Vectors of Spread:
General: Japanese Knotweed can be identified by its tall (1-5 metre), bamboo-like stems, large leaves and small white flowers that bloom in late summer. It is a perennial plant (grows back every year) that dies back every winter, so amazingly its height is achieved in just one growing season.
Flowers: Flowers are white-green and arranged in plume-like branched clusters along the stem and leaf joints.
Stem: Stems are green with red speckles in spring and early summer and then change to red/brown during summer. They resemble bamboo, being hollow, upright and surrounded by thin, papery sheaths.
Leaves: Leaves are cordate (heart-shaped) and measure 8-10 cm wide by 10-15 cm long at maturity. Foliage is arranged in a distinctive zigzag pattern along the stem.
Caution: Knotweed plants within the Sea to Sky are frequently treated with herbicides so please refrain from harvesting (ie berry picking) near plants. The herbicides we use are non-residual (do not persist in soil), however, it is best to be cautious.
Red Osier Dogwood (Cornus stolonifera): leaves are oval with prominent parallel veins converging at the leaf tips. Flowers arranged in a flat-topped terminal cluster (2-4 cm wide).
Ocean Spray (Holodiscus discolor): leaves are egg-shaped to broadly triangular and coarsely toothed or shallowly pinnately lobed. Stems are slender and hairy.
Habitat and Origin
Knotweed was introduced to BC as a garden ornamental in the 1900s. In its native habitat of eastern Asia, knotweed lives in harsh volcanic slopes, where it plays an important role as a colonizing species. In coastal BC, knotweed thrives due to a lack of predators and diseases that usually control its population, coupled with its incredible reproductive capabilities. From moist soil to river cobble, and from full to partial sunlight, it can dominate rivers, creeks, roadside ditches, and beaches.
Propagation and Vectors of Spread
Japanese Knotweed spreads rapidly through root systems that may extend from a parent plant up to 20 m laterally and up to 3 m deep. Plants thrive in moist, freshly disturbed soil, thus, areas prone to seasonal high water or flooding are particularly susceptible. Japanese Knotweed reproduces vegetatively, meaning that new plants sprout from small pieces (as little as 0.7 grams) of stem or root tissue.
Invasive Japanese Knotweed usually spreads when fragments of the roots and stems are moved by waterways or human activities. These activities include moving soil containing knotweed plant material, mowing or cutting knotweed, or dumping of yard waste that contains knotweed. In river corridors, Japanese Knotweed can reproduce from fragments that travel downstream during high-water events.
Stem or root material can produce a new plant in as little as 6 days and as a result, one patch can be the source of many downstream infestations. The rate of spread is exponential and the size of infestations will likely double every 5 years.
Economic and Ecological Impacts
Japanese Knotweed roots can grow through concrete and asphalt, severely damaging infrastructure. Large monocultures of knotweed can also widen irrigation ditches, once again potentially damaging surrounding infrastructure. This results in significant control, management, and repair costs.
- In the United Kingdom, Japanese Knotweed reduces property values and, in some cases, people have been unable to secure a mortgage or insure their property due to infestations. It is conceivably only a matter of time before this is the case within the Sea to Sky corridor.
- It can grow through small cracks in pavement, concrete, or drainage structures, reducing the structural integrity of public infrastructure – a huge potential burden to taxpayers.
- It is a safety concern along roadways as it reduces sightlines at intersections and along roadsides due to its rapid growth.
- Hydrological changes can cause excessive widening of stream channels, undercutting existing adjacent roads and highways.
Knotweed threatens biodiversity and disrupts food chains by reducing available habitat, increasing soil erosion potential, and shading out other plant species.
- Although minor insect grazing has been observed on invasive knotweed, no wildlife species are known to feed on it so devastates habitat for our native wildlife.
- Dense monocultures of knotweed displace native plant communities, including those containing rare and endangered species.
- Knotweed appears to exude allelopathic substances (biochemicals) that negatively effect growth and development of surrounding native vegetation.
- Soils with knotweed infestations are prone to erosion, as roots lack the well-developed hairs required to stabilize stream bank soil. This destabilization, combined with the die-back of above-ground growth, further exposes soil to the elements during winter months. This exposed ground is more susceptible to erosion during peak low events, such as winter rains, and will increase risk of sedimentation in waterways, potentially impacting fish populations and water quality.
- May cause flooding by clogging river and stream channels with its large stalks, thus changing natural erosion and deposition patterns.
- Displaces the lower, slower growing native plants beneath its extensive canopy through shading, competition for moisture and nutrients, and its densely matted litter.
- By limiting the amount of sunlight available below the water, knotweed infestations impact freshwater food chains. Many invertebrates, especially aquatic insects that prefer woody plant leaves (as opposed to knotweed leaves), move elsewhere.
What Can I Do?
Learn to identify knotweed: use the images presented in this profile page and head over to our Knotweed Resources page for more information
What to do if you spot it: You can report any Japanese Knotweed sighting by clicking here.
What KNOT to do: The best approach to controlling the spread of knotweed is PREVENTION.
- Plant Japanese Knotweed in a garden, no matter how well-contained its enclosure may seem.
- Move soil that has been contaminated with knotweed.
- Mow or weed-whack knotweed plants, as the fragments can propagate as new colonies Frequent mowing may also accelerate shoot development, leading plants to spread laterally from the parent plant.
Control: Mechanical control options, such as mowing or hand pulling, are NOT recommended. Refer to the Metro Vancouver Best Management Practices document to learn more about how to control knotweed, or contact SSISC to discuss site specific requirements.
- Center for Invasive Species and Ecosystem Health, Controlling Knotweed in the Pacific Northwest, https://www.invasive.org/gist/moredocs/polspp01.pdf
- Electronic Atlas of the Flora of BC, Reynoutria japonica, http://linnet.geog.ubc.ca/Atlas/Atlas.aspx?sciname=Reynoutria%20japonica
- Invasive Species Council of BC, Knotweed info page, https://bcinvasives.ca/invasive-species/identify/invasive-plants/knotweed
- Invasive Species Council of BC, Knotweeds TIPS Factsheet, https://bcinvasives.ca/resources/tips/knotweed
- Knot on My Property, http://www.knotonmyproperty.com/
- Metro Vancouver and Invasive Species Council of Metro Vancouver, Best Management Practices for Knotweed Species in the Metro Vancouver Region, http://www.metrovancouver.org/services/regional-planning/RPAC/invasive-species/RPACRegionalInvasiveSpeciesPublications/KnotweedBMP.pdf
For more information, please head to our Knotweed resources page.