PotWatch: Finding Flatworms

PotWatch is a campaign to highlight how importing potted plants can accidentally establish invasive species in the UK.

We are particularly concerned about the invasion of non-native flatworms, and we need your help to find out just how far they have spread.

What is a flatworm?

Land flatworms have smooth bodies covered in mucus and range in shape from flattened to cylindrical. Unlike earthworms and leeches, flatworm bodies are not segmented, and they can be separated from slugs by the lack of tentacles and completely smooth bodies. Land flatworms can be found in dark, damp places such as in the soil, leaf litter, at the bases of plants or under logs, stones, and other objects.

There are at least three native species of land flatworms in the UK, but more than ten non-native species. We have an identification guide to flatworms here.

Why are we interested in finding out more about flatworms?

Non-native land flatworms have been accidentally introduced to the UK in imported pot plants and newly arrived species continue to be discovered. Once introduced these flatworms can reproduce rapidly, cannot be eradicated, and pose a risk to native soil invertebrates such as earthworms by feeding on them.

Scientists have found that in some areas, non-native flatworms can reduce local earthworm populations by 20% – this could have a huge impact on soil health and agriculture, as well as our native soil wildlife.

By taking part in our survey, you will help us to better understand how non-native flatworms are spreading around the UK, and you may alert us to a newly arrived species.

Pot Plants?

Over £1 billion of live plants are imported into the UK every year, for the vast majority, there are no biosecurity measures to check the soil for eggs or hibernating animals. Non-native land flatworms have been accidentally introduced to the UK in imported pot plants and newly arrived species continue to be discovered.

Local horticulturalists are capable of growing plants for domestic markets, so almost all international trade in live plants is unnecessary. Eradicating invasive species after they have become established can be expensive or impossible, and so preventing the spread of invasive non-native species is key to limiting harm. Improved biosecurity practices are essential. Until there is a proven way to sterilise both pot plants and the potting medium, and this is implemented, cross-border trade in pot plants should be prohibited.

There are no recorded natural enemies and no biological or pesticide control methods for non-native flatworms, the key control measure is to prevent their introduction to new areas.

Consumers should buy locally grown potted plants to avoid aiding and abetting biosecurity breaches. Peat use and neonicotinoid contamination are other environmental issues associated with the pot plant industry. As this campaign develops, we will be looking more closely into these other aspects.


New Zealand Flatworm (Arthurdendyus triangulatus) © Katy Martin

Get involved in our flatworm survey

Spend some time searching for flatworms in a nearby outdoor space – it could be your back garden, or somewhere you visit during your daily exercise. Look in damp, dark places such as under wood, stones and plastic, and underneath plant pots. The mucus covering the flatworm can cause skin irritation so wear gloves if handling any flatworms you find.

We also want you to check any newly purchased plant pots for any unwanted guests – if you find a species that is not a flatworm in a new potted plant, please email us here.

Other invasive species that are likely to have arrived in the pot plant trade include:

  • Rosemary Leaf-beetle
  • Spanish Slug
  • Lily Beetle
  • Oak Processionary Moth
  • Asian Hornet
  • Harlequin Ladybird
  • Three-lined Balkan Slug
  • Yellow and green cellar slugs

If you find a flatworm please take a picture. Use our flatworm guide to help identify the species, but do not worry if you can’t, our team will take a look.

Then please answer our questions below to send us your photo and information on your survey. Complete the questions whether you have found a flatworm or not – we are interested to know where flatworms are not as well as where they are!

  • DD slash MM slash YYYY
  • Hidden
  • (please take a look at our identification guide to the right)
  • Drop files here or
    Accepted file types: jpg, png, Max. file size: 100 MB, Max. files: 10.
    • This field is for validation purposes and should be left unchanged.
    Obama Flatworm (Obama nungara) © Richard Lewington

    Explore the map and click on the circles to find out more about the survey submission.

    What should I do with non-native flatworms?

    • Avoid spreading flatworms 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 flatworms can attach themselves to these things via their mucus.
    • Destroy non-native flatworms by, for example, squashing them or by dropping them into hot or very salty water. Do not use pesticides or chemical-based products that could cause wider harm to the environment.

    Buy locally grown plant species rather than imports

    • Buying locally grown plants, or bare-rooted plants is the best way to ensure we are not spreading non-native species. Plants should be regularly checked to ensure they are free from stowaways and garden centres should be following the Code of Practice to prevent the spread of non-native Flatworms.
    • Alternatively, grow plants from seed or cuttings. See our gardening for bugs pages for more wildlife-friendly ideas.

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