Knotweeds

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.

Leaves:

• Japanese knotweed  has egg-shaped leaves 3-10 cm wide and 10 – 15 cm long, and the veins on the underside of the leaves have blunt knobs.
• Bohemian knotweed leaves are egg-shaped, 5-30cm long, and the undersides have short, stiff hairs.
• Giant knotweed leaves are heart-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.

Knotweeds originate from Asia, including Japan, Korea, China and Pakistan. They were introduced to B.C. in the early 1900’s as garden ornamentals.

In their native range, knotweeds live on harsh volcanic slopes, where they play an important role as a colonizing species. In coastal B.C., knotweeds thrive due to a lack of predators and diseases, 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. They can tolerate a range of conditions and pH ranges, including areas with heavy metal and salt pollution. Knotweeds’ preference for moist, freshly-disturbed soil also makes areas prone to seasonal high water or flooding particularly susceptible. 

Ecological impacts:

Knotweeds threaten biodiversity and disrupt food chains by reducing available habitat, increasing soil erosion potential, and shading out other plant species.

Habitat Reduction

• Although minor insect grazing has been observed on invasive knotweed plants, no wildlife species are known to significantly 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. This disrupts food chains and threatens biodiversity.
• Knotweed species appear to exude allelopathic substances (biochemicals) that negatively impact the growth and development of surrounding native vegetation.

Erosion

• Knotweed-infested soils are prone to erosion, as the 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 increases the risk of sedimentation in waterways, potentially impacting fish populations and water quality.
• Knotweed may cause flooding by clogging river and stream channels with its large stalks, thus changing natural erosion and deposition patterns.

Shading

• Knotweed species displace the lower, slower-growing native plants beneath their extensive canopy through shading, competition for moisture and nutrients, and the production of densely matted plant 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), tend to move elsewhere.

Economic impacts:

Knotweeds roots are extremely persistent. They 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. Without concerted action, 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. Knotweed plants can also reduce the integrity of rail beds and compromise train safety.
• Knotweed is a safety concern along roadways as it reduces sight lines at intersections and along roadsides due to its rapid growth.
• Hydrological changes caused by knotweed can bring on excessive widening of stream channels, undercutting existing adjacent roads and highways.
• Knotweeds can restrict access to waterbodies, impacting aquatic recreationalists like anglers or boaters.

Knotweeds are difficult to control and require a dedicated, multi-year approach.

Chemical control:

• A site-specific, multi-year treatment plan, using herbicide, is the most effective control method for knotweed infestations. Targeted application methods, such as foliar spray or stem injection, attack both above-ground vegetation and root systems.
• We recommend that any herbicide application is carried out by a person holding a valid BC Pesticide Applicator Certificate and experience treating knotweed. Before selecting and applying herbicides, you must review and follow herbicide labels and application rates; municipal, regional, provincial and federal laws and regulations; species-specific treatment recommendations, and site-specific goals and objectives.

Mechanical control:

• It is NOT recommended to use mechanical treatments on knotweeds, due to limited efficacy of such treatments.

Biological Control:

• There are no biocontrol agents currently available in Canada for knotweed control, however a sap sucker psyllid (Aphalara itadori) is currently being assessed by Agriculture and Agri-Food Canada.

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.