Invasive non-native species

Our gardens have been greatly enriched by the introduction of plants from abroad but a small number have proved highly invasive in the UK, threatening natural habitats and native species. The control of these species is difficult and costly, yet many are widely available with little indication of the damage they can do if they are allowed to escape from gardens or are disposed of carelessly. After habitat destruction, invasive non-native species are the most serious threat to global biodiversity.

Himalayan balsam

Quick facts

Clearing the Olympic site of Japanese knotweed has been estimated at £70 million
There are 1,402 non-native plants in the UK, but only 108 (8%) are considered invasive
Approximately 60% of invasive plants come from horticulture
It is a criminal offence to plant or cause to grow a non-native invasive species in the wild

What are non-native invasive plants?

Non-native species are those that occur outside their natural range due to direct or indirect introduction by humans. If the introduced plants or animals persist in natural or unmanaged habitats, they are termed ‘naturalised’.

Many naturalised species do not present a problem but some that spread and outcompete native species can threaten ecosystems, habitats or native species – resulting in environmental or economic damage. These are considered to be invasive either due to lack of natural control mechanisms (such as predators); rapid rate of spread (by seed or vegetatively) or suppression of other species (such as allelopathy – as with black walnut – or competition for resources).

Invasive species can be plants, animals, or other groups such as fungi or algae that cause disease or pest problems.

The problems

Non-native invasive species can:

  • Change ecosystems and habitats and have non-biotic effects, such as reducing or impeding water flow leading to flooding, or changing the pH or the chemical composition of the soil, or lock up nutrients
  • Outcompete native species either by habitat change or by spreading so rapidly as to crowd out slower growing species, threatening the long-term survival of species
  • Take a long time to become invasive. Many of the plants now considered invasive have been growing in the UK for over 100 years and for much of that time showed no sign of becoming a problem
  • Be expensive to eradicate. It is also very costly to restore degraded habitat, if it can be done at all

Invasive plants

FACT: There are 1,402 non-native plants established in the wild in Great Britain, of which 108 (8%) are stated to have a negative impact.

What UK legislation covers invasive non-native plants?

The Wildlife and Countryside Act (1981) recognised the need to control certain species of invasive plants and animals already causing a problem in the UK, listing them in Schedule 9. Originally only giant hogweed (Heracleum mantegazzianum) and Japanese knotweed (Fallopia japonica) were listed. However, in April 2010 a further 36 plants were added onto Schedule 9 (see below for a download of the list). A recent amendment to the Wildlife and Countryside Act has a new provision to ban specific plants from sale. In early 2013 Defra announced a ban on sale of five of the worst invasive water plants in the UK which will come into force in April 2014. The five species that will be banned from sale are:

Azolla filiculoides
Crassula helmsii
Hydrocotyle ranunuculoides
Ludwigia grandiflora
Myriophyllum aquaticum

The development of policy and legislation in relation to the environment is one of the areas that falls under the devolved administrations;

  • Wales: covered by the Wildlife & Countryside Act but with separate amendments
  • Northern Ireland: covered by the Wildlife and Natural Environment Act (Northern Ireland) 2011. This includes a provision for the ban on sale of animal and plant species listed in Schedule 9, as specified in an order issued by Northern Ireland Executive Department of the Environment. At the present time it is not clear whether any such order has been made
  • Scotland: a new Wildlife and Natural Environment Act (Scotland) 2011 is now in force making it illegal to plant any non-native plant in the wild in Scotland

FACT: It is a criminal offence to plant or cause to grow a non-native invasive species that is listed on Schedule 9 in the wild which carries penalties of up to £5,000 fine and/or 2 years imprisonment.

Republic of Ireland

The Irish Government adopted in 2011 the European Communities (Birds and Natural Habitats) Regulations in which Schedule 3 provides a list of species which it is an offence to (a) cause to grow, disperse or spread in the wild and (b) possess, propagate, offer, distribute or import with the intention of making available for sale. However the relevant clauses of the legislation have not yet been made effective and will only be so by public announcement by the relevant Minister.

What is in place to help tackle the problem?

The Convention on Biological Diversity (1992) included a requirement for signatories to prevent the introduction of, control or eradicate those alien species which threaten ecosystems, habitats or species. This led to the formation of the Global Invasive Species Programme in 1997 which published the Global Strategy on Invasive Alien Species in 2001.

Within the UK, legislation on non native species was reviewed in 2001 which led to the formation of;

Alongside these developments, working with the horticulture industry, the Department for the Environment, Food and Rural Affairs (Defra), published the Horticulture Code of Practice in 2005 which provides non-binding guidance to horticulture professionals and gardeners on dealing with non-native invasive species.


