A paean to peonies: The Chinese have been cultivating them for years. A nursery in Cheshire is mad for them. But just what's so special about the tree peony? Anna Pavord takes a fresh look

In the autumn of 1961, Dezhong Chen resigned from his job as factory accountant in the Chinese province of Gansu and took off for the hills. The government of the time had called for 'educated urban youth to go and work in the country and mountain areas to strengthen agriculture'; Dezhong Chen's idealistic plan was to return to his home town, Peace, and revitalise the barren land of this poor, drought-stricken area by sourcing and planting trees and shrubs that could survive the harsh conditions. After years of home study, and a degree in forestry, he began to put together a collection of native plants that he felt could do the job. Surprisingly - at least to those who cosset them in their English gardens - it included several kinds of tree peony.

These big woody peonies have been cultivated in China for thousands of years, as much for their medicinal uses as for their beauty - the bark from the root is highly prized in Chinese medicine. In his Peace Peony Nursery, Chen began to gather in various local types of purple-blotched mudan (as Peking has become Beijing, so moutan has shifted to mudan), and by crossing them created a whole new race of tree peonies that were tough, vigorous and versatile enough for his purpose. Painfully, through the course of 10 winters, he dug a channel 150 yards long to bring water from a spring to the drought-prone nursery where the annual rainfall is only 35cm and there are just 155 frost-free days in the year.

Gradually these Gansu Mudan, as they are now officially called (see The Plant Finder for the 19 named varieties on sale over here), began to acquire a reputation, not only in China, but beyond. In France, Robert and Nicole Pardo began growing them at La Pivoine Bleue, their nursery at Montégut just north of the Pyrenees. In England, they have become an 'obsession' for Will McLewin of the Phedar Research and Experimental Nursery in Cheshire. It began, he says, 'with the conviction that most of what was said about Paeonia rockii and 'Rock's peony' was inaccurate, and that there were many similar or associated plants that should be better known and more widely available.'

The fabled P. rockii is named after the American plant hunter Joseph Rock, who is said to have seen this exotic tree peony with vast white flowers growing in the garden of a Buddhist monastery in Gansu province. McLewin, who with Chen has now written a definitive account of the peonies he has called the Gansu Mudan ( Peony rockii and Gansu Mudan, Wellesley Cambridge Press), tells a different story. He debunks a few myths and instead stitches together a complicated trail showing that few of the tree peonies that bear Rock's name are really the wild P. rockii and that many of the ones in cultivation have descended from ancient Chinese hybrids.

We are talking here about a shrubby peony (tree is a misnomer, as McLewin is quick to point out) with woody stems that are a permanent part of its structure. Its cousin, the herbaceous peony, dies down to the ground each autumn, producing fresh, juicy shoots in spring. The growth on a tree peony is upright and rather sparse, though a mature plant can be up to seven feet high and wide. The flowers are enormous, sometimes a foot across and the Chinese species P. rockii is thought particularly desirable because the blooms are pure white, with dramatic blackish-purple blotches at the base.

McLewin provides an exhaustive list showing how the wild P. rockii from the Xinglong mountains differs from the cultivated Gansu Mudan. That the flowering period is 15 days, compared with 24 for the cultivars, is one reason why the Gansu Mudan is far more attractive to most gardeners than the wild species. The photographs which take up a large chunk of the McLewin/Chen book (150 in a total of 181 pages) are an even more potent draw.

The Chinese classify these peonies by colour - nine different shades. As a gardener, I'd say the choice is white or pink, but the variation on those two themes is immensely subtle, almost as delicate, intricate and sophisticated as a tulip's. As with the tulip, much of the drama comes from the contrast between the overall colour of the petal and the basal blotch. In some cultivars, the dark blotch bleeds through like raspberry juice into the surrounding colour of the petal. In others it is neatly confined to a series of thumbprints. Some petals are smooth-edged, others are as ruffled as a ballerina's tutu.

As well as the nine colours, the Chinese designate six forms of tree peony flower: single, lotus, rose, anemone, crown and globular. In general, going by the pictures alone, I liked the singles best, especially those such as 'Xiong Mao' (bear cat or panda in English) and 'Yu Shan Lun Jin' (which translates roughly as feather fan silk braid).

Conditions at the Chinese nursery are very much more extreme than at McLewin's Cheshire nursery where winter temperatures don't often drop below -6C and there are rarely more than 38 days of frost. Average rainfall there is 90cm, almost three times as much as Chen can expect in Gansu. The French nursery provides a third set of different conditions, a Mediterranean climate with mild winters and hot, dry summers. Although the heat and drought of summer sometimes forces the tree peonies into premature dormancy there, they emerge unchecked the following spring.

The message for gardeners, therefore, is that this race of tree peonies is quite awesomely tolerant of the worst that nature can throw at it. The ideal, says McLewin, is deep, rich, open loam that drains well and a position in sun but where air can circulate freely through the bush. But 'on the whole,' he adds comfortingly, 'any half-decent site will do'. Like herbaceous peonies, tree peonies resent being moved, but early autumn is the safest time to do it. They don't need regular pruning, though dead wood can be cut out if necessary in late spring or early summer.

I've no experience of these Gansu Mudan, but would expect the foliage to be as much of an asset as the flowers. That's certainly the case with a different Chinese tree peony in our garden, P. delavayi, which has small, deep red flowers and handsome, jagged leaves on long stalks. I learnt with this one that it pays to be patient. Ours had to be moved, to accommodate building works. Gradually, after it had been shifted to a new site, all the woody stems died back, but from the base the following year sprang five new shoots of miraculous vigour.

'Peony rockii and Gansu Mudan' (£37.50, including postage and packing) is available from Will McLewin, Phedar Research and Experimental Nursery, 42 Bunkers Hill, Romiley, Stockport, Cheshire SK6 3DS; 0161 430 3772 or mclewin@phedar.com. He can also send a list of Gansu Mudan available from his nursery