Tweed project staff on lookout for another 'invader'

by OUR ENVIRONMENT EDITOR

Having successfully tackled a range of invasive non-native species across the River Tweed catchment over the last 22 years ranging from giant hogweed to American skunk cabbage, a specialist team is now ready to take on 'the worst aquatic weed in the UK' should it arrive uninvited in the Scottish Borders.

Floating pennywort, another unwanted immigrant from the Americas, has been confirmed in a nearby river system, according to the Tweed Forum which has been battling to contain if not eradicate fast growing foreign plants via the Tweed Invasives Project [TIP] since 2002.

In its annual report for 2023/24, the Forum outlines the vital work of the TIP in preventing the non-native species from smothering native vegetation along the Borders watercourses. 

Trustees of the Forum point out that with the most recent GB Invasive Non-Native Species (INNS) strategy estimating the cost of these species to the British economy at nearly £1.9 billion each year, as well as being one of the top five causes of biodiversity loss globally, the threat posed is clear.

"The Tweed Invasives Project, currently working on five target INNS within the Tweed catchment, aligns strongly with the aims of the national strategy", says the report, "in particular surveillance efforts to prevent the introduction of INNS from external sources".

Part of that work - with funding from two separate sources - will result in a revised and updated Tweed biosecurity plan to be published next year. This will provide a framework for the Forum and partner organisations to protect the freshwater environment from these harmful species by means of early detection and other measures.

"Recent early detection/prevention measures have included collaborating with North-east partners to develop an approach in dealing with floating pennywort if it is detected in the Tweed catchment".

The aquatic weed was introduced into the UK in the 1980s through the ornamental pond/water feature trade. Conservation agencies describe it as a strong contender for the title of worst aquatic weed in the country, having spread rapidly from garden ponds into waterways.

The Canal and River Trust warns: "It grows very rapidly in late summer, up to 20 centimetres per day, and is responsible for swamping waterways, blocking water flow, crowding out native plants and taking oxygen from fish and insects. It's becoming more and more of a problem across the country, blocking canals, rivers and other waterways.

"Like Japanese knotweed, floating pennywort also has the ability to grow from miniscule fragments, making its removal incredibly difficult and expensive."

In addition to its negative environmental impacts, the economic costs of this species can be high. Losses in tourism revenue are caused through disrupted fishing, river navigation and water sports. The weed also impacts commercial fisheries and blocks pipes and pumps which damages waterworks and can lead to flooding. Across Great Britain and Europe, these costs are estimated to exceed £25 million per year.

Meanwhile The Tweed Forum's continuing campaign against those invasives already present on the main river and its tributaries saw staff and project volunteers control 14,295 giant hogweed plants last year.

"Stands of American skunk cabbage have also responded to the ongoing control programme and the remaining large stands of Japanese knotweed are fragmented and much reduced." And the 'vigour' of Himalayan balsam has also been reduced, allowing native vegetation to co-exist with this 'smothering INNS'..

The Tweed project sends results of its work to the science team at CABI [ Centre for Agriculture & Bioscience International].

The Forum report concludes: "Surveys show that native vegetation is re-establishing at INNS control sites across the project area, demonstrating that nature can recover wherever those overpowering species have been removed".