IUCN threat status:

Not evaluated


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Distribution: Giant knotweed has a discontinuous distribution in North America. In eastern North America, giant knotweed occurs from Tennessee and North Carolina north into eastern Canada. Some states in the Great Lakes region (e.g., Indiana) and New England (e.g., New Hampshire) lack giant knotweed. Giant knotweed is also found in Louisiana. In western North America, giant knotweed occurs from California north to Alaska, with populations also in Idaho and Montana.

Japanese knotweed is more widely distributed than giant knotweed. The Plants Database reports Japanese knotweed occurring in almost all of the United States with the exceptions of Florida, Alabama, Texas, New Mexico, Arizona, Wyoming, and North Dakota. However, one source documented Japanese knotweed in Arizona [39]. Japanese knotweed occurs throughout Canada, with the exceptions of Labrador, Saskatchewan, Alberta, Northwest Territories, and Yukon. Plants Database provides distribution maps of giant and Japanese knotweed.

As of this writing (2010), the distribution of Bohemian knotweed in North America is not well known. The Plants Database reports Bohemian knotweed occurring in British Columbia, Ontario, and Quebec (2010). However, sources included in this profile have also documented Bohemian knotweed in New York [97,107] and Washington [31,109,111,153]. It is likely that the distribution of all 3 knotweeds is expanding in the United States.

Introduction to North America: Both giant [49,85,98,121] and Japanese [46,98] knotweed are native to Asia. During the late 19th century, giant [42] and Japanese [98] knotweed were introduced to North America as ornamental plants. Giant knotweed was also promoted as a soil binder [42] and fodder plant [136]. One source reports Japanese knotweed established from seeds released from ship ballast in New York [6]. Giant [98] and Japanese knotweed [6,49,51,85,142,146] escaped cultivation, with herbarium records indicating that Japanese knotweed escaped cultivation at least 115 times in North America prior to 2003 [6].

Rate of spread: There is some information available regarding the rate of spread of Japanese knotweed, though as of this writing (2010) information was limited for Bohemian knotweed and lacking for giant knotweed. After initial introductions, Japanese knotweed populations displayed a 50-year lag time prior to exponential population growth. As of 2006, spread rates in the United States were increasing rapidly, while those in Canada leveled off in the 1970s [6]. In Washington, Japanese knotweed was established in one county in 1960; by 2000, it was established in more than 50 counties [127]. Along the Hoh River in northwestern Washington, one Bohemian knotweed plant was transported downstream in a winter storm event. Approximately 4 years after this event, 9,600 stems were located within 20 river miles of where this plant established. Five years after the flooding event, 18,585 stems were mapped within the same 20 river miles [111].

Means of spread: The spread of all 3 knotweeds is linked to the ability of both aboveground and belowground parts to sprout when separated from the parent plant (see Vegetative regeneration). Humans spread the plants through dumping yard waste [31,40], roadside mowing or construction projects [40], or using fill dirt from riparian areas [8]. Plants of all 3 knotweeds that escape cultivation and establish in riparian areas may spread when plant parts are transported downstream ([2,93,112,136], review by [7]). Spread by seed is rare, though it has been suggested for Japanese knotweed [141,151], and seedlings of giant [87], Japanese [16,44,74], and Bohemian [115] knotweed have been observed.


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