Gardens: Benmore Botanic Garden, Dunoon, Argyll

The first is from the path down the hill above, which affords a striking view over the 20ft glass-roofed structure.

From here you can see how the building has been tucked into the rocky hillside, taking advantage of the naturally moist microclimate that promotes the perfect internal growing conditions required by a range of rare and unusual ferns.

The second, more easily accessible, route is from below. Here, as you walk up the newly laid path that winds between stone-edged beds and a narrow burn planted with ferns from China, Japan, Tasmania and New Zealand, you wonder how building materials were moved 60ft up the side of the hill. The answer, says Peter Baxter, curator at Benmore since 1995, is that everything had to be lifted in by crane.

Both routes lead to the wooden door, flanked by a bed of rare British ferns, oblong woodsia (Woodsia ilvensis), a reminder that this project is as much about conservation of the world's oldest and most vulnerable species as about history.

The door opens into the vault at the foot of the fernery from where stone steps curve up into the airy space dominated by tall, elegant Dicksonia squarrosa, the New Zealand native planted in the central bed beside Cyathea spinulosa.

You are immediately transported back to an era when the Victorian craze for fern collecting, pteridomania, was at its height.

Built in 1870 by James Duncan, a sugar broker and refiner from Greenock, owner of Benmore and a keen plantsman, the fernery fell into disrepair before the 1920s. Baxter had a challenge on his hands: "There was Rhododendron ponticum growing in the building alongside brambles, ivy, conifers and elder. The roof had collapsed."

Once these were cleared, the restoration team were surprised to find that much of the internal layout, including the grotto, pond and a raised terrace, remained along with the flights of steps to the top terrace. Finding the bare bones of the structural layout was helpful, Baxter points out, as no plans remained to illustrate the original barrel shape of the roof, once supported on cast iron arches: only the shape of the gables suggested its proportions.

"It was agreed that the roof should retain its original form, but that the new roof should be seen as a modern intervention, something clearly of its time," he says.

The new structure, designed by Glasgow-based MAST Architects, also benefited from modern technology, although it proved unnecessary to replace the original boiler. During the renovations a new water storage tank was concealed on the hill above, fed from a stream.

Fund-raising for the 550,000 project was a challenge. "The idea of restoring the fernery had been under discussion for several years but it wasn't until 2007 that a definite plan was put in place," says Baxter.

The Heritage Lottery Fund met 40 per cent of the cost. Additional money was raised by the Trustees of the Younger (Benmore) Trust, an RBGE members' appeal, Loch Lomond and the Trossachs National Park, the British Pteridological Society, the Scottish Government and the support of the private individuals.

Growing began early in 2004. RBGE experts, including Andrew Ensoll, drew up a planting list and stock was grown at the RBGE from spores, some collected in the wild. Todea barbara barbara, however was grown from spores from the 1,000-year-old specimen at the Ascog Hall fernery on Bute.

"As the project advanced, we honed down the list to create as diverse a collection as possible," Baxter says. "Currently the planting – with more than 70 species – is slightly experimental as we learn about the preferred habits of some species."

They found humidity rose once the plants were brought in and soon identified several distinct micro-climates; sunlight is excluded from the vault area and in some places the light is filtered through the foliage of tall plants, such as the Australian native Cyathea cooperi and its cousin, the fuzzy, bronze-stemmed C cooper.

Closer inspection reveals striking fiddle-heads of Hawaiian native Sadleria cyatheoides unfolding in the higher beds. Scattered around are under-plantings of flat-leaved Blechnum tabulare and groups of bronze Maidenhair ferns, Adiantum. In the top bed the bronze foliage of Doodia media ssp australis also catches the eye. The origins of some species make intriguing stories: the open fern Blechnum cycadifolium originates from Juan Fernandez Island, one of a group of islands off Chile that includes Robinson Crusoe Island.

As the project unfolded, happy discoveries were made. "When the sun shines through from the south, it bounces off the water and lights up the roof of the grotto," Baxter says. "We had thought the east wall would dry up but were surprised to find that it remains moist and that water drips down."

The relatively young planting has a light feel, a quality Baxter says the team wishes to retain as far as possible. "We don't want the planting to become dense, we want to be able to see the form of the different species. We think that is important. The Victorians tended to cram things in and we want to take a step back from that."

New material, he says, is being accumulated all the time and the team aims to be flexible and is keen to experiment. "We could play it safer by putting in things we know would be 100 per cent but we want to try new things," Baxter says.

Benmore Botanic Garden, Dunoon, Argyll PA23 8QU, tel: 01369 706261, www.rbge.org.uk/benmore is open until the end of October.

Wheelchair access to the fernery is not possible but the exhibition, Pteridomania: The Renaissance showing in the Benmore Gallery gives a flavour of the restoration.

This article was first published in The Scotsman on 17 October 2009