When composer William Walton and his young bride, Susana, came to the island of Ischia in 1949, they had no idea that they would end up staying there forever. Walton was simply looking for somewhere quiet where he could work on his lyric opera, Troilus and Cressida, and the idea of spending the winter in a warm Mediterranean hideaway had obvious appeal.

However, they both found Ischia so congenial that they eventually decided to make their permanent home on an expanse of secluded, rocky land on the north-west coast of the island, overlooking the Gulf of Gaeta. We bought the ground because it faces the setting sun and is therefore blessed with extra hours of sunlight, explains Lady Walton.

The Waltons built their villa against the vertical cliff face that rose in the heart of the property and Lady Walton was finally able to indulge her passion for gardening. She set about creating their own private garden of Eden. A less determined person might have been daunted by the task of converting a site that a visiting friend, the actor Laurence Olivier, described as a stone quarry, but Lady Walton threw herself heart and soul into the project. The work was to last a lifetime. It involved coaxing cuttings to grow in crevices, seeding sheer rock faces and altering the level of the valley floor. I seem to remember buying over 70 trucks of soil, she writes in the guidebook for visitors to the garden.

In 1991, eight years after her husband died, Lady Walton decided to open her creation, La Mortella named after the myrtle shrubs, which grow wild on the hillside to the public, and it is now one of Ischias principal attractions. In 2004, it was awarded the United States company Briggs & Strattons annual prize for the most beautiful garden in Italy.

A split-level tropical paradise containing over 800 rare plants from four continents, the garden was designed by the famous British landscape gardener, Russell Page. It is, in many ways, a secretive place, full of surprises with a maze of shady paths and flights of stone steps meandering through lush undergrowth and opening onto unexpected and enchanting vistas of pools, fountains and intriguing features.

The garden has also become a memorial to William Walton, who died in 1983. His ashes are buried on the summit of a high rocky outcrop, denominated Williams Rock. The museum and recital room adjacent to the house contain mementoes of his life and work, such as his piano, his walking sticks and spectacles, and the last notes of music that he wrote. Busts of the three Sitwells, the eminent British family of poets and writers who were Waltons early patrons, grace the entrance hall.

Some of the plants, such as the New Zealand Christmas bush (Metrosideros excelsus) and the tree fern collection that grow in the shade of the Liriodendron tulipifera trees near the water lily pond and fountain, were sent to Lady Walton by her husband when he was on tour abroad. A small, eight-sided fountain set in an octagonal court paved with volcanic rock from nearby Vesuvius was Pages present to the composer on his 80th birthday. Sadly, neither Walton nor Page lived long enough to see the work completed.

Waltons musical compositions are commemorated in the monolithic Temple of the Sun, which was created inside an ancient cisterna where rainwater was collected. The central chamber, featuring Apollo, the god of sun, music and poetry, has inscriptions from two of Waltons best-known works: Praise ye the Lord of gold from Belshazzars Feast and How can I sleep when love is waking from Troilus and Cressida.

The sculptures and mythological figures in the three rooms of the temple are by sculptor Simon Verity, who also contributed the Roman-style fountainhead inside the greenhouse, which is one of the gardens most fascinating features. A curtain of trailing creepers and bright orchids frame the pool inside the glasshouse, which is filled with the spreading pie-plate leaves of Victoria amazonica.

The garden will never be finished, admits Lady Walton with a smile. Her latest project is the new Greek theatre in the upper part of the garden above the house, at present under construction. It is to be used for concerts by full-blown 100-piece symphony orchestras, which will perform in the breathtaking setting of the lush, tropical garden overlooking the sea. Performances are due to start next year.

La Mortella already offers a programme of recitals every spring in the concert hall. Lady Walton is proud of the villas contribution to the world of music: We are the only ones on the island with a recital hall that meets the needs of an orchestra. The acoustics are perfect.

Lady Walton, who is 79, says she never thinks of age. William was 25 years older than me. It doesnt mean anything.

Wandering through the garden, however, you may come across her personal memorial, concealed inside a formation of hedges in a secluded corner. In the centre, a shining basin of stainless steel reflects the sky and personifies the Mirror of the Soul, as Lady Walton decribes it in her guide to the garden. The inscription around it reads: To Susana who loved with tenderness, worked with passion and believed in eternity.

Fondazione William Walton La Mortella.

Via F. Calise 39, Forio, Ischia, tel. 081986220, www.ischia.it/mortella. Open April-Nov. Tues, Thurs, Sat, Sun. 09.00-19.00. Entry 10.