- Damaged infrastructure – Knotweed can grow through concrete and have been seen to damage roads and buildings.
- Sightline and Safety issues – Knotweed can grow up to 2 m tall, and if they are located at culverts of ditches along the road, the plants can obstruct sightlines for vehicles. This can cause major safety concerns for drivers and lead to potentially dangerous driving conditions.
- Erosion and Destabilization of drainage and stream-banks – Knotweed root systems do not hold soil like other plants, and when they grow along water, the sediment can be easily washed away.
- Wildfire risk – Knotweed dies back every winter, leaving dried stalks which are very fibrous that do not decay quickly. The stalks stay at the site and can end up adding to fire risk as fuel.
- Flood Risk – Knotweed plants growing at drainage culverts or ditches, or at streams and creeks, can reduce the space for water to flow. Along with the sediment that are not being held by the roots, this can lead to flooding if the plants continue spreading and blocking water movement.
- Habitat loss and environmental harm – Knotweed grows very quickly, up to 10 cm per day. A piece of plant fragment as small as 2 cm can grow a whole new plant. Because of this, knotweed creates dense infestations that crowd out native plants. This interruption to the food web leads to fewer food sources and decreased feeding areas for salmon, other fish and waterfowl species.
- Loss of recreation area – Knotweed’s root system can get up to 20 m across and 3 m deep, and it can grow back more aggressively if mowed or cut. If the knotweed keeps spreading, it can quickly take over and reduce the amount of park space that people can readily use and access.
Why is Knotweed a problem?
If left to grow and spread, knotweed can cause lasting damage to city infrastructure and our natural environment. The impacts which Abbotsford is trying to prevent are:
What is Abbotsford currently doing about Knotweed?
Abbotsford has been successfully managing knotweed along roadsides and other Abbotsford-managed areas through the Fraser Valley Regional District Weed Control program. Many knotweed sites have been significantly reduced or eliminated with these treatments, but knotweed control can take several years before successes are seen.
Why does Abbotsford need a Pesticide Use Permit?
Currently, knotweed located too close to waterbodies, like sloughs or culverts, cannot be treated under the Provincial BC Integrated Pest Management Act, which requires a Pesticide-Free Zone (PFZ) buffer of 1 m. But there are several knotweed sites in Abbotsford located in that PFZ area which need to be controlled. This is why Abbotsford has applied for a Pesticide Use Permit, which would allow for herbicide use to control the invasive knotweed plants in that buffer area. The permit will allow for the treatments at the proposed sites only for a period of 3-years.
Why is Abbotsford using herbicide to control Knotweed?
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.
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 sites that Abbotsford is hoping to control are 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.
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.
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.
What about using goats to control knotweed?
Since successful control of knotweed relies on targeting the root system, goats grazing on the above-ground plant parts of knotweed would not affect the knotweed roots – it would only be another form of mechanical control. This would not be an effective method on these knotweed sites, and may even stimulate the plant to grow back more aggressively.
How is Abbotsford planning to control Knotweed?
For this knotweed control work, three methods of herbicide application will be used: Stem injection, Foliar spray, and Wipe-on. The herbicide glyphosate will 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 onto it.
The herbicide application method chosen will be based on the site, and will depend on site conditions, such as proximity to walking foot-traffic, water, other infrastructure.
Herbicide is carefully applied onto the knotweed leaves using low flow backpack or hand-held sprayers and spray wands.
Tarps will be used as herbicide drift shielding precaution to eliminate contact with either non-target vegetation or adjacent water.
Herbicide is deposited directly into the hollow knotweed stems using hand-held injection devices. All stems of the plants will be injected, which requires the stems to be ≥ 0.5” for application to occur.
Risk of pesticide drift should be eliminated due to depositing herbicide directly within the plant.
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.
Where are the Knotweed sites located?
There are 40 proposed sites currently anticipated for the treatments conducted under this Pesticide Use Permit. These sites were chosen based on reports from the public and from Abbotsford staff as they encounter these problem knotweed plants while maintaining the Abbotsford drainage and parks facilities. All of these site locations have knotweed plants growing at drainage culverts/ditches, along dykes of canals and sloughs, and adjacent to creeks/streams where they have potential to cause lasting negative impacts and need to be controlled.
These sites will be revisited to confirm again that they are suitable for treatments this spring, and adjustments will be made based on these site visits as well as based on public feedback.
When will treatment occur?
The best time to control the knotweed plants are while the plants are actively growing in order to trans-locate the herbicide into the roots. Treatments are anticipated to start during late-September 2021 until September 2024.
There will be one treatment in Fall 2021, and subsequent treatments will begin in late May or June, repeated at 6 to 8 week intervals as needed until the plants go dormant (late May-June, August-September, and October-November).
Timing will adjust to ensure that work is being done outside of the salmon spawning window, and will consider when water levels are at their lowest during the dry part of the summer. All treated sites will be re-visited after treatment to monitor and assess the efficacy of initial treatments, and re-treated if necessary or possible.