Skip to main content
Contact Us

Regular monitoring of urban greenspace may be important to ensure a site has not been colonised by a particular invasive species. It can be helpful to identify the nearest population and decide if any steps could be taken to stop individuals of that species reaching the site. Some sites could be managed in a way that deters colonisation, for example by altering habitat management.


The term ‘invasive species’ refers to non-native plants and animals that have been introduced into wild habitats; either deliberately or inadvertently. These species are often able to proliferate rapidly by out-competing native species that occupy similar ecological niches, and often cause significant economic damage.

Practical considerations

Invasive plants

Invasive plants that may cause problems in the UK include:

  • Japanese knotweed (Fallopia japonica)
  • Himalayan balsam (Impatiens glandulifera)
  • Rhododendron (Rhododendron ponticum)
  • Giant hogweed (Heracleum mantegazzianum).

The first three are a threat to native flora and habitats, as they are aggressive and form dense stands that exclude other plants. Giant hogweed is less aggressive to other plants, but is poisonous and can cause severe skin reactions.

Invasive animals

  • Grey squirrels (Sciurus carolinensis), which outcompete red squirrels and damage young trees
  • American mink (Mustela vison), which threaten birds and mammals inhabiting river systems
  • Ruddy ducks (Oxyura jamaicensis), which may outcompete other duck species for resources
  • American signal crayfish (Pacifastacus leniusculus), which outcompete and threaten the indigenous crayfish population
  • Muntjac deer (Muntiacus reevesi), which are becoming more abundant in suburban greenspace, and cause damage to tree and shrub shoots.

Some invasive species are covered by the Wildlife and Countryside Act (1981). It is illegal to release most of them into the wild, even when taken from the wild.

Many brownfield and contaminated sites have been colonised by Japanese Knotweed (Fallopia japonica). This non-native species was introduced to the UK in the 19th Century and is extremely invasive. Under the Wildlife and Countryside Act (1981) in is an offence to plant or otherwise cause this plant to grow in the wild (Schedule 9, Section 14). Japanese Knotweed is also classed as a ‘controlled waste’ under the Environmental Protection Act 1990. This status means that it must be disposed of at a licensed landfill. Current recommendations require that it is buried at a depth of at least five metres.

Additional information

If an invasive species is present on a site, identify whether removing it will be important, based on the impact it is having, what species will replace it, and the trade-off between resources and effectiveness.

In the case of plants, it is important to avoid inadvertently spreading the species by attempting to destroy it. Japanese knotweed can regrow from very small root sections carried on boots or machinery; and Himalayan balsam seeds are small and carried in explosive pods.

Removing animals from a site usually presents ethical issues, especially in an urban setting, where the local community may be quite strongly involved with the site and its fauna.


Forest Research is a member of the IUFRO Unit (Unit 7.03.12) which has been established to examine global forestry issues related to the unwanted international movement of alien invasive species, including fungi, insects, nematodes, and plants.

Forest Research can provide specialist advice on deterring deer and grey squirrels from sites in order to protect trees (for example by using fencing).

Further information

Edwards, C. (2006). Managing and controlling invasive rhododendron (PDF-1570K). Forestry Commission Practice Guide 17. Forestry Commission, Edinburgh.

Mayle, B., Ferryman, M., Pepper, H. (2007). Controlling grey squirrel damage to woodlands. Forestry Commission Practice Note 4 (Revised). Forestry Commission, Edinburgh.

Trout, R.C. & Pepper, H.W. (2006). Forest fencing (PDF-2200K). Forestry Commission Technical Guide 2, Forestry Commission, Edinburgh.

Mayle, B., Pepper, H., Ferryman, M. (2004). Controlling grey squirrel damage to woodlands. Forestry Commission Practice Note 4 (Revised). Forestry Commission, Edinburgh.

Mayle, B. A. (1999). Managing deer in the countryside (PDF-7700K). Forestry Commission Practice Note 6, Forestry Commission, Edinburgh.

Tools & Resources
In this section
Tools & Resources