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Invasive Non-Native Species & Horticulture

Dr Matt Elliot of Woodland Trust and David Smith of Buglife explore the connection between horticulture and INNS, and share how we can get involved to help stop the spread from our very own gardens.

This blog is part of our series for Invasive Non-Native Species Week 2021, an annual event to raise awareness of invasive species and how everyone can help to prevent their spread. Find out more about INNS Week 2021 and how to get involved here.

May 2021

Trees and other plants, fungi, insect and bacterial species survive in a delicate ecological balance within natural landscapes. Unfortunately, the entry of a new species into this system can significantly disturb this balance. This is particularly the case with non-native species because they often arrive without the insects or animals that would usually keep them in check in their home range.

An example of this is Rhododendron ponticum which has become a severe problem across the UK over the last century. This species was originally introduced as an ornamental garden plant but the climate in the UK has proven to be particularly suitable for it and it has spread across the country. It forms dense thickets, is poisonous to most herbivores and releases millions of seeds each year. In addition, a recent study found that native ground flora does not recolonize effected areas, even 30 years after the Rhododendron was cleared.

Like most introduced invasive plant species, Rhododendron is very expensive to manage and will never be completely removed from our delicate natural landscapes. It is therefore very important that we carefully consider the species we plant in our gardens, treating potentially invasive species with extreme caution. To help with this, the Royal Horticultural Society and Plantlife have produced guides on gardening without invasive species which can be downloaded here.

As well as the plants themselves, have you ever considered what might be hitching a ride on a plant or the soil they grow in? Plants from all over the world have found their way into our gardens to provide colour and variety that we might not otherwise get from native species. But plants and soils sourced from overseas also have the potential to bring with them, unwanted guests. Invertebrates living amongst foliage and in soils are spreading out around the world and causing significant damage to their new homes.

Not all introduced species become invasive, and the amount of harm caused varies from one species to the next, but each new species that gets introduced is a risk. Species such as the Oak processionary moth (Thaumetopoea processionea) can strip oak trees bare and even cause rashes and eye irritations to people, while invasive slugs such as the Spanish slug (Arion vulgaris) can destroy garden plants and crops.

Buglife continue to be concerned by non-native flatworms. More than 10 species have found their way to the UK so far; many of them prey upon our native earthworms and land snails, endangering soil fertility, food production and wildlife. New Zealand (Arthurdendyus triangulates) and Australian flatworms (Australoplana sanguinea) are already well established in the UK and have reduced numbers of earthworms in some areas by 20%. Earthworms are vital for soil health, often referred to as ecosystem engineers, as they create soil structure and break down organic matter, helping to aerate and fertilise soils. Losing them leads to impact higher up the food chain.

The New Guinea flatworm (Platydemus manokwari), while yet to reach the UK, is already found in France and is 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 do the same here. New Guinea flatworms are particularly effective at causing the extinction of species because they can eat all the available prey and then fast for months or years. While fasting, the flatworms digest their own tissues, shrinking to a fraction of their former size.

Buglife want to understand how far flatworm species have spread. We are asking you to go out and have a look in your garden or nearby parks to see if there are any flatworms present and report back to us via our PotWatch campaign. You can also help stop the spread of non-native species by purchasing locally grown, or bare-rooted plants and by not exchanging plants or growing medium with other gardeners if you know an invasive species is present.

Eradicating invasive species after they have become established can be expensive or in some cases impossible, so preventing the spread is key to limiting harm. The Government must improve biosecurity measures to minimise the risk of pests and diseases being introduced into the UK. Until there is a proven way to sterilise both pot plants and growing medium, and this is implemented, cross-border trade in pot plants and soil should be prohibited.

Dr Matt Elliot is Policy Advisor (Tree Health & Invasives) at Woodland Trust, and David Smith is Social Change and Advocacy Officer at Buglife.

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The opinions expressed in this blog are the author's and not necessarily those of the wider Link membership.