Invasive plant species


Indian Balsam Impatiens glandulifera © Ita McCobb

What are invasive non-native plants?

Invasive non-native species are considered to represent one of the greatest threats not just to biodiversity but also to the economic health of our planet.

It is important to distinguish between a non-native plant species and an invasive plant species since not all non-native species are invasive and native species can become invasive.

A non-native plant is one that has been introduced by man either deliberately or accidentally to an area out of its natural range of distribution. Some can co-exist in their new environment without impacting negatively on native species.

An invasive species is one that can cause economic, environmental and/or human, animal or plant health problems. Plant species can be invasive in some regions but not in others.

The problem with plants that are both non-native and invasive is that they become well established generating a negative impact on the local ecosystem and species.

These species represent a global problem that impacts particularly severely on small island countries and other geographically isolated regions.

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Japanese knotweed Fallopia japonica in flower © Ita McCobb

How are invasive plants introduced?

Physical barriers such as oceans and mountains that would have acted as a barrier to plants’ natural dispersal are now no longer effective since some species arrive accidentally. With the increase in transportation of goods around the world plant seeds can unintentionally be transported with merchandise into a new area by ship, plane, train or truck.

Other species are introduced deliberately as crops or garden plants. Many have become established in the wild through natural seed dispersal or by inappropriate disposal of garden waste.

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Invasive plant characteristics

Invasive non-native species tend to share successful characteristics such as their method of reproduction, growth rate, growth form and persistence.

They tend to spread rapidly by seed or vegetative propagation and are more likely to be clonal than native plants. They grow rapidly and tolerate a wide range of environmental conditions competing aggressively for resources such as food, water and habitat, lacking both natural enemies or pests in the new ecosystem.

Other important characteristics are that they are often tall in stature and have long seed dormancy.

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Cogon grass Imperata cylindrica  © Ita McCobb

Impact of invasive non-native plants

The environmental impact of non-native invasive species can range from simply putting native plants at risk by occupying large sections of available habitat to breeding with related native or other invasive species and reducing native genetic diversity.

When non-native species are introduced into an ecosystem in which they did not evolve naturally they can often spread out of control, since they don’t experience their natural native controls.

No habitat or region of our planet is immune from the threat of invasive species. Islands and their incredibly rich biodiversity are particularly susceptible to invasion because they are no longer isolated due to global trade and transport.

Freshwater ecosystems and estuaries are also particularly vulnerable as an invasive plant can quickly cover an area cutting out light and starving aquatic life of oxygen, as well as obstructing boat passage, damaging recreational areas, restricting water intake pipes and potentially increasing flood risk.

Soil erosion and sediment loss can increase. This is common when native plants with fibrous roots are replaced with invasive broad-leaved plants with tap roots.

The frequency and intensity of wildfires is another impact of some invasive non-native plants, particularly invasive grasses that act as highly flammable tinder when dry, creating charred areas ideal for invasive species.

When non-native species are introduced into an ecosystem in which they did not evolve naturally they can often spread out of control, since they don’t experience their natural native controls.

Agricultural production can be threatened with invasive plants reducing yields, interfering with harvesting operations and reducing land and property values.

On grazing ground invasive non-native plants can compete for and restrict access to food and water for animals and alter soil chemistry and nutrient composition. They have little value as a food source and some are toxic to animals.

Hybridization can also be a problem because hybrids can spread faster than the original species and therefore outcompete and displace other plants.

In addition to harming the environment some invasive non-native plants cause major economic losses to countries around the world as well as costing billions to control and eradicate.

Certain species now thrive in new areas due to increased temperatures and changing conditions due to climate change.

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Stags-horn sumach Rhus typhina © Ita McCobb

Control and eradication of invasive plants

One of the most important ways of controlling invasive non-native plants is through awareness and education. In the UK there are campaigns to raise public awareness of the problem. The general public is also becoming involved in monitoring programmes. The Plant tracker phone app shows how to identify species and allows people to submit geo-located photographic records of various invasive plant species as well as including a Confusions species gallery.

Early detection and eradication is more cost effective and less risky than later intervention. For example, systematic surveillance of entry pathways, such as mapping plant species distribution around airports or container ports, is used to inform rapid eradication strategies in some countries, such as New Zealand.

Prevention, however, is not always applicable since in many cases the plants are already widespread and causing numerous impacts. Successful management of the various species requires an understanding of how they grow and the ecology of the sites where they occur.

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General methods of control

There are four basic methods of control: mechanical, chemical, natural and environmental.

  • Mechanical control uses various methods to uproot or cut the plants.
  • Chemical control uses specific herbicides.
  • Natural control (Biological control) uses natural enemies such as insects and pathogens that have co-evolved with the species in their region of origin.
  • Environmental control alters the environment to make it less suitable for the growth of the particular species.

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Water hyacinth Eichhornia crassipes © US dept agriculture

Worst invasive species

Japanese knotweed Fallopia japonica is causing serious problems across large parts of Europe and North America and has even found its way to Australia and New Zealand. It has outcompeted native plants in habitats from roadsides and riverbanks to derelict land, and has even caused structural damage.

Giant hogweed Heracleum mantegazzianum forms dense colonies along riverbanks and areas of wasteland suppressing the growth of native plants. It produces a toxic sap that causes blistering and severe skin irritation.

Cogon grass Imperata cylindrica is “one of the top ten worst weeds in the world”. It spreads through its extensive rhizome system as well as wind-blown seeds. It adapts to poor soils, is drought tolerant and increases fire frequency.

Water hyacinth Eichhornia crassipes is “one of the worst aquatic weeds in the world”. It forms dense colonies that block sunlight and crowd out native species while obstructing waterways. It is fast growing, with populations known to double in as little as 12 days!

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References: The Environment Agency UK;; British Ecological Society; Houses of Parliament, Parliamentary Office of Science & Technology, Dr Jonathan Wentworth, Invasive Alien Plant Species, July 2013; The United States National Arboretum; Global Invasive Species Database; United States Department of Agriculture (USDA), National Invasive Species Information Center.