Knotweed Species (Polygonum sp.) | Region 10 | US EPA

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Knotweed Species (Polygonum sp.)

Three of the knotweed species leaves side by side, starting from the left is the Giant Knotweed, the middle is Japanese knotweed, and on the right is Himalayan knotweed. - Photo Courtesy of Whatcom Co. Noxious Weed Board.

Knotweed is thought to have been brought to the United States for the Columbian Exhibition in the late 1890s. Its pretty leaves and white flowers make it attractive for horticulture, and until a few years ago it was even sold in nurseries. Four species, Japanese knotweed ( Fallopia japonica), Giant knotweed (Fallopia sachalinensis), their hybrid, Bohemian knotweed (Fallopia bohemica), and Himalayan knotweed (Persicaria wallichii) are found in the Pacific Northwest.

Knotweed is a semi-woody perennial with densely spaced jointed bamboo-like stalks, sometimes up to 9 feet tall, and its dense and spreading rhizome mat grows as much as 7 feet deep. It rapidly creates monocultures that push out and prevent growth of native vegetation and affecting structure and complexity of streambanks. It spreads by rhizomes, fragments and seeds, and a single one-inch fragment can regenerate a whole new plant. Because it likes moist soil, it is common along streams and roadside ditches, and it is often spread by flowing water. In some areas, headwater infestations provide a continuing supply of plant fragments to downstream corridors, resulting in miles of stream thickly lined with knotweed. While some plants have been in the Pacific Northwest for decades, the 1990s floods are thought to be responsible for the explosion of thousands of new knotweed patches along stream corridors. Knotweed is extremely difficult to kill; established plants are very persistent and seem unaffected by mowing or cutting, and repeated applications of herbicide are needed to kill a clump. Several years of monitoring are needed to ensure no re-growth.

Status in the Pacific Northwest: All four knotweed species are listed on the Washington State Noxious Weed List, and on the Washington State Noxious Weed Seed and Quarantine List, and various species are also listed on the Oregon and Idaho Noxious Weed Lists. There is also a state coordinator in the Washington Department of Agriculture assigned just to this weed. In Washington, since 2004, there has been an annual $500,000 appropriation for knotweed projects, and in 2006, program cooperators contributed slightly more than that amount as well. Huge mapping and eradication efforts are now underway: in 2006, 646 acres of knotweed along 1,564 river miles were treated. The jury is still out on whether total eradication is possible.

What can you do? If you have knotweed on your property, work hard to eradicate it, starting upstream where applicable. After eradication, you will need to monitor for several years to make sure it is dead, as even small pieces can start new plants. For additional information, contact the appropriate state agency:
In Washington: 1-360-902-1853 (State Knotweed Coordinator)
In Oregon: 1-866 INVADER
In Idaho: 1-208-332-8686 (ANS Coordinator)
In Alaska: 1-877-INVASIVE

The links provided may be outside the domain. Control in the PNW,5,What is Knotweed?

Washington State Knotweed Control Program 2006 Report (PDF)


Facts sheets and profiles

P. cuspidatum (Japanese knotweed)

Fact sheets

P. bohemicum (Bohemian knotweed)

P. polystachum. Himalayan knotweed

P. sachalinense - giant knotweed

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