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Are there Japanese knotweed hotspots in the UK?

Published on by Phlorum

Japanese knotweed (Reynoutria japonica) is a perennial, invasive weed, that is non-native to the UK and has several detrimental effects on both UK plants and land. Due to knotweed having no natural competition from UK plant species or threat from wildlife, it has been allowed to grow throughout the UK and its damaging effects are becoming more widespread.

The best time to identify Japanese knotweed is during the growing season, which lies between mid-summer and early autumn. Key identifiers during this period are tall, hollow, bamboo-like stems with purple specks, spade shaped-leaves that alternate along each stem in a zigzag like pattern, and small white flowers that form in a cluster.

Knotweed dispersal

Image credit: NBN Atlas

Where is Japanese knotweed most likely to grow in the UK?

Due to the widespread nature of knotweed in the UK today, it is difficult to pinpoint exact locations where it is more prevalent. The above Japanese knotweed distribution map UK highlights areas where higher levels of knotweed have been recorded, however, due to its intensity across the UK, it cannot easily be categorised into precise zones.

It can be simpler to identify environmental features that commonly accommodate knotweed and also indicate where it is more likely to grow successfully.

For example, in a domestic setting such as a garden, knotweed may have been transported onto the area via propagules (in the form of rhizome fragments) in contaminated, imported soil. Later, propagules can grow into a stand of knotweed with the capabilities of displacing other plant species in the area and spreading further if unknowingly cut or mowed and disturbed.

Furthermore, in a larger setting such as a commercial building site or wooded area, knotweed has the potential to grow in many areas but specifically is often located in areas that have human or vehicle access, due to the likelihood of people or machinery tracking knotweed rhizomes across the surface. There is also the higher probability of vegetation being discarded there that contains contaminated material.

Watercourses are also common routes of knotweed spread due to rhizome fragments being broken (either naturally or manually via human management) and carried downstream until they lodge at a different location and regrow. In addition to this, rhizome fragments can unknowingly be picked up on a commercial site on work boots, machinery etc and moved elsewhere.

Using the above information, it can be stated that there are notable areas of the UK that exhibit higher levels of knotweed, largely associated with their equally high levels of development, such as South Wales, East London and Manchester.

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