Nature experts, from organisations including Buglife and The Wildlife Trusts are calling on the Government to phase-out imports of soils from other countries, to help prevent ‘nature invaders’ from entering the country. This is vital to stop damaging invasive non-native species from hitching a hidden ride in the plants, soils and compost bought in garden centres across the country .
Soil, compost, and potted plants can host a variety of insects and microorganisms (bacteria, fungi, and viruses), and seeds from unwanted foreign plants, which can all outcompete native species. Both garden and freshwater pond plants represent a risk of introducing nature invader stowaways.
356,000 tonnes of plants and soils were imported into the UK during 2021 with a value of nearly £1.2bn. Of this, 287,000 tonnes were imported from EU countries, with much of the soil coming into the UK within potted plants.  Since Brexit the EU does not allow imports of UK soil, either directly or in potted plant containers, with prevention of wildlife contamination a key reason for this policy. The UK, as one of the worst 10% of countries in the world for nature loss, should similarly be taking a preventative approach on soil imports, as invasive species are one of the top five drivers of nature decline.
Richard Benwell, CEO of Wildlife and Countryside Link, said: “Invasive species are a growing threat to wildlife in the UK. It only takes one or two eggs or a few spores hidden in imported soils to introduce organisms that can play havoc on our ecosystems. Once they have a foothold, it’s hard to stop damage spreading, but prevention is possible and so much more affordable.
“Phasing out imports of soils would help prevent further disasters like ash dieback. As part of a robust strategy to hold back and root out invasive species, this is an essential part of any plan to halt the decline of nature.”
Joan Edwards, Director of Policy & Public Affairs at The Wildlife Trusts, said: “Both the UK economy and the natural world are paying a steep price for the impact of invasive species. Nature soil invaders are already hitting British wildlife hard, out-competing and eating species from our beautiful but rare lilies and fritillaries, through to gardeners’ friends like ladybirds and earthworms. The economic cost of invasive species is more than £2 billion a year, impacting shipping, rivers, recreation, timber and even the gin industry. Closing a major route to entry to the UK, by banning soil imports, makes good economic sense and is vital for nature’s recovery.” 
Alisha Anstee, Lead Policy Advocate – Tree Health and Invasive Species at Woodland Trust, said: “The introduction of invasive species including certain tree diseases is one of the most serious threats to trees and other wildlife in the UK. The UK is suffering the impacts of many imported invasive species, some of which can travel undetected in the soil. It is likely that Phytophthora Ramorum, which is causing larch dieback, arrived via the plant trade and this disease is now leading to mass tree loss. Prevention is much easier and cheaper than attempting to find a cure when it comes to dealing with these invasive threats.”
David Smith, Advocacy and Social Change Officer at Buglife, said: “Many species that travel in soils are small, hard to detect and easily transported once on our shores. Invasive species are spreading from our gardens into wild places, impacting native species. We are quite capable of growing our own plants in the UK, effective action should be taken to boost home-grown plant production and reduce our reliance on imports. The Government must stop soil imports and only import bare-rooted plants until there is a proven way to ensure soils are free from pests and diseases.”
While not the only method of arrival, the import of soil and growing media is a significant route for invasive non-native species to enter the UK. A phase-out would reduce the risk of horticultural hitchhiker species entering and establishing in the UK. Current approaches focus on detection, reporting and awareness raising once a species has arrived in the UK. Yet prevention is better than cure ; preventing a species from introducing and establishing in the first place is more cost-efficient than attempting to manage or eradicate it once arrived. It is also more effective at mitigating the economic and environmental damage caused. Members of the public can also take action by buying only locally grown or bare rooted plants.
Current species: Experts have highlighted the destruction of British wildlife already being caused by species including the New Zealand Flatworm, Spanish Slug, Harlequin Ladybird, Red Lily Beetle and pathogens including Ash dieback, Phytophthora austrocedri and Phytophthora ramorum. These are all likely to have arrived in the UK hidden in soil, flower and tree imports.
The New Zealand flatworm probably arrived in the UK in the 1960s via imported soils, compost and plants. It eats native earthworms, reducing populations by up to 20%. The loss of earthworms then negatively impacts soil health, agricultural productivity, and native wildlife such as bird species, with an economic costs of tens of millions of pounds a year. Following its arrival in 2010 the Spanish Slug is now present across the country. It’s a voracious eater of crops, wild and garden plants, and numbers are difficult to control as it’s too slimy for hedgehogs and birds to eat and resistant to poisons.
The Harlequin Ladybird arrived in the UK in 2004, with fruit and plant imports from the EU one of the likely routes of their introduction. It has had a devastating impact on native ladybirds, reducing numbers by around two-thirds over a ten-year period. The Red Lily Beetle arrived in the UK in the 1940s probably as a hitchhiker on imported lilies, this beetle is highly destructive to both lilies and fritillaries, many species of which are in decline in the UK.
Ash dieback, is a fungus that was first recorded in the UK in 2012 and is likely to have been introduced via sapling imports. Its well-known impacts have seen Ash trees across the country wiped-out, with extensive felling of diseased trees necessary for public safety. It’s estimated Ash dieback will eradicate up to 95% of UK Ash trees at an economic cost of around £15 billion. Another fungus-like pathogen Phytophthora austrocedri, which spreads through soil and water, was also likely introduced through imports. First detected in the UK in 2011 it is now present in all regions where Junipers are found. It aggressively impacts conifers, with particularly notable impacts on Junipers, affecting the Scottish gin industry. Similarly, Phytophthora ramorum arrived in 2002, likely via plant imports, and is causing mass tree death, creating huge dieback of larch trees in particular.
