Non-native invasive species

American mink

The problem

Escaping from fur farms, the American mink (Neovison vison) has no natural predators in Britain. As an active hunter, it is highly effective at preying on native water voles, and is one of the main causes of their decline.

What we are doing

Our water vole work has identified areas of mink activity. This information helps us target conservation efforts to support water vole populations and reduce the impact of mink across the Tees Valley.

Floating pennywort

The problem

Originally introduced as a garden pond plant, floating pennywort (Hydrocotyle ranunculoides) forms dense mats on still or slow-moving water. It outcompetes native plants, reduces light levels, lowers water temperature and deoxygenates the water.

What we are doing

Floating pennywort is present at our Coatham Marsh reserve, where it can be seen affecting areas of the Fleet. We have used a combination of mechanical removal and targeted herbicide treatment. As the plant can regrow from small fragments, ongoing management is needed.

Curly waterweed

The problem

Curly waterweed (Lagarosiphon major) is a fast-spreading submerged plant that outcompetes native pond species. It is sometimes sold as an oxygenating plant and can be mislabelled.

What we are doing

This species is currently found in one of our ponds at Preston Farm. It spreads easily from small fragments, making control challenging. Our volunteer teams remove this by hand, although this does not fully prevent regrowth.

Japanese knotweed

The problem

Japanese knotweed (Fallopia japonica) was introduced as an ornamental plant and is now widespread across the UK. It spreads through rhizomes and stem fragments, outcompetes native species, increases riverbank erosion and can damage infrastructure.

What we are doing

We manage Japanese knotweed at our Portrack Marsh reserve using targeted herbicide treatments throughout the growing season, including spraying and stem injection to control regrowth.

Giant hogweed

The problem

Giant hogweed (Heracleum mantegazzianum) is an invasive plant commonly found along riverbanks. It outcompetes native vegetation and poses a risk to human health, as its sap can cause severe skin irritation and sensitivity to sunlight.

What we are doing

This species is present on several of our reserves, including Bowesfield, Preston Farm, Maze Park and Portrack Marsh. We manage it through a combination of stem injection and removal of flower heads to prevent seeding and spread.

Himalayan balsam

The problem

Himalayan balsam (Impatiens glandulifera) spreads rapidly along riverbanks, outcompeting native plants. When it dies back in winter, it leaves banks exposed, increasing erosion and flood risk.

What we are doing

We manage Himalayan balsam at Bowesfield and The Howls through regular volunteer sessions, often known as 'balsam bashing', where plants are pulled out by hand before they set seed.

American signal crayfish

The problem

American signal crayfish (Pacifastacus leniusculus) outcompete native white-clawed crayfish through predation and by spreading crayfish plague. They also damage riverbanks through burrowing.

What we are doing

While we do not run crayfish-specific projects, we follow strict biosecurity measures across all our water-based work. This includes the “Check, Clean, Dry” approach and the use of appropriate disinfectants to prevent the spread of invasive species and disease.