Introduced to the UK during the Victorian era, the spread of Japanese knotweed has been beyond anything anyone could have imagined – and all the more devastating.
WEEDS are simply green plants in the wrong place at the wrong time, but some can establish and advance so rapidly that they smother, swamp and oust virtually all others. These are called invasive weed species and will typically colonise a wide range of ecosystems and environments. Those invading most methodically and with the worst effects are frequently (but not always) alien (exotic) plant species. These introduced plants, now free from the insect pests and diseases which evolved alongside in their natural distributions and often on the other side of the world, are free to run riot.
Three classic examples of alien invasive weeds rampaging across the UK landscape are Japanese knotweed (Fallopia species), Himalayan balsam (Impatiens glandulifera) and giant hogweed (Heracleum mantegazzianum), each introduced by plant collectors, horticulturalists and gardeners hundreds of years ago in what must have seemed like a good idea at the time. This trio of plant species have since graduated into highly-invasive weeds and now occupy significant areas of ecologically-sensitive environments and ecosystems, causing massive amounts of damage.
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All three of these alien invasive weeds have several things in common. They are of Asian origin, prefer damp and wet places, and have no natural predator insects or microbial pathogens in the UK which can exert any meaningful level of natural control. All three require the dedicated application of herbicides to achieve any sustainable, lasting control.
That said, even herbicide treatment will only manage rather than eradicate such weeds because their pervading nature makes it difficult to easily and safely access treatment using standard herbicide application equipment. At risk are watercourses, near to where they most frequently grow, and valued plants, including turf grasses near to and below these tall, invasive weeds. Custom-designed herbicide application techniques are required.
Research into the identification and development of insect pests and plant pathogens which exert a commercially-viable level of biological control is ongoing.
It entails identification and collection of specimens in the host weed plants’ centre of origin and the development of delivery systems (formulations) and application techniques to apply the active ingredient as a commercial biocontrol agent. That’s the relatively easy part. Obtaining permission to introduce equally alien organisms into the UK environment, where they could become pests or diseases in their own right, is a long drawn-out process with relatively low success rates.
Getting to the ‘roots’ of the problem
An invasive label means a weed is invariably damaging and difficult to control, but Japanese knotweed which covers a number of species belonging to the genus Fallopia (family Polygonaceae), native to Japan and parts of China, stands head and shoulders above the rest in speed of colonisation and damage caused. In their native Asian countries and environments, these Fallopia species are primary volcanic colonisers, and generally much less vigorous because they are kept in check by co-evolving species, including insect natural enemies and pathogenic diseases.
The culprit species usually found in the UK is Fallopia japonica var. japonica, introduced from Japan two centuries ago. Japanese knotweed continues to destroy ecosystems and urban infrastructure and is one of few herbaceous weeds which require the direct placement of herbicide inside the plant to ensure effective and lasting control.
The introduction of Japanese knotweed as an ornamental plant for early Victorian gardens must have seemed like a good idea at the time, but the contemporary weed legacy for the UK is huge and still growing. The success of Japanese knotweed is due to its capacity for rapid growth from an in-situ position through enormous, super-active and resilient rhizomes and assisted by the inherent nature of its favoured riparian and urban environments.
The dense growth of Japanese knotweed restricts access for riverbank inspection and increases flood risk through the large volumes of dead stems and leaves washed into rivers and streams. Attempts to remove established stands from riparian areas may cause instability in riverbanks and increase the risk of soil erosion.
Urban infestations cause considerable damage to hard surfaces. Tarmac is no problem for a weed that can force its stems through concrete. Factors favouring and assisting dissemination of propagules are moving water and human activity, including soil excavation, movement into landfill and fly-tipping.
The severity and seriousness of Japanese knotweed is clear to see from the amount of UK legislation used to try and limit its spread over the last five decades. Japanese knotweed is on the list of Schedule 9 alien invasive species under the Wildlife and Countryside Act 1981, making it an offence to plant or cause it to grow in the wild. It is additionally classified as ‘Controlled Waste’ under the Environment Protection Act 1990 and disposed of only at a licensed landfill site in accordance with the Environment Protection Act (Duty of Care) Regulations 1991. More recently, Japanese knotweed has come within the scope of the Anti-social Behaviour Crime and Policing Act 2014, which protects the public against individuals, businesses and other organisations failing to control Japanese knotweed on their property.
A weed plant profile
Red-coloured, spear-like aerial shoots emerge in spring, reaching 30 cm by April and proceeding to form a massive foliar canopy two to three metres tall by summer’s end. Plants produce large numbers of white flowers in summer, but hardly any viable seed, probably due to UK climate limitations on normal sexual reproductive development.
Above ground, stems die back and dry out in autumn, but overwintering rootstock and rhizomes (underground stems) are so fast-growing and all-pervading and consuming that nothing else stands a chance, even during the only ‘knotweed-free window’, which is late autumn and winter. Dead stems and leaves decompose very slowly to form a deep litter that prevents germination of other seeds.
The failure to produce viable seed is more than compensated for by a fast-growing and robust system of rhizomes, which provides the ‘vehicle’ to exploit and dominate environments and vegetative propagation for efficient spread.
