Japanese Knotweed is a troublesome invasive plant species known for its aggressive behavior. It grows at an astonishing rate of up to 10 cm per day, thanks to its extensive root system that burrows deep into the ground. Unlike many other plants, Knotweed doesn’t have any natural enemies to slow down its rapid growth. It spreads mainly through small pieces of plants breaking off and taking root elsewhere, quickly covering large areas and outcompeting native plants. Along rivers and streams, these plant fragments can easily float away, starting new infestations in different places.

After flowering in July/August, Knotweed turns yellow and goes dormant as winter approaches. Its dried stalks remain visible throughout the winter, marking where it has spread.

The invasion of Knotweed poses several serious threats:

• Ecological Disruption: Knotweed grows so fast and thick that it shades out other plants, disrupting the balance of local ecosystems. This harms native plants and animals by reducing their access to sunlight, space, and food.
• Habitat Damage: Knotweed can take over stream banks, harming habitats vital for fish like salmon. This is especially worrying for the T’Sou-ke Nation, whose culture relies on healthy land and water.
• Environmental Risks: Knotweed worsens erosion, destabilizes stream banks, and increases the risk of flooding. Its shallow roots don’t hold soil well, leading to more sediment washing into waterways. Plus, its dry stalks can fuel wildfires, making affected areas even more dangerous.
• Damage to Buildings and Roads: Knotweed’s roots can penetrate through hard surfaces like concrete and asphalt, causing expensive damage to roads, buildings, and other structures.
• Safety Hazards: Standing up to 2 meters tall, Knotweed can block visibility along roads, making driving risky and causing accidents.

The District has been working closely with the T’Sou-ke Nation and Capital Regional District for over five years after the specifies was first identified in Charters Creek.

Unfortunately, surveys of the species inform us that we are losing the battle to control the plant in this area and a new approach is needed. The remaining solution is the application of herbicide. Herbicide application requires a Pesticide Use Application Permit from the Ministry of the Environment and Climate Change Strategy. Initial consultation with the Province indicates permit application submission is the next step to addressing the scale of invasive Knotweed in Sooke.

Once the Ministry of the Environment and Climate Change Strategy reviews the District’s application, further steps will be determined.

In the “toolbox” for controlling invasive plants, there are three control options: Mechanical, Biological, or Chemical. The knotweed site conditions (ie. knotweed plant biology, site proximity to infrastructure, effect on surrounding vegetation) influence which control methods is feasible to be used for the proposed knotweed sites.

Mechanical Control:

Knotweed regrows every year from its roots, so mechanical control methods would need to remove the roots to be effective. With a massive root system that can extend up to 20 m across and 3 m deep, the area that needs to be excavated would be very large, and may even include structures like culverts, sidewalks/roads.

Although cutting or mowing methods are also available, trials have shown that it can also stimulate the plant to grow back more aggressively and encourage the roots to grow further and deeper – making the plant even more difficult to control. Further, if mechanical methods are not carried out with extreme caution, it can create many plant fragments which can create even more infestations. With a plant like knotweed that can grow 10 cm per day, many follow-up visits (weekly or bi-weekly) would be needed to catch any new regrowth, which is not feasible.

Cutting, mowing, or digging might be effective for very small (less than 50 stems), isolated infestations that have not established a large root system yet. But the knotweed site that Sooke is hoping to control is not small or new, and would have a very large root system developed already.

Unfortunately, trials which have attempted to use only mechanical methods (ie. only cutting) to control large knotweed sites have not been successful.

Biocontrol:  

Biological Control, or biocontrol, is the use of an invasive plant’s natural enemies – like insects – to reduce the plant population.

Unfortunately, there currently are no biocontrol agents available for the control of knotweed.

Chemical Control: 

The knotweed root system is the growth-centre of the plant where energy is stored, so it is crucial to target the roots in order to provide effective long-term knotweed control. This means that an effective control needs to be absorbed into the roots – herbicides, which can be absorbed by the plants and translocated to the roots, would be an effective control method.

Herbicides need to be carefully applied to prevent any impacts to surrounding vegetation. There are targeted methods of application, such as backpack spraying using a wand, stem injecting, or wipe-on, which would limit the herbicide use only onto the knotweed plants. Additional items or considerations, like using tarps or applying herbicide when adjacent water levels are low, can further help prevent impacts to surrounding vegetation and prevent the herbicide from coming into contact with soil or water.

Currently, herbicide is the only demonstrated control method that has been successful in controlling large knotweed sites. This success requires only two visits per season by timing the treatments when the plant is actively drawing the herbicides into the roots. In fact, a 2010 Oregon State University study demonstrated 80% control of Japanese Knotweed after 1 year of herbicide treatment.

For this knotweed control work, three methods of herbicide application would need to be used: Stem injection, Foliar spray, and Wipe-on. The herbicide glyphosate would be used as it is the only herbicide whose label allows for it to be applied via the stem injection method, which is a necessary application method as depositing the herbicide directly into the knotweed stems can eliminate the risk of pesticide drift away from the plant.

Studies have also shown that glyphosate is non-persistent in soil and water, which means it does not stay active when it comes into contact with soil and water. Instead, it is degraded by microbial organisms (bacteria and fungi). This prevents leaching and uptake from other plants that did not have the herbicide applied to them.

Foliar Spray:  

Herbicide is carefully applied onto the knotweed leaves using low flow backpack or hand-held sprayers and spray wands.

Tarps would be used as herbicide drift shielding precautions to eliminate contact with either non-target vegetation or adjacent water.

Stem Injection:  

Herbicide is deposited directly into the hollow knotweed stems using hand-held injection devices. All stems of the plants would be injected, which requires the stems to be ≥ 0.5” for application to occur.

The risk of pesticide drift should be eliminated due to depositing herbicide directly within the plant.

Wipe-On: 

Herbicide is wiped on the leaves using wick applicators, which consist of an absorbent pad or brush device.

Risk of pesticide drift should be eliminated due to depositing herbicide directly onto the plant.

Please see:

• Habitat Aqua Technology Sheet
• Habitat Aqua Q & A Information Sheet

The best time to control the knotweed plants is while the plants are actively growing to trans-locate the herbicide into the roots. Given this, it would be anticipated that treatments would take place near September.

In any case, the timing would be adjusted to ensure that work is done outside of the salmon spawning window. Consideration would also be given when water levels are at their lowest.

All treated sites would be re-visited after treatment to monitor and assess the efficacy of initial treatments and re-treated if necessary or possible.