August Study Finds Potential Control Strategy for Infestations of Japanese Knotweed

Removal of moisture has a 100% success rate on killing Japanese knotweed plants and regrowth under lab conditions

 A research study from NUI Galway in collaboration with multinational infrastructure consulting firm AECOM and University of Leeds has found that the removal of moisture could act as a potential control strategy for smaller infestations of Japanese knotweed, which are particularly common in urban settings.

Findings show that incorrect herbicide treatment cannot control the growth and regeneration of this invasive plant, but that fully drying the plant material in a lab environment allowed it to be returned to the soil without risk of regrowth. The research also showed that if there are no nodes attached to the rhizomes (root like underground shoots), there is no regeneration.

Japanese knotweed (Reynoutria japonica) is a problematic invasive plant found in many areas of Europe and North America. Notably, in the UK, the species can cause issues with mortgage acquisition. It can grow up to two to three metres in height and can dominate an area to the exclusion of most other plants. Control of Japanese knotweed is complicated by its ability to regenerate from small fragments of plant material; however, there remains uncertainty about how much rhizome is required and how likely successful regeneration is under different scenarios.

The study, published in the journal PeerJ today (12 August 2021) investigated the ability of crowns (underground mass from which rhizomes and shoots emerge) and rhizomes with different numbers of nodes to regenerate successfully from three sites in Yorkshire and Lancashire in the north of England. Two of the sites had been subject to herbicide treatment for two years prior to sampling and the third site had no history of herbicide treatment.

The study found that the success of regeneration is related to plant fragment size, with larger fragments more likely to successfully regenerate and, for rhizomes, if there is no node, there is no regeneration. Additionally, it was found that the removal of moisture on living material resulted in 0% regeneration after plant material was dried and replanted.

Senior author of the study, Dr Karen Bacon, Lecturer in Plant Ecology, Botany and Plant Sciences, Ryan Institute, NUI Galway, said: “Japanese knotweed is one of the most invasive plant species in the world and has major negative impact on ecology and biodiversity. The findings of this study that showed virtually no difference between the regrowth of treated and untreated Japanese knotweed samples suggest that herbicide treatment, which is often the most suitable approach to tackle the species, is not always being done effectively.

“We also show clearly that the size of the plant fragment is critical to the initial regrowth, with smaller fragments producing much smaller regrown plants. Additionally, if there are no nodes, there is no regeneration, which may suggest potential management strategies in the future. This also highlights that small infestations and plants should not be viewed with the same concern as larger ones and that rapid management should be a goal of tackling this problematic species.”

Dr Bacon added: “Our finding that the removal of moisture has a 100% success rate on killing Japanese knotweed plants and preventing regrowth after they were replanted also raises an important potential means of management for smaller infestations that are common in urban environments. This requires additional field trials, which we hope to undertake in NUI Galway soon.”

When crowns and rhizomes were planted in lab conditions, no significant differences were observed in the new stem diameter, maximum height of stem or maximum growth increments among crowns, when comparing plants that had been treated with herbicide for two years to plants that had no history of herbicide treatment. This shows the importance of monitoring treated areas for regeneration and sustaining treatment over longer periods. Crown material had a higher regenerative capacity, with all traits measured from the planted crowns being significantly greater than those of the planted rhizome fragments.

At least one node was necessary for successful regeneration (regrowth) of rhizomes and the smallest initial fragment weight to regenerate and survive the experiment was 0.5 grams. 0.7 grams was the previously reported smallest fragment to regenerate. It should be noted that such tiny fragments produced only very small and weedy plants that would take many years to regain health and spread significantly, if they survived.

After the 60-day growth experiment, all plant material was subjected to the removal of moisture through air drying by sitting on the lab bench for 38 days until all plant fragments were at their dry weight. They were then replanted in soil and provided with the same nutrient, water and light conditions as the growth phase of the experiment. This resulted in no regrowth (emergence or regeneration) after replanting. These findings suggest that the removal of moisture could be a valuable addition to management strategies for small to medium scale infestations of Japanese knotweed common in urban settings, by providing a new option for dealing with plant material removed from the soil (the disposal of which can be costly).

Dr Mark Fennell, Associate Director at AECOM and co-author of the study, said: “Our latest research sought to add to existing knowledge about how to manage and remove Japanese knotweed. Our key finding, that drying out the plant effectively kills it, should provide reassurance to landowners that the plant is not as indestructible as is often stated. While this invasive species remains a problem plant that can have a negative impact on biodiversity, our research provides a better understanding of the plant, paving the way for the development of more efficient and cost-effective ways of dealing with it. We hope our research helps to challenge some of the popular stigma that surrounds Japanese knotweed.”

The findings from this study coincide with a UK-based review of the advice around how to manage Japanese knotweed in the built environments. Dr Karen Bacon and Dr Mark Fennell are both members of the Japanese knotweed specialist group advising the Royal Institute of Chartered Surveyors. The new guidelines are currently out for public consultation with the hope that final versions may be released later in the summer or early autumn.

Read the full study in PeerJ here: https://peerj.com/articles/11783/. 

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