Invasive Species Series: Japanese Knotweed

In the UK it has been estimated there are nearly 2,000 established non-native species, 10-15% of which cause significant adverse effects. Whether these effects are environmental, economic or social they all come with a price tag and it is estimated £1.7 billion is spent annually on the control of invasive non-native species in the UK.

The most direct implications for our native plants and animals are the threats of predation on, and/or competition with non-native species. In some cases native species are unable to compete and can be displaced by the invasive non-native species entirely, our red squirrel being a relatively well known example.

There is a whole realm of legislation associated with restricting the spread of invasive fauna and flora. Implications of ignoring an invasive species on site, not disposing of it correctly or allowing it to spread into the wider area can be severe and could result in fines or legal action*.

The principal piece of legislation dealing with non-native species however is the Wildlife and Countryside Act 1981 (WCA) (as amended). Section 14(1) of the WCA makes it illegal to release or allow to escape into the wild any animal which is not ordinarily resident in Great Britain and is not a regular visitor to Great Britain in a wild state, or is listed in Schedule 9 to the Act. It is also illegal to plant or otherwise cause to grow in the wild any plant listed in Schedule 9 to the Act. Additionally, for some species (such as Japanese knotweed), plant material removed off site is regarded as controlled waste and comes under part II of the Environmental Protection Act 1990.

The First Invasive Non-Native Species of the Series: Japanese Knotweed

One of the most common and well documented invasive species that we come across regularly on development sites, is Japanese knotweed and this species alone is estimated to cost the British economy approximately £166 million per year (The Great Britain Invasive Non-native Species Strategy August 2015).

It is recognised as one of the most invasive plants in the UK and is listed on Schedule 9 of the WCA. It was introduced from Japan in 1825 as an ornamental plant, however it has rapidly spread, overwhelming other native plants. It is more often found on urban, disturbed sites, and is commonly found on construction sites. It grows rapidly and has been documented to have grown through walls, tarmac and concrete so can cause expensive damage to property.

Instead of producing seeds it forms underground rhizomes (which is like a stem) and the plant can re-grow from just a tiny fragment of rhizome. The main causes of spread are via topsoil movement, construction traffic and from dumped garden waste. This also means it is costly to eradicate from a site as disposal of contaminated soil is expensive. It is estimated that eradicating it from construction sites can cost well over £1,000 per square metre, however the cost of not eradicating the plant in terms of structural and/or legal risk could be far higher.

How to identify Japanese Knotweed

It has a distinctive, zig zag, hollow, bamboo-like stem, covered in purple speckles, often reaching 2-3 m in height. The leaves of the mature plant are up to 12 cm in length, with a broad oval-triangular shape and a flat base. The leaves are arranged alternatively on arching stems. Knotweed flowers late in the season (August to October). It has numerous small creamy-white flowers hanging in branched spikes. The underground rhizomes are thick and woody with a knotty appearance and when broken reveal a bright orange-coloured centre. The rhizome system may extend to, and beyond, a depth of at least 2m and extend 7m laterally from a parent plant.

(Images Courtesy of Pixabay)

What to do if you find Japanese Knotweed

When undertaking Preliminary Ecological Appraisal/Phase 1 Habitat Surveys, our ecologists will look for evidence of invasive species.  If Japanese knotweed is identified on your site, it is important to dispose of it using a specialist contractor who is trained to work with this species. It should be done following a documented method statement allowing a paper trail to prove the works were carried out in accordance to best practice. This will mean should reinfection occur there is documented evidence to show no legislation has been breached and no offence committed as every effort had been made to eradicate it from site.

*More information about relevant legislation can be found on the GB Non-Native Species Secretariat website