Identification and Reproduction

Identification:

There are four species of knotweeds in BC: Japanese knotweed (Fallopia japonica), Giant knotweed (Fallopia sachalinese), Bohemian knotweed (Fallopia x bohemica), and Himalayan knotweed (Polygonum polystachyum). All of these occur in the Fraser Valley, though Japanese and Bohemian are most common, and are difficult to tell apart.

(Photo from Metro Vancouver Knotweed BMP)

(Photo from GB Non-Native Species Secretariat Factsheet)

All have the following characteristics:

• Large, woody, bamboo-like shrub that grows 1-5 m tall. Stems or canes are hollow, upright, bamboo-like and greenish-brown with reddish speckles. In the winter canes may persist but will turn brown and straw-like. 
• Knotweed grows in dense thickets. 

• Leaves are heart to triangular-shaped on all species except Himalayan knotweed, which has lance-shaped, elongated leaves. Aside from giant knotweed, leaves on other species range from 8-10 cm wide and can grow upto 15 cm in length. Giant knotweed leaves are double the size.
• Japanese knotweed has a distinct zig-zagged branching pattern along the stems. 

• Flowers will be creamy white to light green, branching in upright clusters along the stem and leaf joints. 

• Newly emerging buds will be pink to red in colour and appear in the early spring. They will quickly develop into asparagus-looking stems through the spring. 

For more help identifying Himalayan knotweed please check out our next page. 

Reproduction:

• Knotweeds are perennials that spread primarily vegetatively. The rhizome system may extend from a parent plant up to 20 meters laterally and to 3 meters deep. Root and stem fragments as small as 1 cm can form new plant colonies.
• Fresh stems produce shoots and roots when buried in a soil medium or floated in water. Stems submerged in water can produce viable plants within 6 days. Dispersal occurs through root (rhizome) and stem fragments by human activities or by water.
• Reproduction also occurs by seed in Bohemian knotweed.

Habitat & Ecology

• Thrive on freshly disturbed soil in roadside ditches, low-lying areas, irrigation canals, and other water drainage systems.
• Also found in riparian areas, along stream banks, and in other areas with high soil moisture.
• Knotweeds are able to grow in partial shade or full sun.
• They are extremely aggressive and can push through concrete!

Impacts

Social:

• Creates dangerous situations by obstructing sightlines along roadsides
• Roots do not have fine root hairs, leading to erosion, increasing major flooding risk
• Damages infrastructure like buildings, sidewalks/roads, or drainage systems.
• Reduces property values restricting access, damaging buildings and foundations, impacting insurance eligibility and costs
• Increases costs to private property owners to repair areas affected by invasive species
• Annual old dead canes adds to the wildfire fuel load during the fire season

Ecological:

• Forms dense monocultures, reduces biodiversity
• Increases erosion on streambanks, adding to sedimentation and infilling of waterways
• Destroys natural habitat and riparian feeding areas for fish and other native species.

Management

Knotweed is a plant that goes dormant in the winter - this can impact how and when you decide to conduct treatments. Learn more about Plant Dormancy and why it matters (King County Noxious Weeds News). When knotweed is actively growing, that is when treatments are most effective.

Mechanical/Manual Control:

Mowing and Cutting is NOT RECOMMENDED for controlling knotweed.

Mowing and cutting spread vegetative fragments that can root and start new infestations - a piece as small as 1-cm can produce a new plant. The action of mowing/cutting can also stimulate dormant rhizome buds to grow, causing the plant rhizome/roots to grow deeper and further, regrowing more aggressively every subsequent years. Generally, the more often this plant has been mowed/cut, the deeper the belowground biomass will become, and therefore more challenging to control.

Digging should be used with high caution - Unless this is a small, poorly established patch of knotweed plants, where the physical removal of the roots can be achieved, digging can also stimulate the growth of dormant lateral buds, causing the roots to grow larger. Any digging work should REMOVE the roots, often requiring digging up to 3 m deep.

• Note that digging additionally requires the movement and disposal of soil in specific locations or facilities that will either conduct follow-up herbicide treatments on any regrowing plant, or have the space for deep-burial. There are limited locations that accept knotweed-infested soil. Large quantities may be denied or may need to be transported to the provincial landfill in Cache Creek or beyond. Please see Disposal options

Chemical Control: 

Currently, due to the ineffectiveness of other available options, herbicides listing knotweed on the label are the only known effective method of controlling knotweed with proven results.

If land owners are not comfortable or able to conduct herbicide application on knotweed on their property, there are some local invasive plant management companies that can be hired to complete the work - See list here.

Things to keep in mind:

• Site condition
  • Be aware of the site, the surroundings, and any wildlife potentially on or in the plants
  • Be careful to avoid impacting the surrounding vegetation and environment as much as possible - use tarps as shielding when possible and necessary
  • Always follow the herbicide label instructions!
    
    
• Timing:
  • The best and more targeted application time is during late summer, after plants flower but before they begin to yellow. This is when the knotweed begins to prepare for winter dormancy by taking nutrients (and therefore the herbicide) into the roots.
  • Targeted timing of treatment is most important - more herbicide does not equal greater success.
    
    
• Foliar spray:
  • When applying foliar applications to knotweed, the underside AND upper side of the leaves should be sprayed whenever safe and practicable.
  • Treatment should occur when plants are 1-2 m tall (generally that is when many leaves will have formed). Avoid cutting the plants during this time to allow as many leaves (and therefore surface area) to form
  • The plants need to have enough leaves to take in the herbicide into the roots - the roots are the part of the plant that need to be killed in order to control this plant. Any herbicide that only tackles the above-ground foliage will not control knotweed.
    
    
• Stem-injection:
  • If stem-injecting, treatments should occur when the stems are approximately 2 cm (1-inch) diameter, otherwise the hollow space in the stem will not yet have formed. If the stem is not large enough for the hollow space to develop, stem injection is not possible, and control is limited to foliar spray.
  • Ensure that ALL stems are injected, mark the injected stems with a marker to help keep track

For more information on herbicide application on knotweed please read Metro Vancouver's BMP for the knotweed species (pg. 11). 

Always read and follow the herbicide product label!

Powell River - Make Powell River Knotweed free Video

Step-by-step instructions for herbicide application to control knotweed, referenced from Powell River's Parks, Recreation and Culture, Invasive Species: Knotweed Removal guide):

Methods Used To Remove Japanese Knotweed Video (Robert Denny, UK)

Please be aware that this video is from the UK, so some of the herbicide information is not applicable in Canada. However the video is a nice overview of the herbicide application methods that may be used on knotweed plants, if the label allows it. Always read and follow the label of the product you are using.

Resources

Please refer to the Metro Vancouver's Best Management Practices for the Knotweed Species (pg. 12-24) for specific details on how to control invasive knotweed. 

Download the Invasive Species Council of BC's Factsheet on invasive knotweeds here.

King County has some excellent resources on knotweed identification and management. Note this is a resource from the US and Canadian guidelines and regulations may differ. Be sure to follow product labels. 

Even MORE INFORMATION (ie. research papers, Best Management Practice guides) can be found on our Knotweed Resources page.

Header photo (W.carter). 

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