What damage can be caused by Japanese knotweed?

Japanese knotweed can cause damage to buildings and structures, to underground services and to hard landscape surfaces. The severity is sometimes somewhat overstated, although there are many recorded incidents where Japanese knotweed has indeed caused damage.

Does Japanese knotweed damage hard surfaces such as asphalt?

A definite YES, the most common form of property damage from Japanese knotweed is caused by laying a hard surface such as asphalt, patio slabs, driveway block paving etc. over Japanese knotweed infested ground. The knotweed will have no difficulty forcing its way through any weakness in the formation in its unstoppable quest for light.

Does Japanese knotweed damage underground services?

YES. Underground sewers, drains and land-drains are particularly susceptible to Japanese knotweed. The knotweed rhizome will find its way into the smallest hole on a pipe joint to find water. The rhizome will continue to grow gradually blocking the drain and finally breaking it apart.

We also have anecdotal evidence of Japanese knotweed being spread down surface water drains. Pieces of rhizome break off the parent plant and are conveyed down the pipe, infesting the watercourse with Japanese knotweed at the point of discharge.

Does Japanese knotweed damage buildings?

YES, damage can be caused to buildings by Japanese knotweed, particularly where the knotweed has been left to establish itself over many years. However, the extent is often overstated, and some of the horror stories you may read in the press or on the internet are there to sensationalise and scare you!  

Can Japanese knotweed grow through concrete?

YES and NO! There are many recorded incidents of knotweed growing through concrete. However, what actually happens is that the knotweed finds a weak spot (e.g. a crack in the concrete, or an expansion joint) and grows through it, growing and gradually prizing it apart. Knotweed will find the route of least resistance to get water and light.

Can Japanese knotweed grow in cavity walls?

YES, Japanese knotweed can grow within cavity walls. We have experienced stems and healthy leaves emanating through vents and air bricks located 2m above ground level. When knotweed grows in cavity walls it has the capacity to force the two skins of the wall apart.

There is a recorded incident of knotweed growing within a cavity wall of a single storey building, and forcing its way through the flat roof of the building.

Can Japanese knotweed destroy boundary walls?

Japanese knotweed can cause a fence or wall to lean and ultimately fall.

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