As well as the significant economic and environmental damage being caused by already established invasive species, the groups are also sounding the alarm about the further potential damage from species that could arrive if action on soil imports is not taken soon.
New risk species include the Japanese Beetle, the larvae of which live in soil and could easily be accidently moved with rooted plants. These beetles have caused major damage in parts of North America – to plants including apple, birch, lime and rose. Threats to UK species would include maples, birch, apples and other stone fruits. Current summer temperatures are thought to be too low for the beetles but increasingly hot summers due to climate change could see this change.
Similar to Oak Processionary Moths (which came to the UK in imported trees), Pine Processionary Moths risk spreading hidden in the soil of potted plants when they are pupae. Pupae may be present in the soil all year round as this stage can remain dormant and extend to two or three years, sometimes even longer. Worryingly, inspection is unlikely to detect pupae in the soil. They have the potential to reduce productivity of trees due to defoliation of the needles. It also weakens the tree which means it is more susceptible to other pests and diseases. Importantly these caterpillars also pose a significant threat to human health; they have toxic hairs, which can cause itching skin rashes, eye and throat irritation and breathing problems.
The Argentine Ant has been transported outside of South America through means including soil and plants. These ants have been spotted in London and Birmingham but are not thought to be established. However, a study in 2014 warned that the species could spread across regions of Britain by 2024. Studies in other countries where the species has established have shown they can displace native ants, leading to disruption of pollination and soil health.
New Guinea Flatworms are already found in France and are one of the “100 worst invasive alien species” in the world. It has destroyed populations of native snails across the Indo-Pacific and has the potential to similarly harm UK snail and earthworm populations. As with other flatworms the New Guinea Flatworm is likely to be distributed via the movement of plants and growing media in the horticultural trade. With no recorded natural enemies or biological or pesticide controls for non-native flatworms, the key control measure is to prevent their introduction.
The Government has taken some limited action on tree imports, with trees used in Government tree-planting schemes required to meet biosecurity requirements, requiring accreditation or assessments. However, this only applies to trees in Government schemes, not wider tree imports, and does not address the issue of invasive hitchhikers in imported soil and potted-plants. Coinciding with Invasive Species Week 2023 experts are calling for tougher measures to protect native wildlife, by:
- Phasing out all soil imports – banning soil, peat and compost from other countries, including in potted plant imports, with plants to be transported soil free
- Extending Government tree-planting scheme biosecurity requirements to all trees and potted plants entering the country
- Requiring imported timber to undergo protective measures such as heat treatment to eradicate harmful hidden hitch-hiker species
- Supporting the expansion of UK tree and plant growing for domestic sale, to reduce reliance on horticultural imports
- Supporters of the calls to ban soil imports: Buglife, Woodland Trust, The Wildlife Trusts, Angling Trust
- Plant Health – international trade and controlled consignments, 2017-2021 – experimental statistics publication.
For comparison The UK produced around 4.77M m3 of growing media in 2021. With topsoil weighing about 1.3 tonnes per m3 and compost about 430 kg per m3 the UK produced in the region of 3-6million tonnes of growing media
- See details on the costs of invasive species to the economy here
- Previous research from Wildlife and Countryside Link found that investment in preventing the arrival of invasive non-native species would have a return on investment of £23 for every £1 spent. Spending £6 million a year on preventing invasive plants and animals establishing in the UK would save £2.7 billion for our economy over the next 20 years by preventing damage from new invasive species.
A soil import ban would not only reduce the impacts of invasive species on the UK environment and economy, but would also bring wider benefits for nature and for society. For example, a ban on soil and growing media imports from the EU would increase demand for UK-grown horticultural products, thereby boosting the domestic market.
- A ban would also bring wider benefits for nature. Stockists importing plants for the domestic market are currently given to little or no information about the media that imported plants are grown in. Many of these plants are grown in peat, extracted as a cheap growing medium but responsible for severe damage for peatlands around the world. Two-thirds of peat soil in the UK is imported from Europe. Peat extraction causes peatlands to become net emitters of carbon, devastating peatland biodiversity and contributing to poor water quality and flood risk by destroying hydrological function.
- A ban on soil and growing media imports offers the opportunity to ensure full knowledge of growing media, and to support an emerging industry of peat-free compost production.
5. Advice on how the public help avoid spreading invasive species:
- Buying locally grown or bare-rooted plants, or growing plants from seed or cuttings is the best way to prevent spreading non-native species.
- The Woodland Trust UK & Ireland Sourced and Grown (UKISG) assurance scheme can help consumers to find nurseries where they can buy British.
- Look out for misleading labels – ‘UK sourced’ plants may have been grown outside of the UK for parts of their lifecycle, and could be contaminated with invasive species. Ask if plants have been ‘fully sourced and grown in the UK’ when purchasing.
- The GBNNSS ‘Be Plant Wise’ campaign provides some materials to assist retailers and consumers in avoiding invasive species when purchasing plant materials.
- The Biosecurity for LIFE website details how people can be vigilant to avoid accidentally moving species onto our offshore islands when visiting them, as these islands are especially vulnerable to invasive species.
- If you find a non-native hitchhiker in your garden:
- Report non-native species through recording programmes such as PotWatch or irecord
- Avoid spreading invasive species to other areas by not exchanging plants or growing medium such as compost or soil with other gardeners.
- Be sure to clean your pots and gardening tools after use as small invertebrates can attach themselves to these items.
6. Read more, including on how to log sightings of invasive flatworms, here.
Main image credit: Lily Beetle © Lisa Tewksbury, University of Rhode Island, Bugwood.org