There are a number of Japanese Knotweed eradication options employed to control the weed and they are generally based on herbicide application, physical removal and combination of the two. Each method has pros and cons and the choice of control option is determined by the site conditions, the nature and conditions of each stand, the presence of infested areas adjacent to the site and individual requirements of the client. Appropriate biosecurity measures must be applied when managing the Japanese Knotweed plant material to prevent it from spreading around the site or off-site unless this is a part of management plan and is properly controlled.
This is the most widely used method for Japanese Knotweed eradication due to cost effectiveness and high efficiency of treatment. It is applied in situ and minimises the risk of spreading plant off-site. The systemic type of herbicide is applied to the plant that will circulate inside the plant, will reach the rhizome system and will finally kill the plant completely. However, in contrast to contact type herbicide that kills almost instantly the part of plant where it is applied to, e.g. leaves, the systemic herbicide needs time to come into effect. Thus, single application of herbicide may not be sufficient and treatment can take up to three growing season until re-growth is securely suppressed. After this, further two years of monitoring are highly recommended to act appropriately if re-growth happens.
Application of herbicide must follow strictly the manufacturer notes. The herbicide concentration is the crucial factor of the successful treatment by spraying. If the herbicide concentration too high, this will kill foliage, thus preventing translocation of herbicide to rhizome system. In contrast, low concentration of herbicide will make treatment inefficient. It is also important to apply herbicides in right season when the plants are blooming ant foliage mass is at its maximum that is in August to October for Japanese Knotweed. However, there are a lot of records of successful treatments starting as earlier as from May. Special care should be undertaken to avoid spaying at non-target plants when non-selective herbicide is used.
Finally, when the removal of Japanese Knotweed material treated with herbicide is required it will be classed as hazardous material according to The Hazardous Waste (England and Wales) Regulations 2005 and the removal must meet legal requirements. Please learn about the current law in relation to Japanese Knotweed on our legislation webpage or visit official website to read the Regulations in full.
Several different herbicides with the following active ingredients can be used to kill Japanese Knotweed They include glyphosate, triclopyr, picloram and 2,4-D amine. However, only some formulations of glyphosate and 2,4-D amine are allowed for near water applications after receiving permission from Environmental Agency. Further, picloram was recently banned in the UK due to potential for environmental damage and contamination of watercourses.
This is quite interesting method used for Japanese Knotweed eradication that involves breaking the rhizomes to increase the foliage area to rhizome volume ratio and digging to increase air intake by the rhizomes. After such a treatment, the plant growth rate increases and further treatment with herbicides increases of the efficiency of herbicide translocation to rhizomes. However, this treatment requires removal of ground vegetation at the treated area and the soil after treatment must be compacted back to reduce the potential of new regrowth.
The method employs excavation of Japanese Knotweed and movement of its material including disturbed soil to the area on-site where it will be treated with herbicide. Probably the method is not as cost effective as when merely using the herbicide. It requires additional area on site. Once the Japanese Knotweed material is removed and stockpiled the work on the rest of the site can be continued as normal. The appropriate precaution option should be undertaken to avoid access to the stockpiled area.
The method requires a hole of up to five metres depth on site to bury the Japanese Knotweed material, with a cell membrane to be put prior to burying. Monitoring of the site will be required. Not cheap compared with the herbicide treatment.
The vertical geomembrane will be put to prevent from propagation of growth of rhizomes from adjacent site if properly fitted. Applicable to commercial sites and not cheap because needs at least two metres digging to put membrane in the soil and secure stitching.
Involves excavation of removed materials containing Japanese Knotweed and related contaminated soil followed by screening the material through sieving by the people serving the conveyor belts. May be extremely expensive due to number of people and specific equipment involved in the process.
The variations of excavation methods use transferring Japanese Knotweed plant material off-site to licensed landfill sites.
Environment Agency warns that wherever possible, you should treat Japanese Knotweed in its original location. You should only consider excavating Japanese Knotweed as a last resort, unless this is part of an on-site treatment method. If you use excavation for off-site disposal, you must take great care to avoid excess waste and make sure the excavated Japanese Knotweed does not contaminate surplus soil that is currently free from infestation.
If excavation is chosen for Japanese Knotweed eradication the risk of small pieces of rhizomes to be left at the treatment site is relatively high. The site should be carefully monitored and if the re-growth occurs afterwards the treatment with herbicides may be applied. If the weeds are treated with herbicides the rhizomes may turn to dormant state and additional treatment on the next growing season will be required. Property Care Association (PCA) recommends at least two-year monitoring period without re-growth once the treatment was finished.