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Japanese knotweed

Japanese knotweed (Fallopia japonica) is a tall (>3 metres) shrub with bamboo-like stems. It's leaves are 10 - 15 cm long and flat at the base with a pointed tip, and has small white flowers that bloom in late summer.

It quickly spreads to form dense thickets in a variety of habitats, including dry roadsides and moist stream banks. Small patches can quickly spread into large areas, leaving little room for native species to grow.

Concern over knotweed has been mounting around the world as more and more native habitat is being lost, particularly in riparian areas. It has tremendous regenerative abilities and is recognized by international experts as a "world's worst species".

In the Sea to Sky Corridor knotweed is located mostly south of Whistler, where it is trying to be contained.



In the Squamish Estuary

IMG_5775.jpgAlong Centennial way and the Mamquam River, Squamish

Look alike: Oceanspray



Japanese knotweed

(Fallopia japonica)


(Holodiscus discolour)

Growth form

Herbaceousperennial that dies back completely each year

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Perennial shrub up to 4 m tall

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Spotted green to reddish brown, hollow and bamboo-like

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Woody with solid core


10 – 15 cm long, edges smooth

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3 – 6 cm long, deciduous, lobed or coarsely toothed

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Creamy-white flowers in clusters from end of stem and leaf axils

White to cream, small (to 5 mm across) in lilac-like clusters


Moist to wet disturbed areas around human settlements, fence lines, roadsides

Dry to moist, open sites, mostly at low to middle elevations

Other info




As little as 0.6 of a gram of root material (from roots that have been known to extend up to 20 metres) and/or stem material can produce a new plant in as little as 6 days in the warm, wet conditions that characterize the climate of coastal BC.

In their native habitats, Japanese knotweed lives in harsh volcanic slopes, where they play an important role as a colonizing species.  But here in coastal BC, where there is an absence of predators and diseases, combined with its incredible reproductive capabilities, they possess the ability to thrive in moist soil or river cobble, in full or partial sunlight, and dominate rivers, creeks, roadside ditches and beaches.

Japanese knotweed emerges early in the season and reaches full height by early summer.  The plants flower in late summer or early fall and the aerial shoots die after the first frost.  Knotweed re-absorbs nutrients into its root system, which provides resources to the plant for overwintering, enabling early spring shoot emergence.

Specific negative impact areas:
1.Riparian Systems (streams and rivers)
  • Knotweed can tolerate long periods of submersion and colonize on nutrient poor soils, allowing it to establish and grow on the lower banks of rivers and creeks, where there is little competition.
  • It displaces the lower, slower growing native plants beneath its extensive canopy through shading, competition for moisture and nutrients, its densely matted litter mass and alleopathy.
  • With only a limited amount of the sunlight available to drive freshwater food chains, many invertebrates, especially aquatic insects that prefer woody plant leaves (as opposed to knotweed leaves), move elsewhere.  Further, the growth of smaller, younger trees along stream banks can be inhibited.  Such trees would otherwise grow to overhang the water, providing a platform for insects to fall into the water and feed resident fish.  Such trees also provide shade to cool water temperatures, as well as a provide a continual source of coarse woody debris.
  • Large woody debris creates pool habitats, retains spawning gravels, and provides cover for juvenile salmonids.  The loss of large woody debris can disrupt natural processes, leading to channel incision, loss of side channel fish habitat, loss of pool habitat, decreased retention of spawning gravels, and decreased cover for juvenile salmonids and their prey.  The reduction or modification of riparian vegetation is one cause of decreased large woody debris.
  • Knotweed’s extensive root system lacks the well-developed root hairs necessary to bind and hold in place stream bank soil, especially during peak winter rains.  Further, each winter, the entire plant collapses on itself (and other surrounding plants) leaving a sparse vegetative and bare soil that is vulnerable to erosion.  During peak flow events, banks can erode resulting in sedimentation that can negatively impact human water quality and fish populations.  Flood events are catalysts that spread knotweed stem and root material further downstream, where they eventually lodge, establish and perpetuate their aggressive growth cycle.
  • It is believed to exacerbate flooding by clogging river and stream channels with its large stalks, thus changing natural erosion and deposition patterns.


2. Biodiversity & Wildlife

  • Knotweed forms dense monocultures displacing native plant communities, as well as rare and endangered species.
  • Although minor insect grazing has been observed on invasive knotweeds, no wildlife species are known to feed on it.