Rhizome systems may extend up to seven metres from the parent plant and to a depth of three metres. Rhizome fragments down to 0.7 g can generate new plants and pieces of fresh aerial stem will grow shoots and roots in soil or water. Plants can achieve up to three metres of aerial and six metres of rhizome growth in a single growing season.
New shoots arise from the tiniest pieces of rhizome and cut-stem sections readily root at the nodes, while crowns will survive drying or composting to produce new canes once in contact with water. Attempts to mechanically control Japanese knotweed using strimmers, flails or diggers will invariably aggravate its spread.
Requires an ‘inside’ herbicide job
The application of herbicide is the only sure way to manage Japanese knotweed, but it is easier said than done due to the sheer size and resilience of the rhizomes.
Glyphosate is effective, but how much and for how long depends on the application technique used which is the overriding factor, determining the ultimate success of any chemical control programme against Japanese knotweed. Spraying with contact herbicide will burn off leaves, but rhizomes survive and produce new stems. The only sure way to dispatch Japanese knotweed is to introduce a systemically acting herbicide (such as glyphosate), which is approved for use against Japanese knotweed, and introduced directly into the plant by filling the cut stems or injecting intact stems.
The target area is the lower part of the aerial stem to minimise the distance for downward translocation of herbicide into the rhizomes, the underground part of the stem. The most appropriate timing is from late summer through autumn when rhizomes become the sinks for soluble food and nutrients translocated from the leaves before they senesce and fall off. As such, the introduced systemic herbicide is subject to the same strong basipetal (downward) translocation of soluble chemicals into the rhizome system, which is subsequently killed.
Hand-operated applicators used for stem-injection of the hollow stems with an approved systemically acting herbicide such as glyphosate are the preferred weapons of choice. For stem injection, a single shot (several ml) of herbicide is introduced into the hollow stem just below the first stem node up from soil level, as near as possible to the underground rhizome.
Such injection-mode applicators used for the control of Japanese knotweed are equipped with a sharp and robust needle able to penetrate a tough, full grown Japanese knotweed stem with a minimum of force applied. The needle will be equipped with an exit hole (for the herbicide) two thirds of the way from the needle tip and approximately 10 ml (mm) from its circular brass base.
The needle passes right through the mature stem until stopped by the brass base making contact with the outside of the Japanese knotweed stem. The needle is thus aligned so that the exit hole is safely and securely inside the hollow centre of the stem into which the herbicide passes.
Trying to deal with Japanese knotweed is beset with difficulties at the best of times and full of potential pitfalls if UK government legislation pertaining to this plant and weed is not followed. Anyone faced with a Japanese knotweed problem and attempting to manage and control a weed problem is strongly advised to read the UK government document listed in the reference section below.
What happened to biological control?
Biological control of Japanese knotweed invasions using insect pests and diseases which evolved alongside Japanese knotweed within its natural Far East Asian distribution (and thereby having prevented the plant from attaining pest status) is clearly the most environment-friendly and sustainable way forward. Continued focus on biological control is crucial, especially since serious question marks now hang over the future use of all chemical herbicides. Glyphosate, which is the main chemical in the current arsenal of herbicides for Japanese knotweed control, is still approved for use in a range of UK sectors but at this stage only until 2025. Be that as it may, many UK local authorities have already taken it upon themselves to go chemical-pesticide-free.
Research into biological control kicked off with scientists from the Centre for Agriculture and Biosciences International (CABI) scouring the East Asia native range of Japanese knotweed for potential insect pests and fungal pathogens as future candidates for natural biological control of the plant species in the UK and elsewhere. In 2009, the UK government announced how initial research had identified a promising candidate in the form of a sap-sucking psyllid bug called Aphalara itadori. After gaining the necessary government authority approvals, the first psyllids were released in 2011 at a number of sites in England and two sites in Wales with further releases in the following years.
A report by the government of Wales in 2014, three years after the first release, said low densities of adults had been found at some of the release sites, indicating successful overwintering, but admitted this potential insect pest of Japanese knotweed clearly had difficulty in establishing self-sustaining populations. In an effort to overcome this problem, CABI would conduct future caged field trials to run concurrent to the main trials. Caged trials would involve releasing larger numbers to establish higher population densities with the aim of demonstrating effective damage to the weed.
Media reports during this time were exuberant and ecstatic with headlines like ‘Days of Japanese knotweed numbered’ but clearly over-optimistic because some nine years later the alien invasive weed continues to invade the UK. British hotspots include Bolton and St Helens in north-west England, Bristol in south-west England, and Wales. Indeed, I shall never forget taking the valley line (the Merthyr Line) from Cardiff to Aberdare some 10 years ago and seeing huge amounts of Japanese knotweed growing on the banks of the River Taff.
Things appear to have gone very quiet since then. It is well known that once established, biological control takes a relatively long time to work, but in this case it appears to be taking an inordinate amount of time to research and establish.
Previous experience suggests that biological control can take between five and ten years from initial release until a time when significant and commercially worthwhile control can be achieved. It is now around 12 years since first release of the psyllid bug.
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