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A cracking piece of design: Keeping Temperate House at Kew Gardens open is a fragile business

Horace Walpole, self-appointed arbiter of mid-18th-century architectural taste, didn't think much of efforts to add a voguish dash of the Picturesque to the Royal Botanic Gardens at Kew. As the construction of Sir William Chambers' 10-storey Chinese Pagoda soared so high it could be seen from the antiquarian's lawns at Strawberry Hill in Twickenham, he scornfully observed that "in a fortnight you will be able to see it from Yorkshire". Sir William's mock Roman ruins were similarly derided for being made from "Act-of-Parliament brick".

Nobody scoffs at Kew today, firmly bedded down as an international treasure, a designated World Heritage Site. The words "Kew Gardens" evoke images of shady walks by the Thames, broad aristocratic vistas, and the all-ages-together fun of a walk round the mighty glasshouses. So the recent news that Kew has had to launch a £15m public appeal to fund urgent repairs to the Temperate House, and suggestions that it may have to mothball other structures due to overstretched resources, comes as a shock.

A government-commissioned report found that the condition of the Temperate House "raises health-and-safety concerns for visitors and staff alike", and that repair work must start within three years to prevent its closure. The institution's £28m restoration plans have won support from the Heritage Lottery Fund, with an initial grant of £890,000, pending a further one of £15m. However, while Kew is half-funded by Defra, it is also responsible for raising the remainder of its income from gate revenues and sponsorship, hence the need for public support.

The Grade I-listed Temperate House dates from a later era than when Walpole was casting his disdainful eye over the scene. In fact, Kew's complicated evolution offers an explanation of why the report found it has a backlog of repairs to its heritage features – which number around 50 – in the region of £80m.

Kew began as two adjacent royal estates under George II and his son Frederick, Prince of Wales, the botanic gardens being founded by the latter's widow, Princess Augusta, in 1759. "At that point they were 'stroll-around' gardens, the plantings to be enjoyed in the landscape, rather than for botanical merit," explains Dr Nigel Taylor, curator of the gardens and, in effect, the 24th in the line of head gardeners since Princess Augusta appointed her first.

While George III amalgamated and enlarged the gardens, and employed Sir Joseph Banks as unofficial director of the botanic collection, after the monarch's death Kew went into decline until it was taken into state hands in 1841. "The purpose was twofold," Dr Taylor says. "First, to serve economic botany, moving plants around to establish new agricultural economies in the colonies, and second – and equally important, given the polluted state of Victorian cities – to provide a pleasure park for the common man to enjoy." That dual purpose, as a scientific research institution and place of public enjoyment, remains unique to Kew.

The Temperate House, opened in 1862, was positioned to be the first feature visitors saw as they entered the gates, with the anticipated coming of the first railway station at Kew expected to be at the end of the adjacent avenue. In the event, the station was built some 500 yards to the north leaving, as Dr Taylor says, the glasshouse "somewhat stranded in the landscape". Still, along with its slightly earlier sister, the Palm House, nothing so well epitomises Kew as a place of architectural fantasy, as well as floral and arboreal splendour.

At 628ft long, the Temperate House is the largest surviving Victorian glasshouse in the world, twice the size of the Palm House, with which it shares the same architect, Decimus Burton. But whereas the latter – jointly designed by Burton with the engineer Richard Turner – for all the outlandishness of its curvilinear form, was a relatively pared-down exercise in functional purity, the Temperate House was more orthodox. Rectangular, with pitched roofs, its pillars support immense wrought-iron ribs which make for spectacular viewing from the high gallery running round the interior wall.

Burton, who spanned the Georgian and Victorian periods but who largely eschewed Gothic, adorned the Temperate House – a showhouse of the gardens' largest plants – with a rich mix of decorative motifs, finials, pediments, acanthus leaf capitals, swags of tropical fruits and flowers, Coade stone urns and statues.

"They wouldn't build a glasshouse like that today," observes Greg Redwood, Kew's head of glasshouses. "The effect is similar to the contemporary iron pier pavilions of Eugenius Birch. There's a huge amount of decorative finish that doesn't serve a functional purpose." Fortunately, he says, as an early exercise in cast- and wrought-iron and glass construction, the cautious over-engineering made the building structurally sound. "It's probably why it's still standing. But the trouble is the Victorians hid utilitarian features like drainpipes inside the stone columns. Water ingress round the edge of the building is causing rust on the iron to push against the masonry, which is falling away. Equally, many of the decorative features are made of wood and they are rotting."

The glasshouse last underwent a major restoration in the late 1970s, but Redwood says everyday maintenance is more complicated than it was a century ago. "Victorians didn't have a problem sending a man up into the rafters with a broom to clean the glazing bars with no ropes or any other form of protection, but modern health-and-safety regulations mean you can't keep that up to the same extent."

He admits the idea of restoration, which will almost certainly require frame-by-frame dismantlement, "is both terrifying and exciting in the same breath". Much decoration lost over the years will, he anticipates, be put back. Already parts of the north octagon have had to be closed off, and in the gallery he points out a large patch of rust revealed when a corbel fell off the wall. Much of the decay is actually concealed beneath surfaces, and a climb up the ornately winding staircase through the jungle of vegetation to the roof gallery remains a thrilling experience.

Tim Entwisle, Kew's director of conservation, living collections and estates, seems quietly confident that the fundraising campaign will succeed. "One of the last things I'd do is close one of the glasshouses, because they are a critical part of the gardens, not just as historic buildings, but because they hold the collections through which we tell the stories about plants." He admits, however, that if money is not forthcoming, there is a likelihood of further areas of the Temperate House being closed off.

Out on the terrace, Greg Redwood casts an admiring eye over the façade. "The Palm House is more spectacular because of its unusual shape, but I prefer the Temperate House because I like High Victorian architecture." Had Walpole lived to see it, he may have wielded a critical pen, but as the largest surviving Victorian plant conservatory on the planet, it seems unthinkable that it will be allowed to continue its slow decline, either through lack of government funding or public indifference.

For details of Kew's campaign, visit www.kew.org/support-kew/donate-now/temperate-house-appeal

Keep it in the family

Which architectural surname is most synonymous with London's streetscapes – Nash, Wren, Hawksmoor – or Burton?

James Burton (1761-1837) was a builder responsible for large amounts of still-extant Georgian terraces in Bloomsbury and Fitzrovia. Son Decimus (1800-1881), pictured, went further, with the original buildings of London Zoo and some of the stucco terraces facing Regent's Park, and the Holme villa. He also designed the dainty Ionic colonnade which still acts as the official entrance to Hyde Park, and the Athenaeum in Pall Mall. His glazed conservatories at Kew echo the long-lost Crystal Palace, and helped to shape the design of the great railway sheds of the Victorian age of steam.