IT is not the most spectacular looking plant, but when it comes to rarity the Triangular Club Rush is the UK's botanical equivalent of the tiger.

This unassuming sedge is disappearing fast with the last remaining specimens growing in just a few small clumps beside the River Tamar.

The reason the plant is so close to extinction is not known. Scientists believe it might be due to increased competition from fast-growing reeds or changes in riverbank management.

The UK is the most northerly extent of the Triangular Club Rush's range. Restricted to the upper reaches of tidal estuaries, the plant has a bright green angled stem and can grow from 50cm to 150cm in height. It flowers in August and September.

A survey of the Tamar in 2009 revealed the rare sedge is now only found in four distinct clumps near Calstock. The total area covered by these few surviving plants is less than four square metres.

Concerned at the scale of the decline, scientists from the Environment Agency are making a final attempt to save the species with help from Kew Garden's Millennium Seed Bank, Natural England, the National Trust and ecologist Peter Nicholson from Panscape Environmental Consultants.

The National Trust has given permission for a nursery area to be created at Cotehele where Triangular Club Rush plants will be specially grown for re-planting at other suitable locations in the estuary.

The nursery, at Morden stream, was chosen because it has a suitable habitat offering bank stability, an open aspect and low salinity. It will be managed by National Trust staff from Cotehele.

This latest rescue attempt builds on an earlier project in the 1990s when a number of plants were removed from the Tamar and cultivated by Kew Gardens.

Propagated plants from Kew were then planted at selected sites on the estuary in 1997/1998 in a bid to boost a small population of native Triangular Club Rush around Rumleigh Quay. This work effectively saved the plant from extinction.

Jess Thomasson, of the Environment Agency, said: 'The decline of the Triangular Club Rush is causing serious concern. This is our last chance to save this critically endangered species.

'Having a reservoir of Triangular Club Rush plants at Cotehele will greatly increase the likelihood of success by allowing further planting in historic strongholds on the Tamar.'

Today (Thursday) staff from Kew and Wakehurst Place, the Millennium Seed Bank nursery where the plants were grown, will travel to Cotehele to deliver the latest batch of Triangular Club Rush.