Perched on one of south London's highest ridges, the Horniman's gardens are one of the city's best-kept secrets. The views across the cityscape are extraordinary, stretching toward the Shard on clear days, and the gardens themselves pack a surprising amount in: a formal sunken garden, a working conservation meadow, a small animal enclosure, and an extraordinary Victorian bandstand. The tea terrace alone - looking out over the treetops toward the distant towers - is worth the trip. Best of all, it's free.

One of Scotland's hidden jewels, Caisson Gardens sits in a sheltered glen near Loch Earn, where the microclimate allows a remarkable range of tender and exotic plants to flourish far north of where you'd expect them. The garden is intimate and personal, the kind of place where you can sense the decades of patient work and love behind every planting. It rewards slow walking and close looking - turn a corner and you might find a Himalayan poppy in full, impossible blue, or a border dense with plants you'd associate with warmer latitudes.

The Royal Horticultural Society's flagship garden is essentially a living encyclopaedia of what can be grown in the British Isles. It's vast - a day is barely enough - and encompasses an enormous glasshouse, trial grounds where new plant varieties are rigorously tested, a walled garden, rock garden, orchid house, and a series of themed planting areas that shift dramatically with the seasons. Go in late spring for the rhododendrons and azaleas on Battleston Hill, or in autumn when the borders of the Walled Garden glow amber and crimson. It's also simply excellent at the practical business of teaching people how to garden.

Kew's country outpost in the Sussex Weald, Wakehurst is wilder and more expansive than its London sibling, with ancient woodlands, a bog garden, rock walk, and a series of ornamental gardens surrounding a handsome Elizabethan mansion. Beneath it all - quite literally - lies the Millennium Seed Bank, one of the most ambitious conservation projects in history, housing seeds from tens of thousands of plant species. The combination of spectacular horticulture above ground and world-changing science below gives Wakehurst a particular sense of purpose. The Himalayan glade in early summer is among the finest sights in British gardening.

Few places carry a story quite like Heligan's. These were the extensive pleasure grounds and working gardens of a grand Cornish estate, abandoned after the First World War when the young men who tended them never came home. They lay forgotten under decades of bramble and ivy until their discovery and restoration in the 1990s - a project that became the template for garden restoration projects worldwide. Today they're a place of genuine atmosphere: the walled kitchen garden produces vegetables with Victorian growing methods, the jungle valley shelters palms and tree ferns, and everywhere there's a sense of time suspended, of something rescued from loss.

There is nowhere quite like Kew. It is simultaneously a world-class scientific institution, a living museum of plants from every corner of the globe, and one of Britain's grandest public spaces. The Victorian glasshouses - the Palm House, Temperate House, Princess of Wales Conservatory - are architectural masterpieces that happen to be full of extraordinary plants. The treetop walkway offers a child's-eye view of the canopy. The Japanese garden, the waterlily house, the carpet of spring crocuses under the great trees - Kew offers something at every season. It is a genuine wonder, and the fact that it's accessible by Tube makes it extraordinary.

Created in the 1860s on a barren Highlands peninsula warmed by the Gulf Stream, Inverewe is one of the great horticultural improbabilities - a lush, exotic garden at the same latitude as Moscow. It shelters tree ferns from New Zealand, Himalayan rhododendrons, and southern hemisphere rarities in one of the most dramatic settings imaginable, surrounded by mountains and sea loch.

Perhaps Wales's most celebrated garden, Bodnant tumbles down a hillside above the River Conwy with views to Snowdonia. In May it blazes with laburnum arching over the famous tunnel walk, and the rhododendrons and azaleas across its terraces are among the finest in Britain. The formal Italianate terraces contrast beautifully with the wilder dell below, where ancient trees shade a stream garden.

On the shore of Strangford Lough, Mount Stewart benefits from a remarkably mild microclimate that allows tender plants to thrive that you'd expect only in Cornwall or the southwest of Ireland. Created by Edith, Marchioness of Londonderry in the 1920s, the gardens are intensely personal and full of wit - the Dodo Terrace is populated with stone animals representing the political figures of her circle.