Sustainable Control of Japanese Knotweed

Abstract

Control and management of invasive plants frequently differs from agricultural weed control as plant establishment and development progresses in less intensively managed systems. This is particularly the case for rhizome-forming invasive plants, such as Japanese knotweed (Fallopia japonica var. japonica) as minimum disturbance regimes permit the accumulation of significant below ground reserves that allow recovery from many physical, biological, chemical and integrated control methods. Here, we review the ongoing work of Jones, who established the world's largest and longest running invasive knotweed field trial. Using an integrated weed management (IWM) approach to testing, this research evaluated 19 different control treatments over three years to minimise pesticide use and increase the sustainability of controlling this ecologically and economically damaging species. Through consideration of plant biology, it was found that glyphosate-based herbicide treatments that exploited phenological changes in rhizome source-sink were significantly more effective than all other treatments. These results provide a roadmap to the more effective and efficient control of rhizome-forming invasive plants and emphasise the importance of scale appropriate empirical evidence to inform regulators when considering non-agricultural weed control. Within intensively managed arable agricultural systems, weed control is directed toward immature annual and perennial plants, during a 'critical period' extending for a relatively short time after crop emergence. This is because at this time, resource depletion by weed species may exert a major negative effect upon crop yield. Agronomic weed management may be achieved using a range of weed control methods, including: cultural/preventative (e.g. soil cultivation, disrupting weed establishment), physical (mechanical methods or hand weeding), biological (biocontrol or bioherbicides), chemical (plant protection products; PPPs) and integrated weed management (IWM). True IWM systems combine cultural, physical, biological and/or chemical methods; integrated herbicide management systems use a range of PPPs to mitigate selection of resistant weed populations. In contrast, control of invasive weed species, or invasive alien plants (IAPs) is commonly undertaken in less intensively managed systems, or unmanaged areas such as abandoned agricultural land, riparian areas and brownfield sites. Here, IAPs tend to be large and well-established so that plant persistence and development processes are often unhindered by weed control methods typically applied in agriculture. This is particularly true of rhizome-forming invasive species, such as Japanese knotweed (Fallopia japonica var. japonica) where minimal disturbance regimes permit the long-term development of significant carbohydrate reserves within perennating rhizome organs below the soil surface (>50 cm). Storage reserves permit recovery from repeated intentional disturbance and biological control; while physical size, depth, resilience and strong seasonal changes in source-sink strength of such organs preclude effective chemical control using many herbicides, as insufficient herbicide active ingredient is accumulated within storage tissues. There are strong environmental, ecological and economic cases for the management of IAPs to minimise their negative impacts. However, invasive plant management is hindered by the absence of scale appropriate empirical evidence to support control method selection and plant traits which are effective against control, e.g. rhizome bud bank. Consequently, control programmes may have less than optimal results in terms of economic and environmental sustainability. Terminology is critically important when defining clear programme objectives (i.e. control, management, eradication) and long-term assessment of IAP control and post-treatment habitat recovery is needed to establish best practice. In the case of Japanese knotweed, an approach that works with the seasonal resource translocation between above- and below-ground biomass and adequate herbicide coverage is the key to success. Physical disruption of the plant or increasing use of herbicide application will not give better control and may be less effective and costly. There is increasing public concern (real and perceived) about the widespread use of herbicides, and glyphosate in particular, resulting in increased PPP deregulation and reduced concentration and application rates. Experimental data that define best practice are essential to inform regulators when considering non-agricultural weed control. While the use of PPPs to control perennial IAPs is relatively small, the detrimental environmental, economic and amenity impacts are significant, such that the loss of effective PPPs should be of concern. In the absence of glyphosate, the only effective alternative for Japanese knotweed control would be complete physical excavation and disposal which would be significantly more expensive and arguably more environmentally harmful due to increased associated CO2 emissions and the risk of further spread.