After five years, 10,000 plants uprooted and replanted, 15,000 panes of glass replaced, 69,000 sections of metal, stone and timber repaired or replaced, enough scaffolding to stretch the length of the M25, and £41m spent, the largest Victorian glasshouse in the world is ready to open its doors again. The Temperate House in Kew Gardens is once again, as the naturalist Sir David Attenborough describes it, “a breathtakingly beautiful space”.

The great glass and iron doors to this botanical cathedral, first opened in 1863, closed in 2013 for the most complex restoration project in the history of Kew Gardens, and will reopen to the public on Saturday.

The building once again shelters several unique species that have become extinct in the wild, including the cycad Encephalartos woodii, which survived since the era when its spikes helped protect it from dinosaurs but is now labelled “the loneliest tree in the world” because only male specimens survive.



A gardener waters some of the 10,000 new plants. Photograph: Alicia Canter for the Guardian

Attenborough is one of the many local residents who have missed the great glasshouse terribly and will return to it with joy. “When I had an office job I used to come to Kew at weekends when I was depressed and take deep breaths,” he recalled. “It’s lovely.”

He described Kew as the most important botanical institute in the world. “In some circumstances the only way you can prove that a particular species is that species is to come to Kew and compare what you have with what is here. This is why people come from all over the world come to the Kew herbarium and to these great glasshouses.”

Far more light is now streaming into the glasshouse, partly because the tallest plants, which were brushing the ridge of the roof, have been pruned or replaced with shorter specimens, which has also permitted much denser planting now the space beneath them is no longer shaded out.

Another of the treasures is Dombeya mauritiana, a tree that was thought to be extinct in the wild until Carlos Magdalena led an expedition from Kew, found one growing in the highlands of Mauritius, formed part of a human ladder to reach the lowest branch and take cuttings, and brought it back to make Kew the only place in the world to have succeeded in cultivating the plant.

Bird-of-paradise flower Strelitzia Reginae seen inside Kew Gardens Glasshouse. Photograph: Alicia Canter for the Guardian

Several of the plants are part of international conservation projects to preserve or reintroduce them in the wild, including the Nepalese Taxus wallichiana, source of the anti-cancer drug Taxol.

Richard Barley, the director of horticulture at Kew, said the project was an example of world-class horticulture, science and design working together. “It’s been amazing watching this project unfold, the building emerge gloriously and some of the world’s rarest plants safely reach their home.”

The oldest part of the Grade I-listed house, designed by Decimus Burton in 1860, was regarded as one of the wonders of the age, and crowds flocked to walk the winding paths and marvel at the exotic plants.

However, the full, awesome 628ft (191-metre) length was not completed for decades, and as costs soared the use of cheaper materials and inferior workmanship caused many problems including leaking roofs, corroding metal, rotting timber and decaying stone.

Earlier repair jobs caused further problems. The present work, which cost far more than the original estimate of £34.3m and has been funded by grants from the Heritage Lottery Fund and the government, included reopening ventilators painted shut half a century ago.