3. Recreation, Safety, Infrastructure & Amenity Values

  • Knotweed impedes angler access to rivers.
  • Rapid sprawling growth reduces highway sightlines (visibility).
  • Hydrological changes can lead to over widen stream channels, undercutting existing adjacent roads and highways.
  • It can grow through small cracks in pavement, concrete or drainage structures, reducing structural integrity of public infrastructure – a huge potential burden to tax payers.
  • If embedded into riprap, it is impossible to control without removing such entire structures.
  • There is significant cost associated with treatment and disposal of knotweed, hence land values will be negatively impacted, if a given area becomes knotweed infested.



There is no “silver bullet” or single best control strategy for knotweed – each site is different.  It will likely take 3-5 years to be successful using integrated approaches.  Due to its dispersal method, control projects require coordination with multiple land owners and across jurisdictions.

Mechanical control
Options for mechanical removal of knotweed are prohibitively costly on large infestations due to its tremendous regeneration capabilities, estimated at a minimum of $250,000 per hectare.  If you were to dig up the plant, you would have to make sure all the roots are collected.  This means digging a pit approximately 2m deep and at least 2m beyond the infestation.  In some cases this could not be accomplished without dismantling existing infrastructure.  Another option is to constantly cut the plant every 2-3 weeks, which could potentially exhaust the underground rhizome if continued diligently for 3-5 years.  A final mechanical option is cutting and smothering the plant initially and then cutting all surrounding re-growth for 3-5 years as in the cutting option.  Digging was used to treat the Whistler knotweed sites and a small site in Alice Lake Park.  Long term monitoring will determine the efficacy of this approach for small infestations (up to 15 stems). 
Chemical control

When does the risk of losing a riparian or natural area to destruction from knotweed outweigh the risk from potential herbicide contamination? As a society, we need to weighall the risks and develop best management practices.  Like a doctor prescribing medical drugs to heal a patient when “plenty of rest and fluids” aren’t enough, the selective use of herbicide has been one of the crucial tools used in other parts of the world when controlling knotweed. 

In considering the potential direct effects of any chemical on any biological organism, it is necessary to take into account two fundamental principles of toxicology:

  1. All chemicals are toxic (e.g., herbicides, caffeine, alcohol, nicotine, sodium chloride [table salt]), but some are more toxic than others;
  2. The degree to which a toxicological effect is expressed depends on exposure or dose, both in terms of the actual amount and the time frame over which it occurs (as an analogy, think of the difference in effect resulting from consuming several glasses of alcohol in say an hour, versus the same amount over an entire day)

The herbicide being proposed to control Japanese knotweed in the Sea to Sky Corridor is glyphosate, the same herbicide found in Roundup®.  This is a non-selective herbicide, which means it will target all plants, native and invasive.  Also importantly, it is non-residual, which means it will rapidly degrade, principally by micro-organisms, and therefore is non-persistent in soil and water.  The time required for the chemical to dissipate by half ranges from days to a few weeks depending on edaphic (soil characteristics) and climatic conditions.

There are three key reasons why this herbicide is being considered: i) its excellent record of efficacy and reliability in controlling knotweed; ii) its relatively favourable environmental behaviour profile (e.g. non-persistent in soils, vegetation and water, non-bioaccumulatory, very low leaching potential) and; iii) its relatively low innate toxicity to humans and wildlife.

Options for targeted application are as follows:

  1. Stem injection: herbicide is injected directly into each hollow stem cavity (3-5 ml of a 48% glyphosate solution; suggested for populations up to 300 stems in high use areas);
  2. Cut and fill: herbicide is injected directly into the hollow stem cavity after the above ground plant material has been cut down (3-5 ml of a 24% glyphosate solution; suggested for populations up to 300 stems in lower use areas); and
  3.  Foliar spray: herbicide is selectively sprayed from a backpack tank on the surface area of the plant (an 8% glyphosate solution; suggested for populations over 300 stems)

The Sea to Sky Corridor is part of an area that, for the first time, has a Pest Management Plan (PMP) intended to outline an integrated pest management approach to managing invasive species on crown land.  At a municipal level, integrated pest management approaches are decided locally.

If you’re interested in researching this topic further, there are many sources of factual, accurate and peer-reviewed scientific information on the internet. Unfortunately, there is even more unsubstantiated personal opinion, myth and outright inaccuracies or misrepresentations circulating through that medium.  Please always be cautious in accepting any singular piece of information as fact (including this article), and require presentation of solid data to substantiate a viewpoint.  Good science and derivative policies are founded on replication, peer review and the weight of scientific evidence principle.



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