Japanese knotweed (Fallopia japonica, or Polygonum cuspidatum) is regarded by many people as one of the most invasive and undesirable plants in the western hemisphere. Resistant to most weedkillers, devoid of natural enemies and capable of aggressive regrowth from tiny remnants of its root system, it is a gardener's worst nightmare. Japanese knotweed is common alongside roads and rivers throughout the UK and Ireland, where it forms dense thickets. It has also spread relatively unhindered through many parts of Europe, the USA, Canada, New Zealand and Australia. It is particularly prevalent in parts of London, Wales and southern Ireland.

A Potted History

There are no prizes for guessing where Japanese knotweed comes from originally. Its natural habitat is on the volcanic slopes of Japan's mountain ranges, where its spread is well controlled by local pests and parasites. It was introduced to Britain in 1825 as an ornamental plant and for a while it was a must-have for any gardener worth their salt ¹. It was a feature of many Victorian gardens throughout much of the 19th Century. Well-intentioned gardening folk, not quite understanding the true nature of the plant, distributed its cuttings throughout Europe and North America. Everywhere it was planted it grew vigourously: the land was devoid of any predators, pests or parasites to keep its growth in check. It propagated quickly within gardens, eventually escaping such artificial confines and breaking out into open countryside. Since then, its growth has been alarming. The plant is now found almost everywhere in the UK and Ireland: particularly beside roads and rivers and on waste ground. Its introduction into the wild is forbidden under the 1981 Wildlife and Countryside Act in the UK.

Knotweed Spotting

The best time to recognise Japanese knotweed is during mid-summer and early autumn. The plant is characterised by dense clusters of large, heart-shaped leaves, each about the size of the palm of your hand. The plant grows from the ground in multiple woody shoots, which can be green or red in colour. The shoots grow quickly, reaching heights of between one and three metres. Very dense thickets are formed where the weed is long-established. These thickets can spread in size by a few square metres each year, displacing most other plants in the process. The multiple small white flowers, which normally appear in late August and early September, appear together in dense protruding spikes. There is a die-back in winter, but the barren shoots often remain in place throughout the season.

How It Spreads

Interestingly enough, almost all Japanese knotweed outside of Japan is female, possibly cloned from a single introduction. In general, therefore, it does not spread through seed germination. So how has it become such a successful invader? The answer is in the underground stems, known as rhizomes, which can grow horizontally from a single plant to a distance of up to 20 metres. Each rhizome can form multiple new shoots, each genetically identical to its parent. These rhizomes are almost impossible to eliminate. Less than a centimetre of rhizome can swiftly create a viable plant. As people fought to control the weed, rhizomes were carelessly dumped in rivers and along waste ground, further spreading the weed to new localities.

Environmental Damage

Japanese knotweed is an undesirable plant in a number of different ways. Because of the thickness of its thickets and the density of its leaves, it prevents significant amounts of light from reaching the ground, so no other plants can grow in areas where the weed has become established. It finds most soils to its taste and propagates aggressively, displacing native flora in the process. Rare plants and overall biodiversity are threatened by this plant if it is left unchecked. The shoots can cause damage to buildings and structures, as they are capable of breaking through concrete and tarmac. Cemeteries, roads, residential dwellings and historical sites have all been extensively damaged by the plant. Because even a small quantity of rhizome can cause regrowth, it is extremely difficult and very costly to eradicate. In addition, many insects, such as bluebottle flies, are attracted to (and quickly poisoned by) Japanese knotweed. This may have long-term implications for local bird and animal populations.

Getting Rid of It

Japanese knotweed is not an easy plant to kill. Cutting down the shoots only spurs its growth, and attempts to rid the ground of its dense network of rhizomes are enormous undertakings which are unlikely to succeed. It is resistant to most weed killers but can be killed by the chemical glyphosphate - the active ingredient in a number of well-known commercial products. Even so, repeated application over a period of three years is normally required to completely eradicate the weed. Some experts advise using flame-guns on young shoots to control the weed's growth.

Another recommended approach is to steal the plant's light by cutting it down to ground-level and placing a heavy flexible material, such as an old carpet or Plantex membrane, on top of it and around it. The method takes time and it is not advisable to do any gardening in adjacent areas while the weed is dying.

Another approach is biological control - the plant has natural pests in Japan such as the leaf beetle and rust fungus, both of which are effective in controlling its growth there. Introducing these pests into the countries where Japanese Knotweed has run amok would seem, on first glance, to be sensible, except for the fact that this might make a bad problem worse, were the new arrival to attack a native plant even more aggressively than the plant it was intended to control in the first place! Nevertheless, controlled experiments are taking place to ensure that such 'collateral damage' can be minimised.

Because of its high potential for regrowth, the movement of Japanese Knotweed is strictly regulated under various laws and subject to extreme restrictions. Best practice indicates that, where possible, the weed should be burned or buried in situ, so as to minimise the possibility of an outbreak elsewhere.

The Future

Japanese knotweed is already a significant threat to gardens, buildings, roads and agricultural land throughout the western world, but the worst may be yet to come. There is a possibility that the plant could set viable seed by hybridising with related species such as the giant knotweed or Russian vine, thus enabling it to populate new areas in entirely different ways.