Eradicating Japanese Knotweed: Getting Rid of an Invasive Species

Japanese Knotweed (Fallopia japonica) was among many plants introduced to the United Kingdom during the nineteenth century as an ornamental species for Victorian gardens. While many of these species are still popular today, others have become a problem. Defined as invasive non-native species, these plants are now controlled by legislation.

Bright green and with heart-shaped leaves and a cane-like stem resembling bamboo, Japanese knotweed bears white flowers in late summer. Its stems can reach three metres in height and its rapid growth rate – up to 10cm daily – means that it rapidly takes over any areas where it sets root. Since 1940 it has spread rapidly and is now found across most of England and Wales and large parts of Scotland and Northern Ireland.

Why is Japanese Knotweed a Problem?

Unlike giant hogweed, another invasive and fast-growing plant, Japanese knotweed doesn’t pose a risk to human health, but its rapid growth causes both environmental and economic problems. Environmentally it outgrows native plants, competing successfully for nutrients and growing space: the Environment Agency warns that it can block wildlife corridors and even block footpaths.

Its preference for habitats such as rivers and canals can cause flooding problems, while knotweed is capable of cracking concrete, causing particular problems of development sites. The Japanese Knotweed Alliance quotes figures from the Non-Native Species Policy Review estimating that the cost of eradicating the plant across the country would be over £1.5 billion (Japanese Knotweed Alliance, cabi.org, accessed 18 June 2010).

Japanese Knotweed and the Law

So great a problem is Japanese knotweed within the UK that it is one of many plants subject to legislation, most notably the Wildlife and Countryside Act of 1981. Under the terms of the Act it is an offence to allow certain plants, of which Japanese knotweed is one, to grow in the wild. The potential punishment for such an offence is significant: an offender may be fined or imprisoned.

In addition, the disposal of the plants attracts legal controls. The Environment Agency gives the details of appropriate legislation, which includes the Environmental Protection Act, and other hazardous waste regulations. Under these, the procedure for the disposal of contaminated waste, which includes remnants of knotweed or the soil in which it has been grown, is outlined.

Managing Japanese Knotweed

Part of the reason that Japanese knotweed is so strongly controlled is its resistance and its capability for regeneration. The plants spread through their rhizomes (parts of the root) so that removal of the main part of the plant is ineffective unless the entire rootstock is removed as well. Even a small root part left in the soil can generate a new plant within ten days.

Management guidelines for developers or those buying land suggest that it should be checked for knotweed beforehand (noting that the plant isn’t in evidence during the winter). If the plant is present, however, removal is a problem, as it can remain in the soil for a long period of time (the EA quotes an anecdotal figure of 20 years).

Manual controls, such as mowing or grazing, have short-term impacts only. Currently, apart from clearing a site of soil, the only effective solution is application of appropriate weed killers. The EA suggests that glyphosate is effective during flowering and triclopyr and picloram while the plant is growing. It is important that the weedkillers are applied according to the instructions and with care taken with respect to the surrounding environment, especially near water.

So resilient is the plant that even weed killers will need to be reapplied regularly, possibly over a number of years, if control is to be effective. The longevity of the rhizomes within the soil means that a year or more without evidence of regrowth doesn’t necessarily indicate that the knotweed has been eradicated.

Japanese Knotweed; Potential Biological Controls

In its native surroundings, Japanese knotweed is prey to insects. In March 2010, DEFRA approved the release of a sap-sucking insect which feeds on the knotweed and its hybrids but does not damage other plants – particularly important where native species are being ousted by the alien species (BBC ‚”Insect That Fights Japanese Knotweed to be Released”, by Rebecca Morelle, 9 March 2010).

This case is the first in Britain where a natural control method has been used as weed control in the EU (BBC); though it may not be the last. CABI, the organisation responsible for the release of the insects (a non-profit-making consortium with partners from 14 countries) has also investigated the possibility of using another natural control, this time a fungus, which affects knotweed alone. Research is still being undertaken into this possibility.

Japanese knotweed is not a problem in its native country because it is subject to natural controls as part of the ecosystem in which it naturally belongs. If the introduction of its natural predators is a success, it may be that a balance can be restored and the problems which the plant causes can, at least, be reduced.