"Do you get lots of silly queries?" I asked Dave Cooke, one of more than 400 specialist scientists and horticulturists who work at London's Royal Botanic Gardens.

After bounding up the ornate spiral staircase from the bowels of the Victorian Temperate House at Kew to deal with my own far-fetched inquiry, Cooke's silent smirk said it all.

About 70 years ago, my English-born, plant-mad grandmother sent kowhai seeds - swaddled in cotton wool and tucked up in match boxes - by ship to London "because it's a beautiful tree and they don't have a specimen at Kew".

So could he please check Kew's records to see if any of its kowhai trees were germinated from seed sent by a Mrs A. Margaret Loudon sometime in the 1930s or 1940s?

Smirking aside, he took me at my word and began searching Kew's database. His attention to detail should not have surprised me. It is, after all, the trademark of Kew, the world's leader in botanical and environmental research.

To coincide with its 250th birthday this year, the World Heritage Site is relaunching itself as a Breathing Planet Programme.

The year-long garden party at Kew, which spreads over 132 hectares on River Thames floodplains in southwest London, is not just celebrating its incredible history.

It is also showcasing Kew's critical work - including trying to collect and preserve seed from a quarter of the world's plant species by 2020 - to combat threats like climate change and habitat loss.

People are flocking to Kew - a 30-minute train ride from central London and a five-minute stroll through one of the city's most well-hoofed suburbs - to enjoy the festivities, lectures and exhibitions being put on for the anniversary.

Gate sales are up more than 40 per cent and a record 40,000 people visited the gardens on one sunny bank holiday in May.

"It's partly the publicity for our 250th anniversary and partly good weather. But people are also slowing down because of the recession," said head of horticulture Nigel Taylor.

Whatever the reason, Taylor, who joined Kew in 1977 and worked for 14 years as an international plant-hunter, is delighted people are reconnecting with nature.

Kew's mere size merits a few days of wandering. It contains unrivalled collections of plants from all over the earth - many, like rubber and tea, were acquired by the British Empire in the early 1900s to create new agricultural economies and others have been used to fight diseases like malaria, leukaemia and Aids.

In the Rhododendron Dell - the last remaining area of Kew landscaped by Capability Brown - are 700 species of the exaggerated blooms native to the Himalayas, China, Russia and Alaska.

Kew's 15,000 catalogue trees include "old lions" like an acacia planted in 1762 and others that withstood the Great Storm of 1987 when the garden lost one third of its mature trees.

Unlike those 700 fated trees, the Chinese Pagoda - the kingdom's tallest building when it was built in 1762, which narrowly escaped German bombers in World War II - still stands today, along with the Tea Pavilion which was built after suffragettes burnt down its predecessor in 1913.

Kew Palace, built in 1631, is still Britain's smallest, and the gardens boast Europe's largest compost heap, which recycles 99 per cent of the gardens' organic waste.

The 48,000sq m Temperate House, designed by Decimus Burton for Kew's burgeoning collection of semi-hardy plants, is Britain's largest glasshouse.

Twice the size of the palm house - where you can see one of the world's longest-living pot plant, a 228-year-old African cycad - the temperate house took 38 years and £11 million (NZ$26.2 million) in today's money to complete.

No wonder Kew's modern administrators were relieved when its latest architectural folly, a treetop walkway designed by the architects behind the London Eye, was built more or less on time and on budget.

While critics can accuse Kew of exploiting other countries' plant resources in the past, Taylor says the poachers have definitely turned gamekeepers.

Kew's Millennium Seed Bank is already on course to collect 10 per cent of the world's wild species by the end of 2009 - a year ahead of its target.

At the bank's underground vault in West Sussex, seeds are collected, counted, dried and mostly frozen at -20C to preserve them for hundreds of years.

Kew's pre-1940s archives are patchy. But its database indicates the gardens' first kowhai tree may have been propagated from seed in early 1947.

So maybe my grandmother's seed-saving efforts were not in vain after all.

Getting there: Cathay Pacific offers daily flights from Auckland to London via Hong Kong. Special fares are available.

Kew Gardens: To find out more about the Royal Botanical Gardens, see: kew.org
Further information: You can get general information on visiting Britain at: visitbritain.co.nz
Deborah Telford flew to London with Cathay Pacific and was a guest of VisitBritain at Kew Gardens.