expense of salmon and other animals.Knotweed roots can grow so vigorously that theyerupt through blacktop and damage foundationsand other infrastructure.Control: Knotweed’s extensive roots andvigorous growth makes it extremely difficult tocontrol. Although small patches can be dug up,it may take several years to eradicate, and moststands require repeated applications of herbicideover several years. County noxious weed controlboards can provide advice and assistance.W-32