The garden of earthly delights

Claude Monet's post-1900 paintings, as seen in the Royal Academy's "Monet in the 20th Century" exhibition, serve to highlight Monet's other, less well-known love: his real passion was gardening, and his legacy is Giverny. At Giverny, his home for over 40 years, he often swapped his brushes for a trowel, and, using bulbs and shrubs from his "palette", created banks of colour that constantly inspired and challenged his artistic eye.

Although 60 years old at the turn of the century (the start of the RA's exhibition), with the independent "impressionist" exhibitions that established his position a distant memory, Monet was by no means past his best. He outlived Post-Impressionism, Fauvism and Cubism, working diligently to the impressionist credo of capturing the transitory effects of light and atmosphere. In the 1890s, he focused increasingly on one motif at a time - Rouen cathedral, haystacks, poplars - and spent months away from home, painting, trying to capture the changing moods of nature by painting dozens of views of the same subject at different times of the day. But he found it harder and harder to stay away from home: he wrote to his wife in 1892, "I pine for Giverny. Everything must be so beautiful there in this glorious weather." Although not yet painting his garden, he was already transforming and preparing it to be his last, and longest-lasting, motif.

By 1900, Monet was the grand seigneur of contemporary art, wealthy through continued sales and stock-market nous, and able to indulge in large- scale gardening projects. Even before he bought his farmhouse, while he was renting it, he had started to work on the garden. By his death in 1926, he had built studios, acquired new land, transformed the vegetable garden into an informal bower, and created a Japanese-style sanctuary, at the heart of which he made a serpentine water-lily pond by diverting a tributary of the Seine. (Not all went smoothly - he had an ongoing battle with local residents, who thought his exotic plants would poison their water source.)

After years of disrepair Monet's house and gardens, now the Fondation Claude Monet, reopened to the public in 1980. In Monet's lifetime however, access was through personal invitation only and, perhaps not surprisingly, art- historical myths about the garden became established. It was suggested that Monet built the gardens to wall himself off from the world (hard when, at the time, a railway bisected his land and First World War munitions trains rumbled past towards the front, which was sometimes only 20 miles away), or that he stayed at home because his eyesight was failing, painting in an increasingly abstract way. True, his eyesight was failing - he had cataracts, and underwent an operation when he was 83 to clear them - but the giant water-lily panels he left to the nation, which now fill two rooms in the Orangerie in Paris, are testament to the quality and quantity of his work up to a month before his death.

In 1907, Marcel Proust wrote that if he ever saw Monet's garden, he imagined that he "would find it a place of tints and hues more than of blossoms ... a garden of colours laid out in a manner different from nature's ... more a transposition of art than a subject for paintings, being really a finished painting rendered in nature and illuminated by the eye of a great artist". He couldn't have been more accurate. It is said that Monet read more horticultural catalogues and papers than writing on art, and with the help of his six gardeners, planted bulbs that would flower in cycles, and in carefully arranged colour harmonies. In the Clos Normand - previously the old orchard - vivid orange nasturtiums grow over blue-green rockery plants, blood-red dahlias sway against lime foliage, and shining yellow sunflowers and irises rise between softly nodding purply delphiniums; today, the garden is planted following the same instructions Monet repeatedly left for his own gardeners and family. Walking down the central allee with its raised beds awash with colour, it is as if you are strolling through his inspiration, or, in fact, his most vivid painting.

He was immensely precious about his plants, and cared for them as much as he did for his family. When he was away he would write long letters to Alice, his second wife, thanking her for taking care of his "beloved" flowers, and listing jobs for the garden: "There's no pressing need to lift the gladioli; but when you do, I recommend hardy perennials like anemones". If a plant was ailing, Monet would talk about it in human terms: "What you say about my poor little roses is very worrying, and I fear plenty of other disasters. Did anyone think at least to cover the Japanese peonies? It would have been plain murder not to do so."

Monet bought land beyond the railway line that flanked the Clos Normand, and built an underpass so he could start work on it. Now, walking between the gardens, the impressionist palette of the Clos Normand is left behind, and the garden that was to inspire his most ambitous works is entered through a bamboo forest.

The water-garden was planted carefully, and it became Monet's muse. He used rich purple irises to darken the shadows under the trees, and geraniums to add red highlights to sunny green banks. The water-lilies, with their deep red hearts, solved Monet's problem of painting the surface of still water (which by itself is invisible), and the remainder of the pond was left to act as a mirror, reflecting and capturing the scudding clouds above. Monet had created a subject that was composed of all the elements he had wanted to paint throughout his life, and he had little need to look up or beyond the pond again.

For both gardens, Monet spared no expense in buying trees, shrubs and bulbs from abroad - particularly from Japan. His house overflowed with Japanese prints, and he collected avidly, particularly prints of plants, such as Hokusai's flower series. His gardens are filled with Japanese maples and cherry trees, anemones, bamboo thickets and water-lilies. Monet didn't follow the traditions of Japan to the letter when it came to creating his water-garden. His oriental-style bridge is green; Japanese garden bridges are always red - but when the garden is taken as a whole, it follows the broader parameters of Japanese garden design. The water-garden is asymmetrical, and designed to be seen from many viewpoints. Monet used fewer types of plants in this part of the garden, and its simplicity echoes the economical use of materials in a Japanese garden, the purpose of which was to allow a spiritual communion with nature.

In his garden Monet was able to experiment with the colour, textures and harmonies he would later explore in his canvases. The garden is something more than his motif. It is the original work; the paintings are his attempt to get to grips with its changing light, atmosphere and seasons. It is a fitting legacy for a painter of the outdoors, and a visit is a necessary experience to understand the majority of his 20th-century paintings, the fusion of his two passions.

'Monet in the 20th Century': Royal Academy of Arts, W1 (0171 300 8000), 23 January to 18 April. Fondation Claude Monet in Giverny opens 1 April (00 33 2 32 51 28 21). 'Monet's Garden', BBC2, Sundays to 14 February.