Digging ponds in the countryside can deliver “unprecedented” gains for nature, experts said after a study showed they significantly boosted rare plants.

A nine-year project found that creating just 20 clean water ponds across 10 square kilometres (four square miles) of farmland increased the number of wetland plant species by more than a quarter (26%).

The number of regionally rare plants almost trebled, with an increase of 181%, while species that had gone extinct in the local area returned, the research found.

The study, published recently in the journal Biological Conservation, tested a range of measures for improving freshwater habitat and wildlife in the countryside in three catchment areas in Leicestershire.

These included putting woody debris in streams, damming up ditches to create pools that slowed run-off and building interception ponds to filter out nutrients and other pollutants.

The project also created clean water ponds, focusing on digging them in low intensity pasture, scrub or woodland where they would fill up with clean water and avoid pollution running off from agriculture or roads.

In the absence of any measures the number of wetland plant species would have declined by 9% over the nine years of the project, the study said.

The experts said the clean water ponds were many times more effective than the other measures tested out.

They were also one of the cheapest interventions, costing just £1,500-£2,000 each to dig, and helped bring back rare species such as marsh arrowgrass, bristle club-rush and mare’s tail.

Lead author Penny Williams, from Freshwater Habitats Trust, said: “The gains we saw are unprecedented for freshwater and are, by a long way, the largest recorded improvements in freshwater diversity seen from adding land management measures to countryside landscapes.

“Our previous work had already shown that ponds were a secret treasure in the British countryside – with a value out of proportion to their tiny size – however the scale of benefits from adding new ponds took all of us by surprise.”

This is such an important result: freshwater biodiversity is under threat both across the UK and the globe

Dr Jeremy Biggs, Freshwater Habitats Trust director, said the study proved it was possible to increase freshwater diversity significantly at a regional scale.

“This is such an important result: freshwater biodiversity is under threat both across the UK and the globe.

“Climate change will wreak even more havoc in future years and up to now we have found very few ways to combat losses and make the countryside more resilient.”

The project team say creating clean water ponds specifically targeted to boost nature could help stem and even reverse the ongoing declines in freshwater plant species in agricultural landscapes.

The results come from the Water Friendly Farming project by the Freshwater Habitats Trust, Game and Wildlife Conservation Trust, the University of York, the Environment Agency and landowners in three Leicestershire catchments.