Why would anyone open their private garden gate and let strangers come tramping in? What possesses owners of fabulous green hideaways to offer all-comers the chance to tread on their specimen plants? And, how come when the hordes arrive, they are greeted with tea and home-baked cakes?

Delightful companions: a bluebell and hosta embrace in a London garden.

The answer, as Mrs Whittington of London’s Southwood Lodge will tell you, is to raise funds for charity. As the owner of a fabulous terraced plot on Highgate Hill above the city, she has earned more than £40,000 for good causes over the past 25 years. Her method? Inviting admirers of her oasis to share it for two afternoons every year. Mrs Whittington’s garden is one of the most popular of that quintessentially British institution, the National Gardens Scheme. May and June are high points of its calendar, when, for a modest donation, glorious gardens all over the UK welcome visitors in. So from spring bulbs through to autumn leaves I keep an eye on what’s open where. Yellow posters lead me to amazing gardens that usually have an interesting owner in tow.

A seat shaded by ceanothus faces landscaped pools and cascading water.

Last weekend it was the turn of four gardens in Spitalfields attached to 17th-century silk weavers’ homes. The route between them started at Fournier Street, close to the home of besuited artists Gilbert and George, who are regularly seen striding about. Then came Brick Lane where the Bangladeshi community have put down roots. One minute I was inhaling the subtle scents of magnolia flowers, the next it was chicken vindaloo.

A green oasis behind a Spitalfields house.

Each of the gardens had its own personality. One radiated echoes of the Alhambra, with a fountain at its centre and formal hedging. Another had been landscaped with deep pools and a waterfall splashing on stones in front of a basement room. A rustic arbour faced a state-of-the-art kitchen. How things have changed since it was a backyard where Huguenots took a break from their looms.

The Gherkin peers at the peonies.

Most impressive of all was the formal garden filled with blowsy peonies defiantly fluttering among clipped hedges and billiard-table grass. Despite being right at the heart of London it was so peaceful you imagined yourself at a country village rectory. But lest you forgot you were in the most dynamic city on earth, you only had to look up. There to remind you was The Gherkin, peeking over the ancient garden wall.

Southwood Lodge is open again on Sunday 2 June. Don’t miss it.