Fighting Back Against Invasive Plants on the Test and Itchen
Have you ever heard of Invasive Non-Native Species (INNS)? These are plants and animals introduced—intentionally or not—into environments where they don't naturally belong. In the UK, INNS pose significant threats to our ecosystems, economy, and conservation efforts.
Among the most problematic are invasive plants. They spread quickly along rivers and watercourses, particularly sensitive habitats like the chalk streams of the Test and Itchen.
Beautiful, But Destructive
Many invasive plants were introduced for their ornamental value. Their beauty isn’t a coincidence—species like Himalayan balsam and Japanese knotweed were brought to the UK for their rapid growth and aesthetic appeal. Others, like Japanese knotweed, were used for practical reasons such as soil stabilisation along riverbanks and railway embankments.
But these plants can cause unseen chaos. Japanese knotweed, for example, spreads underground up to seven metres deep, can break through concrete, and has no native predators to keep it in check.
What Happens When INNS Reach Our Rivers?
When INNS take hold in the riparian zone (the interface between land and river), the effects multiply:
- Monocultures form, crowding out native plants.
- This in turn impacts invertebrates and wildlife that rely on native species for food and shelter.
- Annual species like Himalayan balsam die back in winter, leaving riverbanks bare and vulnerable to erosion.
- Increased erosion leads to sediment build-up, which smothers the gravel beds that fish need for spawning.
- Floating plants like Floating pennywort reduce oxygen levels and water flow.
- As invasive plants die and decompose, they release excess nutrients, triggering algal blooms and eutrophication—particularly harmful in our naturally low-nutrient chalk streams.
A Coordinated Response Across the Catchment
At Wessex Rivers Trust, we’re responding to the challenge through a multi-year, catchment-wide project targeting 17 high-risk species. By coordinating local stakeholders and conducting ongoing monitoring, we’re working to ensure the long-term health of the Test and Itchen catchments.
Turning to Nature for a Solution
A promising area of work is the use of biological controls—natural solutions that can help keep invasive species in check:
We're supporting research into rust fungi to combat Himalayan balsam along the River Itchen and Bow Lake tributary. This method offers a longer-term solution compared to manual removal alone.
In the headwaters of the Test and Itchen, particularly the Dever and Anton tributaries, biological controls are being introduced to manage Water Fern.
These nature-based approaches reduce our reliance on herbicides and offer a sustainable way to manage INNS over time.
How You Can Help
Community support is crucial. That’s why public awareness and volunteer engagement are at the heart of this project. We’re running educational campaigns and volunteer programmes to help people recognise invasive species and take part in removal efforts.
By building a stronger sense of local stewardship, we can create resilient communities that support ongoing conservation.
Get Involved
Look out for our next round of volunteer event dates in June—and sign up to help protect the Test and Itchen. These iconic chalk streams are irreplaceable and need our help to thrive.
