It is a sharp and blustery sunny day, and Beth Chatto is at home near Colchester, Essex, looking out on a garden still seething with colour and vigour, even at this late stage in the year. Of all the 20th-century horticulturalists in England, she is perhaps one of the best known and most enduring. Her celebrated garden and nursery of 3,000 plants, her books, her international lectures, her medals (ten consecutive gold medals at the Chelsea Flower Show) and, most treasured of all, her OBE received from the Prince of Wales in 2002, all testify to a life of rare achievement. The life is such that the Garden Museum in Lambeth, South London, is marking the opening of its new building with a retrospective of her life and work.

“Beth Chatto is England's most influential living gardener,” says Christopher Woodward, the director of the museum. “Her garden is as complex in its ideas and influences as any major work of art, music or literature.”

Chatto herself confesses to being a little uncomfortable at being the focus of attention. She is slim and alert, with peppery curls, a polished pumpkin face and a fruity air of plenty. When she smiles, about 50 of her 84 years disappear. She sits in a favourite armchair looking out on a sensuously beautiful garden. Opulent clusters of red, green and golden leaves rise and fall in a progression of shapes, following patterns of themes and repetitions. In the 1960s she carved this garden out of dull agricultural land. It took years to establish, and now she says her challenge is “to prevent it from becoming an old lady's garden”.

The land she began with was profoundly unpromising for gardening. “I had boggy, soggy meadow, gravelly slopes, bone-dry shade beneath 300-year-old oak trees and searing droughts. Even native weeds curled up and died... but I was determined to turn these problem areas into advantages.”

She persevered, following her theory that “plants are like people. They don't take kindly to being thrust into the nearest hole in the ground. If you have a plant that originally thrived in the wild in wet shady areas, then you shouldn't try to grow it in hot, dry, sunny areas. You have to adapt your planting to the conditions you have.”

This may seem obvious to us now, but during the 1960s, when Chatto was formulating this approach, it was quite radical. “My parents were keen gardeners and they followed the conventional gardening patterns of the period, in the 1920s and 1930s. They planted ‘proper' garden plants, not species as they would grow in the wild. Things have moved on from then, but in fact a lot of my ideas really came from my husband.”

Chatto was married to Andrew Chatto, who died, aged 90, in 1999. The grandson of the publisher, he was a fruit farmer and scholar, intensely interested in the natural habitat of garden plants, a science known these days as plant association or plant ecology. “His work and research were utterly inspiring for me,” she says. “For the 65 years of our marriage, we lived it, discussed it endlessly.”

For holidays, before children came along, they went trekking in the mountains of Europe and Asia, backpacks full of notebooks and pressing boards.

When her husband's fruit farm failed and had to be sold in 1969, it was Beth Chatto who took up the slack, starting with a propagating unit, which grew into the nursery, and beginning slowly to strike out and inspire others through her displays, books and lectures.

Her influences have been broadly artistic. Cedric Morris, the Welsh painter, had a profound influence, both through his species collection and through his oil paintings, which gave her lessons in composition and design. She leaps up and shows me a painting of a Mediterranean village on her wall. “You see the shapes and patterns, the sun on the roofs, the church spire. I've stared at that picture for hours, thinking about the collections of shapes and masses and textures. Well, out there in my garden I have my clock tower in the form of that cypress, and I have the shapes and accents and textures, the rising and falling lines and diagonals that catch your eye like a painting.”

Chatto has been influenced by artists in many media, including the set designers at Glyndebourne and the music of Beethoven. “I make pictures with my garden. I don't go round with a set square, but I'm putting shapes in here and there, finding the right balance. Cedric Morris inspired my palette, but also my ideas for forms. And even without leaves or flowers in midwinter, the garden has its own form. It becomes a bit like a black and white pencil drawing... I also find I listen to Beethoven a lot and it helps with the construction. His music has themes and repetitions, dark and pale notes, swelling movements and quieter, more tranquil parts, and I find that I plant with a similar approach. Like a symphony, I think a garden needs form, theory and coherence.”

So what of the future, with global warming and, for many of us, a dry and diminished soil base with which to work? “No one can tell,” she says, “but it seems obvious that many of us will have to abandon some of the plants we have cherished in the past, to learn to plant and rely on plants that have become adapted by nature to drought conditions.”

Chatto is characteristically undeterred by the prospect of an uncertain gardening future: “If winter frosts disappear I can visualise using some of our succulent collection to provide features in place of plants that already warn me they may not be able to carry on. Actually, most of the plants in our nursery are as tough as old boots. There would be little point in selling anything that might perish in the care of a less experienced gardener.”

Chatto has many admirers. In the exhibition catalogue, Germaine Greer declares that Chatto is “one of the people I love and admire most in the world”; and the garden photographer Jerry Harpur claims that Chatto began no less than a planting revolution. “Not only had she gathered, with the advice of her husband, masses of perennial species that not everyone knew about, but she had also become a genius at putting them together as never before, naturalistically.”

Dan Pearson, the garden designer, recalls seeing her Chelsea display in the mid-Seventies and, although he was only ten, realising that it was unique. “At that point there was nothing like it... combining plants with such informal artistry... her palette was unusual too, with wild plants and the best selections of them driving to the naturalistic aesthetic.”

Brought together for the exhibition, Chatto's diaries, letters, notebooks, sketches and personal photographs will reveal something of the private world of this remarkable plantswoman.

Beth Chatto: A Retrospective, the Garden Museum, London SE1 (020-7401 8865; www.gardenmuseum.org.uk), until April 19