New Zealand Flatworm Control in the Garden

Use these tips to identify New Zealand flatworms, understand how they spread, and find ways to control and kill them in your garden.

Have you noticed fewer worms in your garden or no worms at all? If you live in northern Britain, including Scotland, Northern Ireland, The Isle of Man, and the Faroe Islands, then you may have the New Zealand flatworm. Measuring 5-17 cm in length, these slimy invasive invertebrates feed exclusively on earthworms and have no known predators here. European earthworms have no defense against them either, and the flatworms hunt and eat them to the point of a population crash.

As we gardeners know, soil without earthworms can be very poor soil indeed. They’re the great recyclers and come up to the surface to feed and bring nutrients down. Without earthworms, dead plant material lies on the surface, sometimes creating a thatch. The soil also becomes compacted, and wildlife such as garden birds and hedgehogs lose a food source. A New Zealand flatworm infestation can be disastrous on a small scale, but as they spread across Britain, they’re becoming a more serious threat.

New Zealand Flatworm Identification

• New Zealand flatworm Arthurdendyus triangulatus
• Large land flatworm native to New Zealand
• 5-17 cm long and about 1 cm wide
• Brown-purple color on the top with speckled cream edges and underside.
• Nocturnal hunters, and take cover in moist places during the day
• Often found under plant pots, stones, or other items left on the ground
• Feeds exclusively on earthworms
• Asexual, so one flatworm can create many
• Reproduces by laying small (8 mm long) eggs
• Eggs are at first red, then change in color to black after a few days

New Zealand Flatworm at our Allotment

I think I counted about ten earthworms at my allotment plot last year. Though strange, I figured that with all the digging and compost I was adding to the soil that it wouldn’t be long before worms would be spilling out of my beds. But it’s now the beginning of June and I can honestly say that I haven’t seen a single worm this entire year. Others noticed the absence of worms on their plots too and one member decided to investigate. As a result, we’ve learned about the New Zealand flatworm and have had it formally identified on our site. Before this, not a single person knew about what the flatworm was, what it does, and how it spreads. Lack of knowledge continues to be the main reason that the flatworm spreads.

One of the first clues that you might have New Zealand flatworm is few or no earthworms. Flatworms are voracious eaters of our garden worms and at night glide along on a slimy trail searching them out from the soil surface. Once they find a worm’s burrow, they slip down it and eat the worm. Each flatworm eats one earthworm per week and can lay up to nine eggs per season. Each egg can contain between 1-14 baby flatworms (source).

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How the New Zealand Flatworm Spreads

As you can gather, the New Zealand flatworm hails from New Zealand and is thought to have been accidentally introduced via potted plants. Plants are potted up and shipped all over the world, mainly for the gardening industry but also for education and science. Since the first place to identify the New Zealand flatworm in Britain was the Edinburgh Botanical Garden, it’s considered likely to have been the place where the flatworm then spread to other parts of Britain.

New Zealand flatworms are flat and can curl up to the size of a fifty pence piece, taking up hardly any space at all. They also like to shelter under pots and stones during the day and can easily stick to surfaces. If they are stuck to a pot that’s then lifted up and moved to another garden, they will take up residence. If you’ve identified flatworms in your garden, please do not gift or sell potted plants to someone else.

Topsoil Contaminated with New Zealand Flatworm

Flatworms and their eggs can also spread through topsoil, so if you’re considering landscaping work, be extremely careful. It is unlikely that topsoil deliveries are screened for flatworms. If you don’t have flatworms in your garden now, then topsoil might introduce them to your garden.

For a long time, the New Zealand flatworm was mainly found in private gardens and allotments. They’re now becoming more common on agricultural land. In farming, machinery is often moved from field to field or even lent or leased to other farms. New Zealand flatworms and their eggs can stick to this equipment, either directly or in clumps of topsoil. They can also hide under and stick to large silage bales, which are wrapped in polythene plastic. These bales are often left in one place, then picked up and moved to new pastures to feed livestock.

Allotments are Breeding Grounds for the New Zealand Flatworm

There are controls that we can take to reduce New Zealand flatworms in our gardens. However, if your neighbor has them too, then they can spread right back over the boundary line. This couldn’t be more true than in an allotment setting, where many people are gardening side by side. A study conducted at the Slopefield Allotments in 2016 found that 90% of plots had the New Zealand flatworm. The study also found that plots that had a lot of clutter were far likelier to have more flatworms. Clutter includes stones, piles of wood, weed control fabric, plastic, etc.

Clutter on the ground creates day-time refuges for New Zealand flatworms. The more hiding places they have, the more flatworms can live on your land. Keeping your plot tidy, and removing features such as raised beds, large stones, and piles of wood can help control their numbers.

New Zealand Flatworm Control

• Remove features that flatworms can find shelter under
• Avoid introducing topsoil and potted plants from friends or swaps
• Garden centers are unlikely to have flatworms, but it is possible
• Destroy New Zealand flatworms when they are found
• Kill New Zealand flatworms by grinding between stones, boiling water, burning, or putting them in a container with salt or salty-water
• Do not touch the flatworms with your bare hands. Their mucus is a skin irritant.

No-Dig Gardening Controls New Zealand Flatworms

Few worms in my garden mean that the soil is missing its great tiller. Initially, I consoled myself with the thought that I was digging the soil instead of worms. Then I, like many other people, became interested in no-dig gardening. The idea behind it is to leave the soil structure as intact as possible, and rely on worms and other soil organisms to do the tilling for you. It’s eco-friendly, low-effort, and relies on a lot of compost. And worms of course.

I spoke to Charles Dowding about it some years ago, and he claims that no-dig can work without worms. That other organisms in the soil can do the job of bringing nutrients from the surface down into the soil. He also shared the anecdote of a garden in Northern Ireland that he visited. They have New Zealand flatworms at the site, but since transitioning to no-dig they’ve seen earthworms starting to come back. I’ve embedded the video of my interview with Charles Dowding above for you to watch.

Final Thoughts on the New Zealand Flatworm

For the past sixty years, gardeners have had to deal with New Zealand flatworms. Over that time we’ve become more aware of them, what they do, and how to control them. If you spot it in your garden, do your best to pick them off. Set traps for them, such as squares of polythene pinned to the ground. Look for them and their eggs regularly, and destroy them when you find them. It’s unlikely that you’ll be able to completely eradicate New Zealand flatworms from the garden completely, but you can do a good job of keeping their numbers down.