Author: Mike Clough

Date Posted: Wednesday 29th April 2015

Is Japanese Knotweed a big problem in Japan? …This is a question I get asked on a regular basis, with people wondering whether the Japanese have some secret strategy for dealing with this problematic plant. Other regular questions are… ‘is it poisonous?’ and… ‘could I feed it my wife to get rid of her…?’.

In Japan although sometimes regarded as a weed Fallopia japonica is nowhere near as problematic as it is when it has accidentally been introduced. Japanese Knotweed is a ‘ruderal’ species meaning it is one of the first plants to colonise bare ground. The dwarf variety var.compacta is often the first plant to colonise volcanic lava and ash fields where its tolerance of sulphur dioxide enables it to survive where other plants would simply be unable to cope. In these types of habitats var.compacta is often the only plant to be seen.

In the UK Japanese Knotweed is very much the aggressor – easily outgrowing the native herbaceous communities with its prolific growth rate (upwards of 30cm a week). In its native Japan the situation is rather different with knotweed being just another plant within a huge herb community all fighting for dominance with the other plants native to the region.

The Japanese Knotweed plants in Japan also have the local pests and diseases to contend with and suffer predation from a whole range of invertebrates and fungi. It is when a plant is introduced to a new setting without these pests and diseases that they thrive and out-compete the indigenous population.

It is to this end that CABI has researched the introduction of a variety of insects and plant pathogens to try and come up with an answer to the continued spread of Japanese Knotweed. The basic premise of biological control is very simple – you simply introduce something that predates on the target in its native setting – into the new environment in which the target has become a problem. Whilst the premise is simple, the possible side effects and ramifications of introducing further alien species is highly complex.

Once a new species has been let loose – it’s very difficult to put it back in the box!

What about the possible knock on effects? Introduce an insect to eat Japanese Knotweed, then a wasp begins to eat the insect and becomes over dominant? …or something else begins to eat the wasp and the whole balance of the ecosystem changes…

History would suggest that there have been some successful Biological Control programmes against alien species. In Britain Japanese Knotweed was introduced without any natural enemies and only a few of our native species will touch it. We therefore get these enormous stands of knotweed which grow tall and healthy – untouched by any insect or disease. These plants then pour their resources into creating the storage rhizomes beneath the ground allowing them to both survive over winter and to defend themselves against surface applied herbicides which are mainly foliar applied and often just kill the above ground growth.

The systemic herbicides will at times, only kill part of these extensive systems and may even encourage the germination of buds on the periphery of the rhizome network (Bailey).

In Japan all parts of the plant are under attack, even the rhizomes are targeted – the leaves will usually show damage and it would be rare to find a leaf in perfect condition. The plant is also attacked by various fungi with varying degrees of specificity.

The Japanese Knotweed in the UK is a clone of the original plant brought in to Kew in the 1840’s – it has no close relatives amongst our native flora or our farmed crops – and as such has been identified as a perfect target for bio-control.

So the answer to the question – is Japanese Knotweed as problematic in its native setting? – would be a resounding ‘no’…due to pests and diseases it has a tough time in Japan…and taking our cue from this… we intend to make it feel just as uncomfortable in the UK.

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Mike Clough

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