The Stamford Millstream has been an important feature of Stamford for almost 1000 years, first mentioned in the Domesday Book back in 1086. The current channel was cut around 1640, feeding the Grade II listed King’s Mill with water from the River Welland at Tinwell.
Martin Ballard, East Mercia Rivers Trustee said, “Due to lack of flow, an infestation of Water Fern (Azolla filiculoides) occurred on the Millstream. Native to the Americas, it was introduced to the UK in the 19th century. A sales ban was issued in 2014 as it had become widespread on slow moving and stationary water bodies. Water Fern causes ecological damage by reducing light and oxygen reaching submerged flora and fauna, so it outcompetes native aquatic plants. It can block drainage systems and inhibits leisure activities such as angling and boating, and poses a risk to livestock and people when mistaken for solid land.”
Water Fern is highly invasive and grows rapidly, prior to the repair of Tinwell pump the fern had almost covered the Millstream. Whilst it still lingers in large patches, the return of the flow has resulted in the plant spreading downstream onto the Welland. Manual or mechanical control is expensive and leads to short-term clearance and must be repeated as the Water Fern recovers. Chemical control faces the same challenges with additional risks to the aquatic habitat. There is no organism native to the UK that can control Water Fern.
However, the tiny North American weevil (Stenopelmus rufinasus), when released as a biological control agent, has been used to control the weed very successfully in the UK. The weevil is already present here (first recorded in 1921) and are known to feed exclusively on Water Fern, so are host-specific and won’t harm other native plants.
The water fern population may initially increases after the release, as it will take some time for the weevil populations to reach the levels necessary to exert control. Once weevil populations have reached critical levels the weed disappears quite rapidly. The first indications of successful weevil establishment are a change in colour from green to brown, this will occur in patches at first, and open water may develop between them. Typically, first indications of Azolla control are apparent after 8-12 weeks.
East Mercia Rivers Trust worked with the Stamford Millstream Group to organise a release event in July 2023 with funding supplied by Stamford Town Council. Whilst it is evident the plant has spread onto the river Welland EMRT and SMG will monitor progress and consider if further releases are needed across the Witham and Welland, where it has become established.
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