Common Name: Japanese knotweed
Scientific Name: Polygonum cuspidatum
Symbol: POCU6
Group: Dicot
Family: Polygonaceae
Growth habit: Subshrub/Shrub/Forb/Herb
Duration: Perennial

What does it look like?

Japanese knotweed grows up to 3 meters tall, with smooth, stout, bamboo-like stems swollen at joints where the leaf meets the stem. Leaf sizes may vary, but the average is 6 inches long, 3-4 inches wide, and growing on arching branches. Blooms appear in late summer and early autumn in clusters of creamy white flowers. Dead stems persist throughout the winter, are orange to brown colored, and form extremely dense, intricate root systems. New growth emerges from these stems in the spring, often in denser thickets than the previous season (JKA 2004).

Where does it grow?

Native to Japan, Taiwan, Korea and eastern China, Japanese knotweed was introduced to new areas as an ornamental and has since widely escaped cultivation. It currently exists in thirty-six states in the United States and is known to grow in the North Coast Ranges of northern California (APWG 2004).

Japanese knotweed thrives in moist, disturbed places and can tolerate a variety of adverse conditions including full shade, high temperatures, high salinity, moisture, and even drought. It is spread by natural and human activities, such as high-water flows in riparian areas, which carry fragments of plants to uninfested areas (APWG 2004).

Is it in our watershed?

In the lower Mattole watershed, there are currently six sites where Japanese knotweed exists. They rank as high-priority removal projects.

Why is it a problem?

Once established, plants grow vigorously and create dense colonies that exclude other vegetation. Japanese knotweed produces a thick layer of leaf litter and dead stems that prevent native seeds from germinating. This species has also been known to reduce biodiversity by out-shading native vegetation while increasing the risk of soil erosion and bank instability after removal projects in riparian areas (JKA 2004).

How do you get rid of it?

Manual/Mechanical Removal: When plants are small, it is best to dig out small clumps to prevent new colonies from establishing. A pulaski can be used to dig up mature plants, while smaller plants can be hand pulled in moist areas. It is important to remove all stems and root systems because new growth can sprout from rhizomes left behind. After pulling, dry and burn the plants for best results. Continuous cutting of plants (4 or more times per season) along with heavy shading by black plastic or nearby vegetation can deplete the energy reserves of Japanese knotweed and help control infestations (JFP 2004). It often takes up to 10 years of cutting (once a month during the growing season) to completely exhaust the reserves, and while pulling is more effective than cutting, both techniques require great care when disposing of rhizomes and plant cuttings. Root systems should be bagged and disposed in contained trash dumpsters to prevent resprouting.

Grazing: Japanese knotweed is palatable to sheep, goats, cattle, and horses. Grazing by these types of livestock has successfully controlled this weed. However, because it continues to grow if grazing ceases, grazing is not a suitable method for complete eradication (JKA 2004).

Author:
Unity Peterson, Invasive Plant Program Coordinator, Mattole Restoration Council, Petrolia, CA.

Photographs:
Unity Peterson 2005

References:
Alien Plant Working Group (APWG). 2004. Japanese knotweed (fact sheet). Plant Conservation Alliance, Alien Plant Working Group, Washington, D.C. http://www.nps.gov/plants/alien/fact/pocu1.htm

JFP. 2004. Jepson Flora Project. Jepson Online Interchange for California Floristics. The Regents of the University of California. http://ucjeps.berkeley.edu/interchange.html

JKA. 2004. Japanese Knotweed Alliance.
http://www.cabi-bioscience.org/html/japanese_knotweed_alliance.htm