By Sally Nancarrow BBC News, Surrey

A lake in front of the glasshouse forms a reservoir for the garden

But the new glasshouse at the Royal Horticultural Society (RHS) garden in Surrey is a giant flat-pack.

"It isn't custom made but a conventional off-the-peg greenhouse - just manufactured to our requirements," said the garden's curator Jim Gardiner.

"The cost of an architect-designed glasshouse would have been phenomenal."

The Wisley glasshouse will be officially opened by the Queen and the Duke of Edinburgh on 26 June, but opens to the public on Friday.

The project to build it, to replace the existing dilapidated collection of glasshouses, was launched to mark the gardening charity's bicentenary in 2002.

Where it differs from most large glasshouses, it is similar in size to the Princess of Wales Conservatory at Kew, is that it focuses on cultivated garden plants.

To see it come true is the peak of my lifetime's achievement," Curator Jim Gardiner Glasshouse picture gallery

"Our whole emphasis is on displaying plants that are attractive and have the potential of being grown by our visitors - not on conservation," said Mr Gardiner.

"We want to put out a message about plants but at the garden end.

"It is about telling people about plants, about practical advice and showing their potential."

Hundreds of thousands of gardening enthusiasts visit the RHS's major flower shows every year at Chelsea, Hampton Court and Tatton Park.

The new glasshouse has the capacity to hold 750 people.

A huge reddish-brown rock at the centre from which a 5m (16ft) waterfall cascades is actually glass-reinforced concrete, which also accommodates a lift to give access to disabled visitors.

It also houses the Root Zone, a walk-through cavern which shows how plants' roots interact with soil.

"The inspiration was the gorilla enclosure at New York zoo, which has an area which shows what is going on underground in the Congo, where the gorillas come from," said Mr Gardiner.

The 5m waterfall is built into the central rock structure

Elsewhere, the glasshouse is divided into three computer-controlled climates - dry and moist temperate and tropical.

The large palms were grown in Holland and shipped to Britain but 85% of the plants were grown at Wisley.

Plants will be moved around constantly so there will always be something in flower and displays in a "horticultural theatre" area will change monthly.

Outside, a new lake acts as a reservoir for the garden, holding more than 1.3 million gallons (6 million litres) of water.

There is also a teaching garden, designed by Chelsea gold medal-winner Cleve West, where children can study the natural world.

It links to a classroom area where visiting schoolchildren can prepare and cook food grown outside and "get messy" learning about compost and the soil.

Mr Gardiner, who began working at Wisley just after the 1987 hurricane, has been involved in the project from the early planning stage in 1996.

"To have been in at the beginning and to see it come true is the peak of my lifetime's achievement," he said.