"It's Tokyo minus the stress," is how the southwestern Japanese city of Hiroshima has been described. So, what's its secret? Well, its six rivers certainly help but Hiroshima's real stress-busting powers lie in its unique open spaces.

Explorer and art historian Langdon Warner (the inspiration for Steven Spielberg's Indiana Jones character) imbued Japanese gardens with spiritual symbolism. They were designed, wrote Warner, to "express the highest truths of religion and philosophy precisely as other civilisations have made use of the arts of literature and painting".

You'll see what he meant if you happen upon some of the tiny gardens tucked away beside Hiroshima's temples that most tourists never get to see. Such as that at the intimate Seiganji, exquisitely designed with the economy typical of all Zen art: a little pond, a maple, a stone lantern or two, some shrubs. Its message is clear: this garden is intended for meditation, not walking round or playing in.

Then there's the dry-stone mindscape at Saizoji temple: meticulously raked white gravel and enigmatic islands of rock. But what exactly do they represent? Zen expert D.T. Suzuki maintained that Japanese gardens express the spirit of Zen. "It's not something you can explain," explains Saizoji's Buddhist priest. "You have to experience it." Ah, but of course.

Hiroshima's green spaces come in two types: traditional Zen-influenced landscape gardens and parklands.

The former start outside Hiroshima Airport. Here lies Sankeien, a six-hectare garden replicating in miniature the mountains, villages and inland sea scenery of Hiroshima prefecture. The garden is built in the circular-tour style, which became popular during the Muromachi era (1337-1573), and the first thing you see is a lake, well-stocked with colourful koi carp - an irresistible invitation to buy a bag of fish-food, stroll along the wooden walkway jutting out into the water and trigger a feeding frenzy.

But, I hear you ask, where are all the flowers? Japanese landscape gardens don't go in for masses of blossoms. One blossom at a time is the norm: from camellias in January and plum in February, right up to chrysanthemums in November. Carefully placed rocks, ponds and trees do the rest. Back in the 16th century, some gardeners maintained that the positioning of the rocks was more important than the foliage. Perhaps that's why more than 7,000 tons of rocks were used in the making of Sankeien.

The vital role played by the seasons in Japanese gardens becomes clear on a visit to Hanbe, a garden-restaurant-spa complex, one bright morning in December - the one month when nothing's blooming.

"There's nothing special to see this month," says the kimono-clad lady in the office.

Hanbe boasts 50,000 azaleas, 1,000 maples, koi ponds, islands, waterfalls, cherry trees, iris and hydrangeas, ensuring that - barring December - there's a different natural wonder to savour each month. Even without blossoms, though, Hanbe is magical, from the soothing sound of water gently falling from a waterwheel to the sight an old tanuki (Japanese raccoon dog) grumbling through the undergrowth.

"Gardens should be appreciated as a piece of art," says Adachi Zenko, founder of Adachi Museum of Art, in nearby Shimane prefecture. To accentuate this point, his museum is set in a splendid environment that has won the Journal of Japanese Gardening's best Japanese garden award for 11 years running. This juxtaposing of art and nature is also present in Hiroshima, the town's three major art museums having been located within its three key parks. Together they comprise an art and nature triangle, making Hiroshima the city "where water, green and art intersect".

Shukkeien Garden was built in 1620 as a garden for the villa of Asano Nagaakira, feudal lord of the Hiroshima area, by Ueda Soko, a warrior who became a Buddhist monk, tea-master and landscape gardener.

To enter Shukkeien is like stepping into Dr Who's Tardis: it distorts concepts of time and space, presenting a miniaturised version of the landscape of West Lake in Hangzhou, China, in a space of just 40,000 square metres. Shukkeien contains many of the elements of Zen landscape gardens introduced from China by priest Muso Kokushi (1275-1351): picturesque rocks, rustic pavilions and a large pond with small islands. Around Takuei Pond, with its unique hump-backed Rainbow Bridge, winding paths lead you through mountains, valleys, rice fields, bamboo groves and tea plantations.

It's easy to lose yourself on a little side trail or among the isolated nooks, waterfalls, mossy rocks and lichen-covered stone lanterns. Watching a kingfisher darting into the pond as frisbee-sized turtles bask on the islets, you forget you're a five-minute walk from the bustle of central Hiroshima. And, sheltering in one of those half-hidden tea huts, it's easy to pretend you're the reclusive 17th-century haiku master Matsuo Basho, composing 17-syllable odes to the chattering frogs.

In Shukkeien's Seifukan tea house, with its thatched roof and lyre-shaped window, a different tea ceremony is held each month to celebrate the flowers of each successive season. In September, there's even a tea ceremony for moon-viewing.

Nourishment of a more bodily sort is available at La Cigale restaurant, overlooking Shukkeien's serene greenery in the adjoining Prefectural Art Museum. The museum was built in 1996 and houses a collection of 4,500 works of art.

Another angle of the triangle is formed by Hijiyama Park. Overlooking downtown Hiroshima, this 70-metre hill is a splendid maze of walking trails that snake through densely forested slopes of pine and evergreen oaks. It is crowned by the award-winning Museum of Contemporary Art, opened in 1989, Japan's first public contemporary-art museum. Henry Moore's huge bronze Arch stands before the museum, framing panoramic views of the city.

In spring, the blossoms of 1,300 cherry trees shroud Hijiyama in a soft pink mist. Trees are hung with lanterns, the air is tangy with the smell of grilled squid and chicken from dozens of food stalls, and people have hearty blossom parties beneath trees.

For the final angle of the triangle we dive deep into the heart of downtown, at the Hiroshima Museum of Art, home to a collection of European Impressionists and post-Meiji Japanese paintings, and built as a prayer for peace in 1978. In front of the museum stands a flowering chestnut donated by Pablo Picasso's son, Claude.

From the roof-garden of the multistorey Pacela shopping mall the circular museum looks like a spacecraft adrift in a tranquil sea of green. The museum, together with Central Park and Hiroshima Castle, forms a broad unbroken swathe of greenery right through the centre of Hiroshima.

Tucked away in one corner of Central Park is Yuhua Garden, a Sichuan-style garden built in 1991 by a sister city, Chongqing. It's tiny, and usually empty, but it's a charming spot for a moment of quiet contemplation.

Our garden tour ends where the city began, at Hiroshima Castle. For it was here, in 1589, that feudal lord Mori Terumoto decided to build himself a new castle, and a whole new town to go with it.

Today, the castle's carp-filled moat and tree-shaded grounds provide another much-loved spot for cherry-blossom viewing, when the banks of the moat metamorphose into a fleeting pink epiphany, echoing Basho's famous haiku: "How many, many things they call to mind these cherry-blossoms!"