By Alex Kirby BBC News Online environment correspondent

Moss in Wakehurst Place's Tilgate wood

The reserve - for mosses, liverworts, lichens and filmy ferns - is at Wakehurst Place, Kew's country branch south of London.

It will be opened by the naturalist Dr Francis Rose, after whom it is named.

Some of the species it shelters may be millions of years old.

Rocky home

The first of its kind in Europe, the Francis Rose reserve opens on 16 September.

About two-thirds of the 18 rarest species of cryptogam in Britain are being sheltered by the Kew project.

The cryptogams comprise organisms that produce spores, not seeds. Examples are algae, lichens, fungi, mosses, and ferns.

The new reserve's sandstone outcrops are an ideal site for re-introducing rare species.

Likely candidates for re-introduction include the slender thread moss, known now at only 18 sites across the UK, and Tunbridge filmy fern, which has virtually disappeared everywhere outside the reserve.

Mosses and lichens have no roots, and often survive by living on sandstone, which can absorb the large quantities they need.

Ice age link

Lichens are regarded as sensitive indicators of the health of an area, because of their need for clean air and water. There are 1,800 species in the UK, with between five and 10 new ones discovered every year.

The main problem for many plants in the high weald of Sussex, where the reserve lies, is the rhododendron.

Andy Jackson, the head of Wakehurst Place, said: "It devastates other species and is far worse than leylandii. It's extremely invasive, on the same scale as Japanese knotweed."

Many cryptogams evolved about 400 million years ago.

Dr Rose, a botanist who has spent much of his life studying mosses and lichens, said some of the plants in the reserve had probably been in Britain since the last ice age, and were the oldest native species.

Dr Rose said: "They are relics; they are living archaeology.

"Some may be millions of years old. I would be very sad if the disappeared."