Context of the Study
Japanese Knotweed, Fallopia japonica, is an invasive alien plant species that was deliberately introduced to Belgium in the 1880s as an ornamental, forage, and soil-fixing plant. Extremely prolific due to its rapid growth and ability to reproduce through cuttings (a 0.1-gram mass of rhizome can produce a new plant), the species quickly spread, notably colonizing embankments, wastelands, road verges, etc.
Now widely spread, it is subject to prevention and control measures in Wallonia aimed at limiting its spread. Through its dense cover, it inhibits the growth of other native species and obstructs visibility along roadsides. Furthermore, its rhizomes (underground stems) can damage pavements and degrade infrastructure.
Commissioned as part of a construction project for an apartment complex in the Brussels region, E-BIOM developed an action plan for the management and control of Japanese Knotweed. The proposed management measures are defined considering the future use of the colonized plots and their accessibility.
One of the plots (300 m²), designated for future gardens, is difficult to access by large construction vehicles, making excavation and soil removal practically impossible.
The second plot (70 m²) is designated for the construction of a “talweg” (water collection line) and requires excavation.
Action Plan
In general, E-BIOM recommends acting on the underground organs, with localized rhizome removal typically yielding quick and effective results. In contrast, mitigation techniques on aerial organs (repeated uprooting and mowing, herbicide injection) require interventions over 5 to 10 years. These methods rarely result in the complete elimination of knotweed and can even promote its expansion.
In-situ Management and Treatment of Contaminated Soil
For the plot intended for the future gardens, crushing the contaminated soil with a minimum of two passes helps eliminate as many rhizomes as possible. This treatment must be done deeply and over a buffer zone of 5 to 10 meters around the area, as rhizomes can extend up to 7 meters underground and around the perimeter of the plant mass.
The area should then be covered with a black plastic tarp to inhibit the growth of new plants until the rhizomes decompose (at least 18 months), with a lateral overlap of 5 meters around the target area.
The combination of grinding and tarping of contaminated soil has already proven to be particularly effective: 100% mortality was observed on 11 sites across France, Switzerland, and Germany (rhizome size after grinding was approximately 10 cm).
Excavation management of contaminated soil
If the soil is to be excavated, the target area should ideally have a buffer zone to ensure complete removal of the contaminated soil. The soil can then be treated in-situ following the previously mentioned recommendations or ex-situ through thermal treatment.
In all cases, it is crucial to implement measures throughout the process to prevent the spread of Japanese Knotweed, such as thoroughly cleaning construction machinery.
Post-Treatment Prevention Measures
Once the contaminated soil has been treated, the potential recolonization of Japanese Knotweed can be prevented by planting species with a high competitive potential, such as Common Hazel (Corylus avellana), Commun Ivy (Hedera helix), and Old man’s beard (Clematis vitalba), etc. The site should then be regularly monitored, and any Japanese Knotweed growth must be immediately removed.
Conclusion
The management of invasive exotic species is the subject of much research, and E-BIOM stays up to date with innovative techniques, regularly adding new service providers to its contact list. Our goal is to offer the most relevant methods for the management and control of invasive exotic species.
Other examples of case studies conducted by our laboratory and expertise office:
