Vita Sackville-West and Harold Nicolson were both gay - or what their generation would have called something else entirely - and they were married to each other for forty-nine years. The arrangement was unconventional and openly so. They had children together. They loved other people, sometimes intensely: Vita's affair with Virginia Woolf became literature; Harold's male relationships were quieter but no less real. They wrote each other every day they were apart, sometimes hundreds of letters in a year, and the love that runs through that correspondence is unmistakable. Sissinghurst was their joint project, the third country in their marriage. Harold designed the structure - the walks, the axes, the architectural framework. Vita filled the rooms with plants. They worked from separate towers, met in the middle, and built something together that neither could have made alone.

The White Garden is the famous one. Roses, irises, foxgloves, gypsophila - everything in shades of white, silver, and pale grey, set among silver-leaved plants and dark yew hedges. Rosa mulliganii climbs over an arbour at the centre, providing a vast white cloud in summer. The garden was conceived in the late 1940s and refined into the 1950s, when Sackville-West and Nicolson were in late middle age. It became one of the most copied garden designs in the world. The other rooms each have their own colour discipline: the Cottage Garden in hot reds and yellows, the Purple Border, the Rose Garden in a riot of old varieties Vita collected obsessively. By 1953, John Vass counted 194 different rose varieties on the property. Her preference was for species roses and old garden varieties - bourbons, damasks, gallicas, the deep-scented roses of pre-twentieth-century gardens.

Vita Sackville-West wrote about roses with the precision of a poet and the patience of a gardener. Of Madame Plantier, the noisette climbing in the Orchard, she wrote: I go out and look at her in the moonlight: she gleams, a pear-shaped ghost, contriving to look both matronly and virginal. She loved the bourbon Madame Isaac Pereire, which the rosarian Graham Stuart Thomas called possibly the most powerfully fragrant of all roses. When she first visited Sissinghurst in 1930, she discovered a dark red double-flowered Rosa gallica growing wild in the property - probably planted centuries earlier, now naturalised among the ruins. She kept it, propagated it, and gave it a name. It is now sold worldwide as the cultivar Sissinghurst Castle. Her favourite roses became reference points for the entire English gardening tradition.

Sackville-West disliked having plants named after people, and she and Nicolson once dug up a rose that had been named Lady Sackville after her mother. But Sissinghurst has accumulated its own roster of named cultivars over the decades. There is the Sissinghurst rose, a Pulmonaria Sissinghurst White with large white flowers and white-spotted leaves, an Iris Sissinghurst bred by a Kent gardener in 1969. There is Rosmarinus Sissinghurst Blue, a self-seeded rosemary discovered growing in the Tower steps with upright stems and deeper blue flowers than the common variety. The garden has its quirks of memorial: Pamela Schwerdt and Sibylle Kreutzberger, who ran the garden after Sackville-West's death in 1962, found a remarkable Phlox stolonifera in a florist's shop near the Chelsea Flower Show and named it Violet Vere after Schwerdt's mother on her ninetieth birthday.

Vita worked from her writing room in the Elizabethan tower at the centre of the garden - the surviving fragment of the medieval manor, climbed by a spiral stair, looking out over the Weald in every direction. Her library is still there, intact, the books she actually used to write. The Tower is a place of pilgrimage now for readers of Orlando, which Virginia Woolf wrote as a love letter to her, and for readers of Vita's own poetry and novels. Harold's writing room was elsewhere on the property, more domestic in scale. They visited each other through the day, talked in the evening, wrote at night. The Great Storm of 1987 hit Sissinghurst hard, and the Orchard in particular suffered heavy losses - much of the planting visitors see now is restoration from that disaster. The National Trust took over the garden after Nigel Nicolson, Vita's son, transferred it in 1967. It is open every day. It is impossibly crowded. The roses do not seem to mind.