Japanese, Giant, Bohemian and Himalayan Knotweed
Reynoutria japonica, Fallopia sachalinensis, F. x bohemicum, Persicaria wallichii
All four knotweed species have similar ID characteristics and growth habits; each species is presented in detail below.
Worth noting is that Bohemian Knotweed (Fallopia x bohemicum) is the result of natural hybridization between Japanese and Giant Knotweeds (R. japonica and F. sachalinensis), whereas Himalayan Knotweed is a different genus altogether (Persicaria wallichii).
Knotweed taxonomy has undergone changes over the years. Knotweed species are also known as:
General: Japanese, Bohemian, Giant and Himalayan knotweeds are very similar in their growth characteristics and overall stem appearance. All four are perennial species, with the above-ground vegetation dying off in winter, while the below-ground vegetation lies dormant. The species can be distinguished by their leaf shape and size.
Stems: All four knotweed species have hollow, upright green stems with reddish-brown speckles, which resemble bamboo.
Flowers: All four knotweed species have showy, plume-like, branched flower clusters but the flower colour varies slightly. They bloom in late summer.
- Bohemian Knotweed leaves are egg-shaped, 5-30cm long, and the undersides have short, stiff hairs.
- Giant Knotweed leaves are also egg-shaped but 20-40cm long, and the undersides have long hairs.
- Himalayan Knotweed leaves look most different from the other species, with lance-shaped leaves up to 20cm long. The underside of the leaves is coated with stiff hairs.
- Japanese Knotweed, lastly, has egg-shaped leaves 3-10 cm long, and the veins on the underside of the leaves have blunt knobs.
Similar Native Species
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.
It is important for SSISC to understand the distribution of all knotweeds, in order to manage the species and stop its spread at the landscape scale. Please report any sighting of knotweeds by clicking here.
Habitat and Origin
Knotweeds were introduced to BC as garden ornamentals in the 1900s.
In their native range of eastern Asia, knotweeds live on harsh volcanic slopes, where they play an important role as a colonizing species.
In coastal BC, knotweeds thrive due to a lack of predators and diseases that usually control their population, coupled with their incredible reproductive capabilities. From moist soil to river cobble, and from full to partial sunlight, they can dominate rivers, creeks, roadside ditches, and beaches.
Propagation and Vectors of Spread
Knotweeds spread 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. Knotweeds reproduce vegetatively, meaning that new plants sprout from small pieces (as little as 0.7 grams) of stem or root tissue.
Invasive knotweeds usually spread when fragments of the roots and stems are moved by waterways or human activities. These activities include moving soil containing knotweeds plant material, mowing or cutting knotweeds, or dumping yard waste that contains knotweeds. In river corridors, knotweeds 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
Knotweeds threaten biodiversity and disrupt 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 knotweeds, no wildlife species are known to feed on them so their presence devastates habitat for our native wildlife.
- Dense monocultures of knotweeds displace native plant communities, including those containing rare and endangered species.
- Knotweeds appear to exude allelopathic substances (biochemicals) that negatively impact the growth and development of surrounding native vegetation.
- Soils with knotweeds 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 knotweeds leaves), move elsewhere.
Knotweeds roots can grow through concrete and asphalt, severely damaging infrastructure. Large monocultures of knotweeds can also widen irrigation ditches, once again potentially damaging surrounding infrastructure. This results in significant control, management, and repair costs.
- In the United Kingdom, knotweeds reduce 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 sight lines 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.
What Can I Do?
Learn to identify knotweeds: use the images presented in this profile page and peruse the documents linked below for more information.
What to do if you spot it: You can report any knotweeds sighting by clicking here.
What KNOT to do: The best approach to controlling the spread of knotweeds is PREVENTION.
- Plant knotweeds in a garden, no matter how well-contained its enclosure may seem.
- Move soil that has been contaminated with knotweeds.
- Mow or weed-whack knotweeds 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.
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 knotweeds, or contact SSISC to discuss site specific requirements.
- British Columbia Ministry of Forests and Range, Key to Identification of Invasive Knotweeds in British Columbia, https://www.for.gov.bc.ca/hra/Publications/invasive_plants/Knotweed_key_BC_2007.pdf
- 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, Bohemian Knotweed, https://bcinvasives.ca/invasives/bohemian-knotweed/
- Invasive Species Council of BC, Giant Knotweed, https://bcinvasives.ca/invasives/giant-knotweed/
- Invasive Species Council of BC, Himalayan Knotweed, https://bcinvasives.ca/invasives/himalayan-knotweed/
- Invasive Species Council of BC, Japanese Knotweed, https://bcinvasives.ca/invasives/japanese-knotweed/
- Invasive Species Council of BC, Knotweeds TIPS Factsheet, https://bcinvasives.ca/wp-content/uploads/2021/01/ISCBC-Factsheet-Knotweeds-180216-WEB.pdf
- 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