Invasive plants covered by legislation in the UK and Ireland

Invasive animals

How are invasive non-native animals a problem in the UK?

Unwanted release of pets into the wild: As with plants, the main problem concerns aquatic animals, especially those sold by the aquarium trade. For example:

  • Terrapins and bullfrogs: these grow into large voracious predators that soon outgrow their aquariums. If released into a pond they can significantly reduce the numbers of fish, native amphibians and small mammals, as well as feeding on inverebrate creatures
  • Exotic crayfish: in many streams the native white-clawed crayfish has already been replaced by the American signal crayfish. This carries a fungal disease that has little effect on the signal crayfish but is fatal to the native species

ACTION: Releasing non-native animals into the wild is an offence under the Wildlife and Countryside Act. Unwanted pets should be passed on to wildlife centres or persons who are able to care for these animals, or they should be humanely destroyed.

New pests and diseases from overseas occasionally become established in Britain: If conditions here are suitable there is little that gardeners can do to avoid their spread. There are, however, some relatively immobile non-native species that have acheived a much wider distribution than they could be natural means through the transport of soil and plants by nurserymen and gardeners. For example:

  • New Zealand flatworm and Australian flatworm: they have been established in Britain for more than 40 years and are causing concern as they feed exclusively on earthworms. In some parts of Britain and Ireland they have reduced the earthworm population to very low levels, with consequent adverse effects on soil quality and the native animals that also prey on earthworms. The New Zealand flatworm is mainly found in Scotland, Northern Ireland and the north of England while Australian flatworm is most frequent in south west England

ACTION: Gardeners who have either species of flatworms should avoid moving soil, compost or rooted plants from their gardens to other areas that are currently free of flatworms. Suspected new plant pests or diseases should be reported to the Plant Health and Seeds Inspectorate or send samples to FERA Room 10G A01, Sand Hutton, York, YO41 1LZ.


The control and eradication of invasive species is expensive and difficult:

  • Swansea City Council has estimated that it would cost £1.25 million to treat existing stands of Japanese knotweed in the council area
  • The cost of removing and disposing of Japanese knotweed from the Olympic site in East London has been estimated at £70 million and it has been estimated that in 2003 it cost the UK £1.56 billion
  • Research indicates that it takes at least ten years to eradicate giant hogweed where it has succeeded in seeding, and three to four years to eradicate Japanese knotweed
  • Grey squirrels have largely displaced the native red squirrel, despite repeated attempts to control it

For these reasons preventing the escape in the wild of non-native species is paramount.

What if I own land infested with invasive non-native plants?

  • Most of the terrestrial non-native plants can be controlled eventually with a sustained programme of herbicide application but this is costly. For large scale infestations it may be best to obtain the services of a contractor with the appropriate Certificate of Competance in the application of herbicides for situations where professional pesticides are permitted to be used under regulations made under the Food and Environment Act 1985
  • Where spraying is necessary in areas adjacent to, or over, water bodies, consult with the Environment Agency who can supply a list of fully trained contractors. Unfortunately, the herbicides available are non-selective and will kill all plants. It may then take many years for the native flora to recolonise the area and critical populations of rare plants may never recover
  • Aquatic weeds can be composted or buried in trenches in the garden but many invasive plants are unsuitable for composting as seeds or portions of the root system may survive. When dried they may be burnt
  • Japanese knotweed is regarded as 'controlled waste' under the Environmental Protection Act (Duty of Care) Regulations and, if not burnt, can only be disposed of in registered land-fill sites

Local Councils and county Wildlife Trusts may also be able to offer advice.

Further guidance for gardeners

There are a number of publications and websites aimed at giving advice to gardeners on how to manage plants in gardens that are known to be invasive and providing suggestions for alternative plants to use, especially where aquatic plants are required;

Be Plantwise
Plantlife, What's in your pot?
Plantlife/RHS booklet

Action by gardeners

  • Follow the guidance in the Horticultural Code of Practice
  • Avoid using plants known to be invasive, especially in the case of non-native aquatic species
  • Do not distribute invasive non-native plants that may damage the wider environment to other gardeners
  • Take steps to prevent the escape of invasive non-native plants into the wild
  • Destroy or dispose of invasive non-native plants in a responsible way. Do not introduce them into the wild or into areas where they may escape into the wild

Be aware of your garden and its situation and act accordingly. For instance, avoid Cotoneaster shrubs if you live near downlands or limestone outcrops, as birds will spread the seeds. Also, do not plant skunk cabbage (Lysichiton americanus) along streams, as seeds and fragments will wash downstream.

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