Ornamental plant growing like a weed?

Japanese Knotweed

(a.k.a. Japanese bamboo, Mexican bamboo)

(Polygonum cuspidatum)

Japanese Knotweed was originally introduced to North America as an ornamental plant originating from Asia, and is often times still used in gardens. It is a highly invasive plant found along roadsides and wetland areas where it quickly and efficiently out-competes native vegetation and is extremely difficult to control once established. It is found across the country from regions of British Colombia all the way to Newfoundland.

Photo Credit: Grandpa Cliff 

The Japanese knotweed is a herbaceous, semi-woody perennial member of the Buck-wheat Family. It can grow up to three meters high and has stout, reddish-brown round and hollow stems that grow very close together in clumps resembling bamboo. This ‘bamboo’ appearance gave rise to its common names of Japanese and Mexican bamboo. It has egg shaped leathery leaves that alternate and form at swollen joints along the stem. They are 10-15 cm long and flattened, rounded at the base with a pointed tip and are dark green and lighter green below. The large aggregations of plants produce rhizomes which can extend 13-18 meters in length. Greenish-white flowers bloom in clusters along the stem from August to September.

Primarily, reproduction is vegetative through the rhizomes or by root fragments which readily produce new plants. Japanese knotweed root pieces are known to contaminate new sites through movement of dirt. When growing near water, root fragments can be carried downstream to establish new colonies.
Japanese Knotweed prefers open habitats. It grows in a variety of soil types along roadsides, edges of waterways, neglected gardens and unused areas. It is known to grow in climates experiencing high temperatures and drought. Wetlands and moist, low-lying areas are the most common habitats.

There is concern because with the lack of natural predators, its dense stands, it is capable of crowding out all other vegetation, and can even degrade native plant communities. It spreadssignificant threat to areas adjacent to rivers, streams and other shore-lines where it can cause bank erosion. It can lead to a reduction of biodiversity (through shading out of native plants). It can clog waterways and lower the quality of habitat for wildlife and fish. During its dormant growth stages Japanese knotweed dries and can create a fire hazard.

It is suggested that Japanese knotweed rarely achieves eradication without mechanical disturbance thus control methods should include digging and pulling along with cutting and mowing or herbicide use during the blooming period. Any control method must target the rhizome or root system, even when the top growth is removed. It is very important to carefully dispose of any stem or root material when controlling this plant manually. Hand-pulling is best done when the soil is soft, plants are young, and there are only a few plants. The effort must be persistent and ongoing for an extended period of time. The plant must be pulled by the root crown, trying to remove as much as possible of the rhizomes.


Internationally it is considered one of the worst invasive weeds world-wide.

Works cited:

Field Guide to Noxious and Other Selected Weeds of British Columbia. http://www.agf.gov.bc.ca/cropprot/weedguid/jknotweed2.htm. Retrieved on April 4th, 2012.
Engler, J., Abt, K., & Buhk, C. (2011). Seed characteristics and germination limitations in the highly invasive fallopia japonica s.l. (polygonaceae). Ecological Research, 26(3), 555-562. Retrieved from SCOPUS database.
Newfoundland Nature. http://www.nlnature.com/Canadian-Wildlife/japanese-knotweed-polygonum-cuspidatum--315.aspx. Retrieved on April 4th, 2012.
http://nlinvasives.ca/files/BMP_Japanese_knotweed.pdf. Retrieved on April 4th, 2012.