Problems of Non-Native Invasive Plant Species


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Giant hogweed takes over a river bank: Photo by Gordon Joly

Plants grow naturally within a particular area, or range. Over the centuries, travelers – including specialist botanist and plantsmen – have brought back new and attractive species to their home countries. While some have remained in botanical gardens or cultivated with tender loving care by gardeners, others have adapted rapidly to their new environments – often with problematic results.

What Are Invasive Non-Native Plant Species?

According to the United States Department of Agriculture, non-native invasive species are defined as ‘introduced species that can thrive in areas beyond their natural range of dispersal.’  Over the years, significant numbers of plant species have been introduced from one country to another – for example, Dr Heather McHaffie of the Royal Botanical Gardens in Edinburgh estimates that around half of the UK’s species have at some stage been introduced (interview conducted 1 June, 2011).

While the majority of introduced species are not a problem, and many, such as snowdrops, have become part of the landscape, others have spread and now pose a threat to the native flora. Government departments such as the USDA and the UK’s Department of Environment Food and Rural Affairs (DEFRA) maintain lists of species which are reckoned to be a threat and many countries have legislation in place to deal with them.

Of course, a species which is invasive in one place may not be regarded as a problem within its natural surroundings – or even within separate areas of the same country. Dr McHaffie points out, for example, that Japanese knotweed, which features on both DEFRA’s and USDA’s lists, is not a problem in Japan; whereas the common teasel, which features on the US list, is not a problem in its natural European home.

Why Invasive Non-Native Plant Species are a Problem

Within their own range, plant species belong in a particular ecosystem where they have evolved in balance with other plant, animal and insect species. That balance is maintained by the totality of that ecosystem, as the plants provide food for particular predators. When they find themselves away from their normal range, the absence of predators and competitors gives certain species a key competitive advantage over the native flora.

Dr McHaffie gives the example of Japanese knotweed, which is controlled by natural pests in Japan but which is rampant in areas, such as Europe and America, where conditions are favorable for growth but where the insects which feed on it are absent.  There are other factors in the successful spread of plant invaders such as Japanese knotweed, giant hogweed and Himalayan balsam in an alien environment. As well as an absence of predators, these plants are characterized by high rates of growth and/or significant seed production.

Harmless in Europe, purple loosestrife is a problem in the US: Photo by Liz West

Given their natural advantages, these plants crowd out and choke native species, depriving them of light and nutrients and upsetting the balance of the native flora. They are vigorous colonizers of disturbed ground, spreading rapidly along man-made corridors such as canals and railways, as long as natural watercourses. The USDA doesn’t mince words, declaring that they are ‘characteristically adaptable, aggressive, and have a high reproductive capacity’.

Dealing With the Invaders

Once an invasive species becomes established in the wild, it becomes virtually impossible to control completely. As DEFRA’s strategy notes, ‘measures designed to prevent the introduction of invasive non-native species into Great Britain will not be completely successful in every instance’. While eradication using weed killer is an option and one widely practiced in attempts to control the spread of certain plants, it requires repeated application and may cause damage to native plants as well as the invaders.

Introduction of natural predators (biological controls) which operate within the plant’s natural range is also an option, although as Dr McHaffie points out, such programs require extensive trials and tests to ensure that they do not cause even more damage and that an introduced bio control does not become an introduced ecological disaster.

With this in mind, and given that many non-native plants are popular and established in gardens, prevention and control are recommended where possible. The UK government’s advice to the public urges gardeners to try and limit themselves to the use of native plants and to take care in disposing of anything suspect.

Increasingly, however, governments are introducing legislation preventing the planting or transplanting of particular species – the Wildlife and Countryside Act in England and Wales, for example, (Scotland has separate legislation) has recently been expanded to include 39 named species and planting these in the wild is considered a criminal offense (Royal Horticultural Society) while the Federal Noxious Weeds list in the USA includes over 80 plants which cannot be moved without a permit.

The problem of invasive non-native species is widespread and not confined to plants – animals and insects can also cause ecological damage. Despite guidelines and legislation, however, efforts to eradicate such plants may ultimately prove futile, especially as climate and other environmental changes mean that the ranges of individual species, native or otherwise, continue to fluctuate.


Department of Environment, Food and Rural Affairs. The Invasive Non-Native Species Framework for Great Britain. Accessed 6 June 2011.
DirectGov UK. Controlling Non-Native Wildlife. Accessed 6 June 2011.
National Archive and Records Administration. Noxious Weed Regulations. Accessed 6 June 2011.
Royal Horticultural Society. Invasive non-native species. Accessed 6 June 2011.
US Department of Agriculture. Non-Native Species Information Center. Accessed 6 June 2011.

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