Japanese knotweed comes from Eastern Asia. It was imported as an ornamental in England in the mid-1800s and from there came to the United States.
It reproduces from seeds and pieces of stem or roots. The root or rhizome can reach a length of 40 feet and burrow 9 feet down. A piece of rhizome the size of a fingernail can float down a river and begin to grow once it is deposited on land. Buried rhizomes have grown through 2 inches of asphalt!
Japanese knotweed is also known as Japanese or Mexican bamboo, because the stem becomes woody and has enlarged nodes similar to those of a bamboo fishing pole.
It is a very fast grower that can reach a height of 10 feet and overtop
native vegetation very quickly.
The heart-shaped leaves are arranged alternately along the stem. Although variable in size, the leaves are normally about 6 inches long by 3 to 4 inches wide. Small creamy-white flowers appear in mid-summer.
This plant is a threat to native vegetation because it often forms dense patches, which shade out all other plants. It is a particular threat in riparian areas where it can survive floods and quickly colonize scoured streambanks. Japanese knotweed is difficult to eradicate once it has become established.
Japanese Knotweed has been found in large patches throughout Clatsop County along riverbanks, in fields and in the vacant lots of Astoria, Gearhart, and Seaside.
Knotweed and its relatives -Himalayan and Giant Knotweed- are very difficult to eradicate. There have been many attempts at digging up roots and continuous cutting to eliminate Knotweed, but none have been at all successful. Presently, the most effective eradication method is through the use of approved herbicides. A thorough discussion of the method can be found at http://www.clark.wa.gov/environ/weed.htm
For more information on Japanese knotweed and other weeds, please visit the following Web sites:
California Dept. of Agriculture
http://pi.cdfa.ca.gov/weedinfo
USDA Home page for all plants. Click on "invasive and noxious" or use
Search feature
http://plants.usda.gov
Animal & Plant Health Inspection Service _ Noxious Weed Home page
http://www.aphis.usda.gov/ppq/weeds/
Wildlife Invasive Species Team/Nature Conservancy - Searchable website
http://tncweeds.ucdavis.edu/esadocs.html