Invasive non-native species - indicator two

The data on invasive non-native species in London is outlined below. This is part of the wildlife section of the London State of the Environment Report 2010.

What are invasive non-native species?

Invasive non-native species are those which are not native to the UK and spread rapidly in the wild after introduction as they have no natural predators or ecologically limiting factors.

These species often cause a decline in native species for a number of reasons, including increased competition for habitat and food.

About this indicator

The indicator will look at distribution in selected species, and any changes over time.

Assessment of invasive non-native plant species in London include:

  • Floating Pennywort was first introduced into the UK the 1980s. In some cases this weed can completely cover the river channel. This can lead to a decline in native plant species and deoxygenate the water, killing aquatic life. Weed cover can also restrict flow and cause flood risk issues.
  • Japanese Knotweed was first introduced into the UK in the mid 19th century. This plant grows on the river bank and colonises rapidly using its roots (its seeds are not presently viable in the UK).  Its ability to grow from tiny root fragments, mainly spread by people makes it one of the most widespread invasive weeds in London and across the UK.
  • Giant Hogweed was first introduced into the UK in 1893. This weed also grows rapidly, due to the disposal of seeds by water, colonising river banks. This weed can cause human health issues, from a poisonous sap on the leaves.

Assessment of the distribution of non-native crayfish species in London will include the Signal crayfish, Turkish crayfish, Red Swamp crayfish and the Virile Crayfish.

The data is collated from local records centres, the Environment Agency and other recorded sightings of invasive non-native species.

The invasive non-native species framework strategy was launched in 2008. This strategy outlines the key actions required to tackle problems arising from invasive species, and improve resilience to the impacts.

The London Invasive Species Initiative has been established to help collate data on the distribution and spread of invasive species in London, and develop action plans to address the species of most urgent concern.

Overall trends

Distribution of invasive species

A map showing the distribution of non-native invasive species in London. The data is based on recorded sightings data from the Local Records Centre and the Environment Agency. It shows that Japanese Knotweed is the most common invasive plant species in London

Japanese Knotweed

Japanese Knotweed is the most commonly found invasive weed in London, and has almost doubled its distribution over that last 20 years. It has been recorded on most main rivers in London, but predominantly on the River Roding, River Lee, Pymmes Brook, Beverley Brook, River Brent, River Crane and River Ingrebourne.  It is also widespread on the side of railways and on derelict land, but is there is limited data away from river corridors.

Japanese Knotweed is prominent within the Olympic Park in London, with colonies covering approximately 4 hectares. The Olympic Delivery Authority has been working to treat and remove this invasive weed from the site since 2007. Site specific treatments have been developed including in-situ spraying of herbicides and controlled incineration. Ongoing treatment and monitoring at the site will ensure any outcrops are dealt with as soon as possible.

Floating Pennywort

Floating Pennywort is being found at an increasing number of sites. Currently, it has been found predominantly in the north-east of London, along the River Lee and the River Roding, and also on the River Wandle in south west London. A programme of eradication is taking place on all these rivers.

Giant Hogweed

Giant Hogweed has mainly been recorded on the River Crane, Yeading Brook, River Brent, River Lee and Lee Navigation, and around Rainham Creek and the lower Ingrebourne River. Away from rivers, it is also found along transport corridors and in a number of open spaces scattered across London.


Non-native invasive crayfish are increasing their range in many of London’s waterways.

The most commonly recorded invasive crayfish species in London, the Turkish Crayfish, is predominantly found on the Grand Union Canal in West London. The populations are thought to be stable south of the River Thames in London, and declining in the area to the north of the River Thames in London. They are, however, still abundant in the Serpentine.

The Virile crayfish has been recorded in a number of locations along the River Lee and the Turkey Brook, in Enfield borough. This is the first occurrence of this species in the UK; its potential impacts are largely unknown.

The Signal crayfish is less prominent in London than in the Home Counties, but has been recorded in a number of locations in Hillingdon borough, mainly on the Fray’s river, River Colne and Grand Union Canal, and also on the Lower Lee, and the River Beam in the north-east of London.

The Red Swamp crayfish has only been recorded at Hampstead Heath Ponds and more recently in the nearby Regents Canal, in London. As with the Virile Crayfish this is the only known population of Red Swamp Crayfish in the UK.