Key to the gate of England's secret gardens

One is a grand design of box parterres and an intricate maze in the grounds of Hatfield House, one of the most historic stately homes in England. The other is a tiny back garden in Bristol, less than 20ft square, with a temple folly inspired by the ruins of Pompeii. What do the two have in common? They will both be opening to the public this year as part of the National Gardens Scheme (NGS), just two of the 3,500 gardens in The Yellow Book, the NGS directory, which launched its 2007 edition this week.

According to the NGS chairman Nicholas Payne, one of the organisation's biggest frustrations is that, while The Yellow Book and the garden visiting scheme is well-known, many people assume the money visitors pay goes to the gardener. In fact, all of it - £1.75m in 2006 - goes to charity.

The National Gardens Scheme was set up 80 years ago to raise money primarily for nursing, and this is still its main focus: palliative care for hospice patients, home nursing for cancer sufferers, and relief for carers of all kinds. Someone came up with the idea of asking owners to open their gardens to raise money, and in 1927, 609 gardens opened to the public, raising £8,191. Gertrude Jekyll and Vita Sackville West were among the scheme's early supporters.

In the 80 years since, much has changed. Gardens are getting smaller and herbaceous borders on a grand scale have given way to designs that reflect our concern for the environment and the changing climate. Wildlife, vegetable-growing and ways of getting children interested in horticulture - with treasure hunts, willow mazes or toffee trails, for example - are all themes that run through The Yellow Book 2007.

There are town gardens, country gardens, water gardens and drought-tolerant gardens. New entries include the spectacular East Garden at Hatfield House, and in complete contrast, the Broughton and Bretton allotments in Broughton, Flintshire. Other additions include Chevithorne Barton in Devon, home to the national collection of quercus (oak), with more than 500 specimen trees, and The Antiquary in Bewdley, Worcestershire, home to six decorative rare breed hens.

The Cumbria Wildlife Trust is also opening its organic garden, Plumgarths in Kendal, for the first time, offering the chance to see birds, bats, insects and 150 species of wild flowers, and at the London Wildlife Trust in Peckham, another new entry, you'll be able to see organic vegetable beds and beehives. The unique thing about the NGS, as its new president Zac Goldsmith observes, is that the process of raising the money - the gardens, the sharing of knowledge, the chance to admire our horticultural heritage - is as valuable as the money itself.

And forming the backbone of all this are the garden owners, who work so hard to make the openings a success. Some, like the owners of the tiny Bristol garden, go it alone, while others join forces with the rest of the street or even village. Kew in south-west London boasts three separate "groups", while in Dorsington in Warwickshire, six miles from Stratford-upon-Avon, 14 gardens will open together on 10 June. Anyone can apply to have their garden open under the scheme, but all gardens are inspected by volunteers (on a county-by-county basis) to ensure there is enough interest. Ideally, says the NGS, there should be 45 minutes of interest - a rule that is slightly bent in the case of inner-city gardens, where wow factor is allowed to compensate for size.

The great and the good get in on the act too - the Bishop of Ely opens his garden, there are Oxbridge colleges, and the Royal College of Physicians, in Regent's Park, London, whose garden was replanted last year to display plants used in conventional and herbal medicine around the world, will welcome NGS visitors for the first time this year.

Private garden owners range from the late Frankie Howerd (the garden opens, appropriately, on 1 April) to the nuns at Emmaus House retreat in Bristol.

For enthusiasts, the chance to see gardens never otherwise open to the public is what makes The Yellow Book indispensable. But for anyone who is intrigued by the eclectic, even eccentric, pot-pourri of English life, it provides a fascinating index of expertise and floral extravaganza.

'The Yellow Book' 2007 is available from bookshops, priced £7.99, or direct from the NGS. Go to www.ngs.org.uk, or call the NGS head office on 01483 211535