Naturenet: Invasive Plant Species in the UK

Invasive Plant Species in the UK
by Alina Congreve


Human beings have benefited from the introduction of plant species into the UK for over 2000 years. These have added to the variety and nutrition to our food supply as well as adding colour and diversity to our parks and gardens. However, a small number of introduced plant species are now acknowledged to represent a risk to natural ecosystems. Invasive plants can also represent a risk to economic activities including the ability to develop land effected by invasive plants as well as causing erosion. Concern about invasive species began in the 1950s and a large body of scientific research followed, often looking at particular species that had already become a problem. Now research is much more proactive, focusing on identifying species before they represent a serious risk. 


Invasive species represent a problem because they are able to out-compete native plants.Most plants present no risk of becoming invasive in Britain. To become invasive three events must happen: the plant must arrive; the plant must survive; and the plant must thrive. This means that the new habitat must be a close enough match for the plant’s native habitat that it can survive without human cultivation. The term invasive is used in a number of different ways:

The second definition is most commonly used by ecologists, while the third definition is more common in legislation and policy papers.

For the purposes of this briefing, the term invasive plant species is defined using the International Union for the Conservation of Nature (IUCN) definition:

“an alien species which becomes established in natural or semi-natural ecosystems or habitat, is an agent of change, and threatens native biological diversity”.

Invasive plant species represent a global problem and impact particularly severely on small island countries and other regions, such as mountainous regions that have remained geographically isolated. Physical barriers such as oceans and mountains that would have acted as a barrier to plants natural dispersal are now no longer effective. Some invasive species arrive accidentally e.g. in packing material on ships. Others are deliberately introduced as ornamental plants in gardens or as crops.

Before human settlement it is estimated that about one species of vascular plants and protozoan arrived in the Hawaiian Islands every 50,000 years. After the arrival of the Polynesians this increased to 3–4 species per century. The current rate is about 20 new species annually (Planttalk, 2001). In the UK, our history as a trading centre and our enthusiasm for plant collecting have been important historical features affecting the introduction of invasive plants. While the UK has not suffered as severely as some other regions, invasive plants have nevertheless had a negative impact on our native biodiversity. 

Threat to native habitat

Japanese knotweedNative habitats in Britain face a number of pressures including direct habitat removal, invasive species, pollution and inappropriate management practices. It is widely acknowledge that the second most important threat, after direct habitat removal is from invasive species. Invasive species represent a problem because they are able to out-compete native plants. By taking the light, nutrients and space normally occupied by native plants, invasive species alter the environment. In their new environment invasive species often lack natural predators, competitors and diseases, leaving little to control their growth.

In a study by Pearman and associates (2002) 2,947 vascular plants were found to be living in this country (excluding gardens). Of these 149 were historic introduction (before 1500) and 1402 were recent introductions. This represents about half of the total. Of these 1402 only a small number are recognised as representing a major problem. Flowering plants that are identified as a problem include:

In rivers and ponds invasive species such as the Australian swamp stonecrop present a direct and immediate threat to one of our most threatened native plants – the starfruit. The floating pennywort is another aquatic plant that is causing serious problems by removing oxygen from the water which kills fish, blocking drainage ditches, and causing local flooding. In 1999 English NaturePages marked with this symbol are exclusively written for Naturenet put the cost of removal of this plant at £250,00-£300,000 per year.   

What qualities make a plant invasive?

Looking at the issue globally, invasive plants share certain characteristics. They tend to reproduce vegetatively. When they do reproduce by seeds they tend to have small seeds, short juvenile periods and low variability in seed crops (Kolar and Lodge 2001). Work by Thompson and associates looked specifically at native and non-native plants that had recently expanded their range in Britain. Invasive plants were found to be more likely to be clonal than native plants - this means one plant gives rise to another, and gives rise to another with no sexual reproduction. Crawley and associates, who also worked specifically on the UK, identified tall stature and long seed dormancy as important characteristics of invasive plants. Work in Australian and North American has progressed further with the development of statistical models to predict the likely invasive qualities of plant species. Such models do not currently exist in the UK and the most likely predictor of whether a plant represents a risk of being invasive is whether the plant itself or a close relative has been invasive in other countries.

The future of invasive species in Britain

The case of the Olympic park site illustrates the costs and technical difficulties of control once an invasive species has become established. Control would be more feasible if ‘problem’ species could be identified earlier. Identifying species that could become invasive is not straightforward because plants can react in unpredictable ways in their new environment. Because of this plant conservation charities and scientists (Manchester et al 2000) argue that proposed new species introductions should be subject to thorough ecological characterisation and risk assessment before introduction.

Once plant species have been identified as potential risk species quick and clear mechanisms are then needed to update Schedule 9Pages marked with this symbol are exclusively written for Naturenet. Plant conservation charities argue that there is already a case to add aquatic plants to Schedule 9, which have proved invasive in mainland Europe including the water hyacinth and water chestnut. Other important measures that need to be considered in the future are the ways in which plants arrive in the UK. There is a greater role for ecologists to have an input into boarder controls – to reduce the risk of accidental plant introductions. There is also an important role for garden centres and gardening organisations to avoid the sale and promotion of invasive plants. There is also a strong role for the general public to avoid the accidental release of invasive species into native habitats.

Development on sites affected by invasive species: London 2012

A survey carried out in 2004 identified Japanese knotweed in patches across the Olympic Park. It is concentrated along river and canal banks, by the Velopark and beside the Aquatics Centre site. A number of methods will be used to treat land affected. These include:

The costs of this treatment programme have been estimated at £70 million.

The legislative context

The UK is affected by a number of European and international treaties which refer to invasive plant species:

The Wildlife and Countryside Act 1981Pages marked with this symbol are exclusively written for Naturenet, provides the main controls on the release of non-native species into the environment. The Act contains Schedule 9 Part IIPages marked with this symbol are exclusively written for Naturenet. This is a list of plants, which it is against the law to “plant or otherwise cause to grow in the wild”. Despite this listing, Japanese knotweed has spread to nearly double its distribution over the past 20 years. The responsibility for enforcing the legislation relating to endangered species currently lies with the police. The Countryside and Rights of Way Act of 2000Pages marked with this symbol are exclusively written for Naturenet allowed for stricter enforcement and increased penalties for releasing invasive plant species. However due to difficulties in tracing sources material there have been no prosecutions under the Act. The new technology of DNA profiling could improve this situation – allowing plants in the wild to be matched with garden centre sources. 


Alina Congreve 5 July 2007