Priority: - Contain
Tags: Aquatic | Terrestrial
Identification and Reproduction
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.
- 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 grow through concrete!
- Create dangerous situations by obstructing sightlines along roadsides.
- Damage infrastructure like buildings, sidewalks/roads, or drainage systems.
- Reduce property values restricting access and damaging buildings.
- Increase costs to private property owners to repair areas affected by invasive species.
- Form dense monocultures.
- Increase erosion on streambanks.
- Destroy natural habitat for native species.
Mechanical/Manual Control [Not Recommended]:
NOT RECOMMENDED for controlling knotweed. Mowing will spread vegetative fragments that can root and start new infestations. Manual controls will likely distrupt knotweed and in turn stimulate the plant to grow more aggressively.
Currently, chemical control is the most effective method of controlling knotweed.
If land owners are not comfortable or able to conduct chemical control on knotweed on their property, there are some local invasive plant management companies that can be hired to complete the work - See list here.
- Herbicides containing the active ingredient Glyphosate are effective, as are herbicides containing the active ingredient aminopyralid.
- 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.
- If using stem-injection method of herbicide application - ensure that all stems are injected, and the best time for this method is during late summer, after plants flower but before they begin to yellow. This is when the plant begins to prepare for winter dormancy by taking nutrients (and therefor the herbicide) into the roots.
- If using spray application of herbicides - Treatment should occur when plants are 1-2 m tall (generally that is when many leaves will have formed). If the plants are not big enough, wait until they are - timing of treatment is most important, and more chemical application does not equal greater success.
- A spring herbicide application or cutting first in June or July will set the plant back so that it can be sprayed a second time in September or October (before the first frost) when the plant is taking nutrients into the roots to prepare for winter dormancy.
- Established patches (hundreds or thousands of stems) will almost certainly require foliar treatments over two or more years, sometimes up to several years. Be sure to search for new shoots up to 20 feet or more away from the central patch after herbicide treatment begins.
For more information on chemical control on knotweed please read Metro Vancouver's BMP for the knotweed species (pg. 11).
Always read and follow the chemical product label!
Powell River has provided step-by-step instructions to chemically control knotweed (referenced from Powell River's Parks, Recreation and Culture, Invasive Species: Knotweed Removal guide):
- Cut knotweed plants, leaving stems 2-4 inches above the ground.
- Slide a long wire into the hollow stem and pierce several stem segments.
- Use Round-up and spray the cuts and into the hollow stems.
- Note that knotweed will regrow within several months, repeat procedure once stems reach 3/4" in diameter.
Please refer to the Metro Vancouver's Best Management Practices for the Knotweed Species 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 chemical labels.
Header photo (W.carter).