As a sailor, I respect the sea; as a gardener, I'd very much prefer to do without it, because with a sea view comes a wind, bearing salt on its wings. "The most effective windbreak," according to Your Gardening Questions Answered, "is provided by planting two kinds of hedges, a sturdy wind-resister such as blackthorn or euonymus, set in two rows four feet apart on the side facing the sea: then four feet inside that a hedge of hebes or escallonia to filter out what gets past the outer defences..." But seaside gardeners who plant the cheerless hedges four feet apart, might find that within a few years, they've not only lost half the garden to hedge, but also planted out the view.

In Cornwall, though, you find the creations of a magnificent tribe of masochist gardeners, who with great patience and determination have made gardens in places so steep, so windswept and so full of rock you can scarcely get a knife blade in the ground, let alone a plant. I'm thinking of the c1900 gardeners who made the terraces that tumble down the south side of St Michael's Mount. There could scarcely be a more exposed place, but aeoniums thrive and flower there. So do aloes and agaves, as sculptural in form as the rocks themselves.

It's always exciting, driving to Cornwall. You have the curious feeling that the edge of the map is looming up in front of you rather fast. When you leave the great artery of the A30 and start fiddling into the smaller lanes, the sea, which at high tide glitters all round St Michael's Mount, disappears behind screens of blackened sycamore. The ferns on the banks get bigger, as though you are veering towards some strange remnant of the prehistoric age. Fuchsia, montbretia, hydrangea, which we, up-country, think of as garden plants, leap over the garden fences and settle in among honeysuckle and bramble in the hedges.

In the gardens themselves you find a cast of plants rather different to the ones we are used to. The sea, wrapping round this westernmost county, may create problems with wind and salt but it provides protection, too. Rare shrubs and succulents grow here as though they were native. Or did. The last three winters have created some large holes where southern hemisphere plants such as proteas from South Africa and fascicularias from Chile used to flourish. Gardeners here have learnt the hard way that plants that can cope with salt will not necessarily survive sustained frost.

At Chygurno, set on the side of the steep, narrow valley that runs down to the sea at Lamorna Cove, Robert Moule is still mourning the proteas and New Zealand metrosideros that he lost three years ago when temperatures in this normally mild stretch of coast plunged to -9C. It was his first warning that gardening on the south coast of Cornwall does not mean waving goodbye to chilly winters. The Moules came to Cornwall from Cheshire, almost by accident.

"We got a soil map," explained Dr Moule "and marked up all the places south of Cheshire where we could grow the kinds of plants we like." They were looking for acid soil, not alkaline, and were beginning to run off the bottom of their map when here on the edge of the edge, beyond Penzance, they found what they were looking for.

The granite house, with its wonderful new deck soaring out over the tops of tree ferns, was originally built in 1908 by two suffragettes, founders of the Women's Social and Political Union in Penzance. Lamorna, which seems so remote, had already acquired a reputation as an artists' colony: both Alfred Munnings and Laura Knight lived here, close to other artists of the Newlyn School. But by 1997, when the Moules acquired the place, the house had been empty for 20 years and the square three-acre plot falling steeply away into the valley had drifted back to wilderness. The Cupressus macrocarpa (Monterey cypress) planted in the Forties had become enormous trees with evergreen canopies blocking out the view of the sea. The slope below the trees was an impenetrable jungle.

It's always hard for anyone other than the owners of a garden to understand the physical effort that has gone into its creation. But here, you do. The Moules made nearly all the paths that zig-zag backwards and forwards across the slope, connected by steps shored up with baulks of cleft chestnut. The paths were made wherever they would fit between the boulders and outcrops of granite that loom up in unexpected places. The paths were treacherous when wet, so the Moules decided to gravel them. That sounds simple enough but the paths are too steep for wheelbarrows. Instead, 80 tons of granite chippings was delivered through pipes taped together to make a chute.

The house sits at the top of the garden with the deck looking out onto a bank of crocosmias and red horned poppies, watsonias and soft Stipa tenuissima. After this you plunge into a wonderfully disorientating network of paths, leading under tree trunks and over smooth sheets of granite rock with the vast fronds of tree ferns arching out. The house quickly disappears. Phormiums and cordylines squeeze out between boulders; olive trees suddenly loom up, with spreads of agapanthus and celmisias underneath.

The most important tool here is a metal prodder. Prodding the ground is the only way the Moules can see if the soil is deep enough to sustain whatever it is they want to plant. Rhododendrons, they've learnt, need to be near the bottom of the slope where the soil is deeper. Higher up they have bottle brushes and drimys, towering blue spires of echium and grevillea.

What other tips would Dr Moule pass on to anyone wanting to garden by the sea? Plant small, he said. Big plants just blow out of the ground before their roots can hold them. Take the tops out of your trees. If you don't, the wind will. If you are planting tree ferns, don't waste expensive trunk by burying it in the ground (tree ferns are costed by the length of their trunks – usually about £35 a foot). They only send out roots at ground level. Perch the trunks on top of the ground and guy them until they've grown their own anchors. Don't worry too much about plant associations. The light, very different here close to the sea, changes your perception of what looks right.

The garden at Chygurno, Lamorna, Penzance TR19 6XH is open Wed & Thur in summer, £4. 01736 732153; gardensofcornwall.com

Other seaside gardens to visit

Arundell, West Pentire, Crantock, Cornwall TR8 5SE Set on an exposed headland between two sandy beaches on the north coast. Open today (1-5pm) and by appointment in July and August, £3.50, 01637 831916

Headland, Battery Lane, Polruan-by-Fowey, Cornwall PL23 1PW Clifftop garden with sea views on three sides and path down to swimming cove. Open Thursday (2-6pm) until the end of September, £3, 01726 870243

Tamarisks, Inner Hope Cove, Kingsbridge, Devon TQ7 3HH A third of an acre perched directly above the sea, planted to attract butterflies. By appointment, £3.50, 01837 840217

Blackpool Gardens, Dartmouth, Devon TQ6 0RG Intriguing and wild woodland garden planted by several generations of the Newman family. Next to Blackpool Sands, £3, open daily (10am-4pm) until end September, 01803 770606

Driftwood, 4 Marine Drive, Bishopstone, Sussex Award-winning garden packed with plants. Open by appointment until the end of July, £4, 01323 899296

Sea Close, Cannongate Road, Hythe, Kent CT21 5PX Fine garden surrounding an Arts and Crafts house that overlooks the Channel. Open by appointment, £3, 01303 266093

Trevanne, 26 Canterbury Road, Holland-on-Sea, Essex CO15 5QJ Close to the seafront, with water features and an eye-catching pergola. Open tomorrow (2-5pm), £3

Marigold Cottage, Hotchin Road, Sutton-on-Sea, Lincs LN12 2NP A seaside garden influenced by the Orient. Open 11 September (11am-4pm), £3 , marigoldcottage.webs.com

Lindisfarne Castle Garden, Holy Island, Lindisfarne, Northumberland TD15 2SH Famous garden with planting by Gertrude Jekyll, now owned by the National Trust. Special open day this Monday (1-5pm) with talks by the Head Gardener at 1.30pm, 3pm and 4.30pm, £1.50