During Invasive Species Week (12 - 19 May 2025) Hampshire & Isle of Wight Wildlife Trust are highlighting the potential threat to rivers and wetlands from the rapid spread of water primrose (Ludwigia grandiflora).  

This attractive but ecologically devastating plant is considered a high priority species for eradication, the aim being to prevent it getting a foothold in our waterways like some other aquatic invasive species in the UK.

It can rapidly take hold in freshwater habitats, where it forms thick, impenetrable mats on the surface of ponds, streams, and ditches. These dense green carpets smother native plants and wildlife, block drainage systems, and increase the risk of flooding. Without swift public action to help identify this plant in the wild, the damage could be irreversible.

Joanne Gore, Ecologist at Hampshire & Isle of Wight Wildlife Trust says: “This plant could have drastic consequences for some of our most protected habitats and it is important to control any outbreak as soon as possible.

“Watercourses provide crucial habitats for some of the county’s most at risk species as well as being a place enjoyed by families wishing to experience the natural world. But water primrose can have an adverse effect, dominating these special wild spaces and limiting the bare ground available to rare plants. If we don’t act now – together - we risk losing some of our most precious river habitats forever.”

Originally from South America, water primrose was introduced to the UK through the aquatic plant trade for ornamental ponds. But from garden water features it has escaped into the wild, and is now spreading rapidly across southern England, including right here in Hampshire.

This Invasive Species Week, the Trust is urging everyone - especially pond owners, landowners, anglers, walkers, and local communities - to help stop the spread.

The public is encouraged to report suspected sightings of water primrose directly to Joanne Gore, Ecologist at Hampshire & Isle of Wight Wildlife Trust via email: joanne.gore@hiwwt.org.uk.

Water primrose can be recognised by its bright yellow flowers (which bloom in July to September) and its sprawling stems which reach across the water’s surface and also grow upright. Its preferred habitat is ponds, lakes, and slow-moving streams. It is often confused with harmless native species like brooklime and water mint, so accurate identification is key.

Hampshire & Isle of Wight Wildlife Trust have been leading the fight against this invader for over a decade, with a targeted eradication project at Breamore Marsh near Fordingbridge, where the plant threatens the internationally important Hampshire Avon Site of Special Scientific Interest (SSSI).

Its explosive growth - doubling in size every two weeks - makes it extremely hard to manage once established. Early detection and response is critical.

“If you’re out and about this summer, keep your eyes open,” Jo adds. “It might just look like a pretty yellow flower, but it’s a threat we can’t afford to ignore. Even one plant in a garden pond can quickly escape and wreak havoc downstream. That’s why we’re asking pond owners, gardeners, and land managers to be especially vigilant. Report it. Don’t remove it yourself. Just let us know, and the information can be acted upon.”

This Invasive Species Week, the message is clear: Stop the spread. Stay alert. Take action. By working together, we can protect our native wildlife and protect the future of our rivers and wetlands.

For more information, visit Invasive Species Week