Japanese knotweed

(Fallopia japonica)


Invasive implications

        Japanese knotweed is a perennial shrub from Asia, introduced to North America in the late 1800's as an ornamental. It has tremendous invasive potential due to its rapid growth and reproductive capabilities. In 1966, U.S. botanists considered Japanese knotweed to be one of the most persistent and aggressive of all perennial weeds. Since then, Knotweed (a.k.a. Mexican bamboo) has continued to thrive in a variety of climates including the Pacific Northwest. It flourishes along streamside banks, ditches, wetlands, and disturbed areas. The key problem with this invasive plant is that once established, it displaces virtually all other vegetation. It is difficult if not impossible to eradicate Japanese knotweed although some techniques can help to control this plant.







bullet appears red when emerging in spring
bullet notice the thick organic layer
bullet grows from 1-3m
bullet stems remain erect and hollow
bullet stems turn brown/grey





bullet heart-shaped
bullet length: 5-15cm
bullet width: 5-12cm
bullet alternate
bullet green with red/purple specks
bullet has distinct nodes like bamboo
bullet forms dense clumps
bullet red at leaf attachment sites
bullet shoots grow up to 8cm/day
bullet blooms August/September

greenish-white 2.5-3mm long


three styles, 8-10 minute stamens

bullet outer sepals narrowly winged along the midrib


Key Features

Japanese knotweed forms dense stands which grow up through the preceding years dead thickets. The dead stems and leaf litter decompose very slowly, forming a deep organic layer which prevents native seeds from germinating. Once present at a site, Japanese knotweed increases in area very rapidly and soon forms monoculture stands.

Japanese knotweed is able to grow through concrete, walls and tarmac. This robust weed quickly spreads via its rhizomes; horizontal underground stems reaching 15-20m. New plants can sprout when these rhizomes transport downstream or when relocated by humans in fill dirt. Even tiny fragments of the plant can form new rhizomes, which highlights the importance to remove all parts of the weed when exercising control methods. Japanese knotweed also reproduces to a lesser extent with seeds that spread primarily by waterways (streams, ditches, flooding). Since there are a disproportionate ratio of female to male plants, sexual fertilization via seeds is far inferior to rhizome propagation.

This invasive plant grows in a variety of soils including silt, loam, and sand. Additionally, Japanese knotweed can tolerate adverse conditions including full shade, high temperatures, high salinity, drought and flooding. For the most part, it requires full sunlight, tending not develop as dense of stands in shady areas. Japanese knotweed thrives on disturbance and accordingly invades urban environments where humans routinely disturb and transport soils.


Control Measures

Fallopia japonica can be best controlled through long-term commitment and effort from both local government and volunteers. Perhaps the most cost-effective method focuses on preventing establishment through monitoring areas for new introductions such as in local parks. Once new colonies are identified, individuals/groups must quickly act to eradicate these stands. A key point to remember is that any leftover portions of the root system of Japanese knotweed can re-sprout. Therefore, all parts of the plant must be properly disposed of in strong plastic bags and transported to either landfills or incinerators. A combination of cutting and spot application of glyphosate (Roundup) is likely the superior choice of control as it is effective, cost-efficient, and practical.


Non Chemical Control

  Digging/Grubbing Cutting Grazing
How Use digging tool to remove entire plant (eg. shovel, Pulaski, etc.) Use laupers or similar tool with a "clean" cut Sheep, goats, horses can be used to graze on young shoots, providing the dead stems of previous year's growth were removed
When Easier when plants are less established Early to mid part of growing season Early spring to mid summer


Until plants are no longer sprouting from area At least 3 times in growing season will deplete food reserves Must be continuous to control growth
Pros/Cons Extremely labour intensive, potential to spread plant fragments as underground stem system is extensive


Only suitable for relatively small stands, is cost-effective if volunteers are available May not be feasible on riparian sites as animals may do more harm than good through disturbance


Chemical Control


Spot Application


How Cut back stems to 5cm above ground level. Immediately apply a 25% solution of Roundup or Rodeo and water to the cross section of the stem. If taller than 1.5m, cut plant back and wait until growth reaches this height. Spray using 2% of Roundup or Rodeo with water to wet all foliage. Do not over apply so herbicide drips off leaves.
When In the fall when leaves are translocating to rhizomes is most effective time. October/November when surrounding vegetation is dormant thereby reducing risk to non-target species
Duration Twice per year, may need to repeat over two years Repeated applications over several years may be necessary
Pros/Cons This techniques benefits from being very selective and effective. May not be feasible for large areas. Can control large areas but risks damaging non-target species.


Control Warnings:

Disposal - Make sure to properly discard all plant pieces in thick plastic bags and transport them to a sanitary landfill site or incinerator. Composting is not an appropriate means of disposal as this may result in further distribution. Remember that humans can actually spread invasive plants by taking seeds from one place to another on clothing, tires, equipment, etc.

Chemicals - Although some chemicals are approved for control of invasive plants, extreme caution must be taken as many pesticides are harmful to humans. Permits may be required for chemical use and buffer zones exist beside waterways to protect fish and wildlife. Chemical control is not a long-term solution and therefore should be part of a finite plan and applied sparingly. Please see the following web sites for further information: Provincial: MWLAP Pest Information  Federal: Pest Management Regulatory Agency


Additional Resources