What’s the problem?

So why are people so concerned about Japanese Knotweed?

There are four main reasons why Japanese Knotweed is a problem in Devon and elsewhere:

  • It spreads easily via rhizomes and cut stems or crowns
  • It out-competes native flora
  • It is difficult and expensive to control or eradicate.
  • It can cause structural damage to buildings.

Because of its regenerative properties and invasive habit Japanese Knotweed is listed under the Wildlife and Countryside Act 1981 as a plant that is not to be planted or otherwise introduced into the wild. The Environmental Protection Act 1990 also lists it as ‘controlled waste’ to be disposed of properly. You have a legal obligation not to cause it to spread if it occurs on your land.

Specific problems caused by Japanese knotweed are:

  • Damage to paving and tarmac areas.
  • Damage to flood defence structures.
  • Damage to archaeological sites.
  • Reduction of biodiversity through out-shading native vegetation.
  • Restriction of access to riverbanks for anglers, bank inspection and amenity use.
  • Increases erosion when the bare ground is exposed in the winter.
  • Obstructs visibility and access on roads and paths.
  • Reduction in land values.
  • Increased risk of flooding through dead stems washed into river and stream channels.
  • Increased risk of soil erosion and bank instability following removal of established stands in riparian areas.
  • Accumulation of litter in well established stands
  • Aesthetically unsightly.
  • Expensive to treat (£1/sq.m for a spraying regime over 3 years not including re-landscaping)


A 2004 British Government Review of Non-Native Species Policy gives an estimate of the costs to control knotweed countrywide of £1.56 billion which, although unfeasible, gives an indication of the extent of the problem and the high costs associated with control were it to be attempted. Swansea is one of the worst affected areas in the country and as such has the most experience with the costs associated with managing knotweed. These are some of their cost estimates:

£140,000 for planning department treatment of established populations over the last 6 years.

Using quoted figures of £1 per square metre for spraying and £8 for landscaping estimates for completely treating the current infestation in Swansea would cost around £9.5 million.

At the current rate of treatment (2ha/yr) the current infestation will take 50 years to treat without accounting for its rapid spread (more than 2ha/yr)

The costs of removal from development sites are very large – One 30m x 30m site cost developers an extra £52,785 to deal with the Knotweed on the site.

I’ve got Japanese Knotweed, what do I do?

Don’t panic! But do start to plan how you are going to control, contain or eradicate it. Here is a checklist of actions:

  • Find out more – is the plant really Japanese Knotweed? What can I do to control it? Look at the other links on this web site.
  • Seek advice from the Devon Knotweed Forum members if appropriate.
  • Seek advice from the Environment Agency if treating Japanese Knotweed near a watercourse.
  • Find out the best options for your kind of site.
  • Make a plan of action to control it and expect an expensive, long-term undertaking.
  • Implement treatment, suitable for your site, to control it – early treatment is best.
  • Notify the Devon Biodiversity Records Centre of the location and size of the colony if you live in Devon.
  • Remember you have a legal obligation not to allow it to spread.
  • Remember: Early treatment of a new colony is vital as it can soon invade large areas and this will make it more expensive and difficult to control later.

Japanese Knotweed homepage