(ANSA) - Latina, October 25 - Ninfa, the world-famous garden south of Rome, stands flush in its autumnal glory, bracing for the last visitors who'll have the chance to enjoy its extraordinary array of visual, olfactory and auditory delights this year.

They are in for a special treat as the gardens will be staying open for the turning of the leaves.

The one-off, later-than-usual opening, on Sunday, November 3, will "put on a spectacle to rival anything New England has to offer", curator Lauro Marchetti tells ANSA.

Now is in any case one of the best times to enjoy Ninfa's unique mix of river, ruins, wildlife and plants, once the best-kept secret between Rome and Naples.

The garden, built by three women in the shell of a medieval stronghold where a fleeing pope was anointed, used to be the preserve of VIP guests like the late 'uncrowned king of Italy', carmaking giant Gianni Agnelli, or the British royal family.

But since the last of its long-established owners, the Caetani family, died in 1977, a foundation now opens it to the public as often as the demands of conservation allow.

Two weekends a month, from April to October, lovers of nature, history and beauty come to this flower-covered ruin dubbed "the Pompeii of the Middle Ages" by the 19th-century German historian Ferdinand Gregorovius and considered by many the most romantic garden in the world.

October sees Ninfa at its climactic height, with its wealth of ornamental trees reviving after the onslaught of the summer heat - although Ninfa's climate is always milder than the steamy marshland around it.

As well as its unique present allure, Ninfa's history also fascinates. The River Nymphaeus was first cited by the Roman historian Pliny, who described a temple devoted to water nymphs, giving the place its name. The town went on to flourish between the 8th and 10th centuries, thanks to its leather-tanning mills and its strategic position on the only road south after the Appian Way was flooded. Pope Alexander III was crowned here after escaping an imperial revolt in Rome. But the emperor, Frederick Barbarossa, took revenge by sacking it.

Ninfa came into the hands of the Caetani family around 1300.

But before that century was out, it was razed after a fratricidal siege, and malaria arrived in the Pontine marshes. The town was abandoned, a crumbling, fever-ridden hulk.

The place still bears some marks of its war-torn past - murder-holes in a city-edge bridge, for instance. But mainly the ruined churches and towers serve as an atmospheric stage-set for the plants to tumble over.

Ninfa astonishes by its variety: Himalayan magnolias, Chinese acacias, Japanese cherries, cypresses, birch, hawthorn, American sweet gum and Italy's tallest poplar. Or the more exotic dragon's claw willow, the 'wig' or 'fog' tree, giant-leafed Amazonian river plants across the water from a knobbly and seemingly precarious Roman bridge, a huge bamboo grove that shoots up out of a spring, and the quirky Tillandsia epiphyte that rings the cedars like hand-warmers, drawing nourishment from the air. All kinds of flora seem to do well in Ninfa's river-cooled environment, and the scents of its flowers, herbs and trees enrich the sensory experience.

A wildlife sanctuary since 1976, Ninfa is also home to badgers, dormice, orioles, owls, kingfishers, herons, falcons and, of course, waterfowl. Sparrows, jackdaws and thrushes nest in the Ghibelline castle. Furtive rustlings and seemingly respectful yet joyous twitterings charm the ear. The river's trout are descended from those brought by the Romans, and a fat carp flops out of the lake now and then to add to the treats for eye and ear.

Three Caetani women were the creative forces behind Ninfa's rebirth, starting with Ada Bootle-Wilbrahim, the English wife of Duke Onorato Caetani, after the First World War. A hunter, balloonist and mother of six, Ada was not the kind to put up with malaria. Despite nearly dying of the fever, she kept coming back to Ninfa, planting many of the roses that can be seen today.

On Onorato's death, Ninfa passed to his son Gelasio, an engineer who helped Mussolini to drain the marshes. Under his mother's eye, he reinforced the ruins, cleared brambles, planted trees and restored the Palazzo Comunale as a residence. When he died in 1934, Ninfa became home to his composer brother, Roffredo, whose American wife Marguerite Chapin - a cousin of T.S. Eliot - took up the reins, devoting herself with equal verve to gardening and literature.

The founder of literary reviews which published the first excerpts from Giuseppe Tomasi de Lampedusa's The Leopard and Dylan Thomas's Under Milk Wood, Marguerite gave writers the run of the place. She continued to lay it out in a free English style with no concessions to Italian landscaping apart from a 'jeu d'esprit' she allowed her husband - the crisscrossing water-runs that create an arresting optical effect at the start and end of the present-day tour.

After the deaths of Marguerite and Roffredo at the turn of the 1960s, their daughter Lelia carried on the family vocation, intensifying plantings. A talented painter, she added flowers and trees as if from a palette.

Ninfa today is largely how she conceived it, an all-round artist's vision. Not that Donna Lelia, as she was called, would have liked all to see it. As the last of the Caetanis, she was jealous of her domain and opened it only to the noble few. Lelia felt the sense of danger expressed by her friend Giorgio Bassani in his best-known novel, The Garden of the Finzi-Continis (1962), much of which was written at Ninfa and which was turned into an Oscar-winning film by Vittorio De Sica in 1971.

Some have seen parallels between the intellectual life of Bassani's Jewish family, under threat from Fascism in their walled garden in Ferrara, and the refined seclusion of Lelia and her British ex-diplomat husband, Hubert Howard. But in the end it was only thanks to Howard that Ninfa was opened to a wider public. Despite his own misgivings about wear and tear, the scion of the British aristocracy also saw its importance as a democratic gift to the world. In their will, the Howards set up a foundation to preserve Ninfa in perpetuity.

At Howard's death in 1987, the mantle passed to their spiritual heir Marchetti.

The curator tells ANSA that projects are under way to restore and expand Lelia's rock garden; shore up the perimeter walls and thicken the surrounding gauntlet of hedges for nesting birds to keep out parasites; seek out the "most ancient" rose varieties to be found, in keeping with Ninfa's medieval aura; and, in a European Union-funded scheme, to boost the garden's population of trout, sometimes nicknamed 'Hannibal's trout', a species found in "very few Italian rivers".

"We are convinced the Howards would be happy with all this because what is taking shape is just what they talked about in the 1970s, according to letters we have recently discovered".

"As always, we are following the path they laid down for us"," he says.

Ninfa, in short, is growing as the Howards would have wished, "the most beautiful and romantic garden in the world", according to British expert Charles Quest-Ritson (The English Garden Abroad, Viking, 1991). As Quest-Ritson says, "Ninfa is quite unlike any other garden...so natural that is is difficult to realize that (it) is an abandoned city invaded by a garden rather than a park containing ruins...a place of rare enchantment".

Ninfa's fame has spread and package tours are now advertised in the British press, billed as "Rome and the Gardens of Ninfa" with stops along the way at "other important Italian gardens" like Villa d'Este at Tivoli. It even received a seal of approval from Brussels in a recent heritage survey which concluded that it was rightly "venerated".

All this is a far cry from the days when, hidden away in that unlovely marshland, Ninfa was a secret whispered along the corridors of Rome's great and good. Though the garden has kept some of its mystique of inaccessibility, it is really only a quick train-and-taxi ride, or an hour's drive, from Rome.

Visitors should aim to get there as early as they can. Ninfa still abhors crowds, preferring smaller groups better able to respect the delicacy of its peerless heritage.