What does Japanese Knotweed Damage Look Like?

Take a look at our gallery of Japanese knotweed damage pictures and videos, showing the common types of damage that can be caused.

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Is Japanese Knotweed Damage as Bad as they Say?

There is no doubt that Japanese knotweed causes damage. Some over-react to the problem, whilst others don’t take it seriously enough. It is fair to say that extreme cases of damage have been sensationalised by the media in the past to create a good story, and even to scare people into thinking that their properties are about to fall victim to this invading terror.

The reality is that while Japanese knotweed causes similar amounts of damage to other problematic species such as bamboo or buddleia, it is unfortunately much more difficult to get rid of, giving it a well earned reputation as one of the UK’s most aggressive plants.

Of course, with Japanese knotweed, damage isn’t the only concern – as there are environmental and legal implications too.

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What Evidence is there that Japanese Knotweed Causes Damage?

Specialists across the UK have documented a huge portfolio of evidence of damage caused by Japanese knotweed over the last 25 years. The natural tendency of the media is, of course to highlight the worst-case scenario possible, meaning that the true scale of damage caused by Japanese knotweed is often overstated. That doesn’t mean that it should be ignored, however.

There have been reports in newspapers claiming that a property needed to be demolished as knotweed was growing under it. This is plainly not true, but to say that Japanese knotweed does not cause significant damage to buildings is also not true. As usual the truth lies between these two extremes of views.

A limited study published in 2018 called “Japanese knotweed (Fallopia japonica): An analysis of capacity to cause structural damage (compared to other plants) and typical rhizome extension” concluded that;

“(Japanese knotweed) should not be considered any more of a risk, with respect to capacity to cause structural damage in urban environments, than a range of other species of plant, and less so than many."

While it is true that structural damage may be rare, you will see from our gallery of images that damage in general is much more common.

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How does Japanese Knotweed Cause Damage?

Japanese knotweed is unlikely to cause significant structural damage to buildings indirectly by subsidence, or by collapse. Japanese knotweed does however cause damage, both by its above ground canopy exerting pressure on adjacent walls/fences, but also by its expanding network of underground rhizomes and roots, and, for mature stands, its crown.

Creeping, highly regenerative rhizomes have the ability to exploit weaknesses in built structures, and will then expand over time, exerting pressure on the element to cause damage. The general rule of thumb is that if water can penetrate it, so can knotweed.

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What Type of Damage is Caused by Japanese knotweed?

The damage will range from cosmetic, to minor and if left unchecked to significant damage which could prove costly to repair. Examples of damage include;

  • Complete blockage of drains and downpipes
  • Destruction of asphalt and resin surfaces
  • Collapse of stone and brick walls
  • Damage to cavity walls
  • Growth through expansion joints, growing internally in buildings
  • Lifting of patios
  • Damage to structural integrity, including damp
  • Blockage of chimneys, ducts and air vents

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What does Japanese Knotweed do to a House

The reason that Japanese knotweed is so problematic is that it is very successful at finding weaknesses in structures and exploiting any cracks. It can then cause damage. It can push through areas like cavity walls, drains and anywhere there is a weakness such as a crack or a fissure. Damage that undermines the structural stability of a property is however rare, and would only occur where the problem has been ignored for a significant period of time.

Once it finds its way into infrastructure, Japanese knotweed can cause more damage as it grows, widening gaps and causing mayhem along the way. If you find Japanese knotweed in your garden, it’s imperative that you do something about it as soon as possible.

If you have Japanese knotweed on your property, you have an obligation to declare it when you sell. You also have a duty to prevent it from spreading into the wild, or into adjoining property.

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Can Japanese knotweed grow through Concrete?

The simple answer to this question is no. When people first find Japanese knotweed on their property it often leads to a sense of panic and an attempt to get rid of it. One of the stories that we often see about this invasive weed is that it can grow through concrete but this is actually a myth.

Japanese knotweed is a highly invasive, fast-growing plant and it can cause significant damage which is why it is so important to get rid of it quickly.

No plant can actually get through solid concrete but it will seek out cracks to try and eventually break through. When the plant starts to grow more, it can force apart the surrounding concrete and cause more damage. If you have an area of concrete and it’s intact with no cracks and fissures, you should expect it to stay clear of Japanese knotweed.

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