Top 5 Non-native Invasive Species in the UK

Gloved hand of a man pulling up a root of invasive species rhizome japanese knotweed

Invasive Species and It’s Cost on the UK

Invasive species in Britain are believed to cost the UK approximately £2.2 billion every year in trying to tackle the problem. Over the decades many plants have been introduced into the UK from abroad, most of them are relatively harmless. However, there are a small number that have proved to be highly invasive.

These plants can threaten our native species and destroy natural habitats, because of this its important to dispose of them safely or keep your eye on there growth if you have them on your property. It is not illegal to have any of these plants in your garden or on your land, but it is illegal if you allow or contribute to the plant spreading beyond your property.


It was first introduced to the UK in 1763 for growing in gardens, now it is believed there are very few areas of the UK that are not affected by this non-native invasive species.  Do not be fooled by its beautiful purple flowers and deep green glossy leaves. Each one of these plants can produce sometimes more than one million tiny seeds every year spreading quickly in the wind. It is practically impossible to remove by usual methods such as spraying with herbicides or digging up.

It is classified as an invasive species, because of its fast-growing and spreading nature. It is believed they can be carriers of diseases which can be fatal to trees also their leaves can be toxic to some animals. They also cause damage to riverbanks, meadows and woodlands as the huge bush grows much larger than other plants around it, smothering them and blocking them from crucial sunlight and warmth.

lavender rhododendron flowers close up shot purple HAG5X3W 1
Rhododendron in full bloom with it’s beautiful purple / pinky flowers.

Giant Hogweed

This non-native invasive species isn’t called ‘giant hogweed’ for no reason it really can be huge growing up to a height of 5 metres. Its flowers grow in sort of umbrella arrangement, with large clusters of white flowers, it has a hollow but rigid stem, which is purple spotted, and its leaves are large but grow sparsely. Its usually spotted near riverbanks as it favours wet environments and when its by water it can send its seeds downstream allowing it to spread quickly and overcrowd other natural plants. Each plant can produce between 30-50,000 seeds a year.

It gained a reputation in the 1970’s as many children were finding the plant in the wild and playing with it or touching the plant, these children then started to come out in blisters. It turns out that there is a toxin in the sap which when it touches the skin it allows the Sun’s rays to burn the skin and irritate it which results in bad blistering. Now it is advised to wear full protective clothing when trying to get rid of the plant.

giant hogweed
Giant Hogweed standing tall against all other plant life.

Himalayan Balsam

Himalayan Balsam was introduced to the Britain in 1839 by Victorian plant hunters who fell in love with its delicate purple flowers as well as its exploding seed pods. In the last 50 years it has spread rapidly and become highly established in the UK. It is a problem because it competes with native plants for sunlight, nutrients, pollinators, and space. Which can lead to the other plants around the invasive specie dying and in turn this reduces biodiversity.

To add to its affect on nature it dies back in the winter, the dead leaves can block waterways leading to flooding as well as river bank erosion.

In 2003, the Environment Agency believes it would cost £300 million to completely eradicate Himalayan Balsam completely from the UK. Most traditional methods of removing the plant such as pulling or digging it up or spraying with chemicals do not or cannot be used. The plant usually grows in hard-to-reach places or by the river side so chemicals cannot be used in order to not poison the water. It does not have any natural enemies in it non-native habitat in the UK so it is able to grow quickly with very little resistance, compared to where it naturally grows in the foothills of the Himalayas.

Himalayan Balsam

New Zealand Pigmyweed

New Zealand Pigmyweed is native to coastal regions of Tasmania and New Zealand. It was imported into the UK in 1911 for sale as an oxygenating plant for use in garden ponds and aquaria. It was first recorded in the wild in 1956 in Essex and has continued to spread from there. For the most part it is believed that it has been transferred and spread by hitching a lift on wild birds such as herons or perhaps someone emptying out their aquarium into a body of water.

It is also believed that pieces of the plant can survive on the outside of canoes, boats and even in a damp fold of a wetsuit, meaning that next time you go to another body of water that tiny fragment of pigmyweed can spread.

This invasive specie forms dense mats on the surface of the water and they can go under the water to a depth of 3 metres, this means it can completely shade out the plants below meaning they lose out on sunlight. This results in a decline in oxygen in the water, effecting frogs, newts, and fish. Unfortunately, there is not really a cure for New Zealand Pigmyweed, in a pond the only method is to fill in the infected pond and excavate another nearby. In lakes and canals this is nearly impossible to, so prevention of spreading is the best cure.

New Zealand Pigmyweed
An example of the ‘mats’ of New Zealand Pigmyweed that form on the surface of water.

Japanese Knotweed

Japanese Knotweed is easily recognisable by its zig-zag stems, large deep green leaves, that are shaped like a shovel or a shield with a flat base, there stems are covered in purple spots, similar to bamboo. In late august they flower a beautiful white creamy colour, and this is where their initial attraction came from, once prized for its beauty when it was brought over to the UK in the 1850’s. However, it is now covered by the Wildlife and Countryside Act 1981, listed under Schedule 9 it means it is an offence to plant or cause the plant to grow in the wild.

The most worrying thing about Japanese Knotweed is that it can impact construction materials such as brickworks, concrete, and tarmac. This is why its so problematic as it can damage your home if you do not control it, it can make it a pain to sell your home, can knock value off and can also make it a nightmare if you’re trying to get a mortgage. Another problem is due to how rapidly it spreads and grows it completely outcompetes any other plants growing nearby, through shading them or by taking most of the nutrients from the ground around it.

It’s notoriously tricky to get rid of and most of the time specialist treatment is needed, the most common way is through injection of herbicides but this usually takes a few years of treatment to completely get rid of Japanese Knotweed.

Japanese Knotweed
Japanese Knotweed showing it’s ability to grow through anything with ease.

If you’d like to find out more information of Japanese Knotweed, the worst of all the Invasive Species, check out this blog post to find out more information about the plant and how it can affect the value of your property. Or if you think you’ve spotted this plant on your property but you aren’t sure get in touch and we can help you identify it.

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