There is a saying about compost ingredients that goes something like this: under barley-straw there is copper; under wheat-straw, silver; but under bracken, there is gold. At Rowallane in Co Down there is gold in plenty. Here, head gardener Mike Snowden works his particular alchemy turning bracken fronds - along with sharp sand, loam and slow-release fertiliser - into potting compost. This magical substance is used in the propagation of the rare rhododendrons and other special plants that inhabit the 50 acres of gardens near Saintfield.
Besides the hundreds of candy-coloured rhododendrons, there are dozens of Pieris with clusters of pale blossom, like lengths of expensively-strung pearls. The rhododendrons were grown from seed collected by either Ernest Wilson or Reginald Farrer, according to Snowden, while the Pieris were Frank Kingdon-Ward collections. The fruits of the great plant-hunters and growers spread through this garden with the same profusion as the wildflowers that seed happily in the carefully-managed grass. A grove of prehistoric-looking, prickly Mahonia, for instance, is from the same batch of seedlings that produced `Charity', the well-known winter-flowering, scented shrub raised by the renowned Slieve Donard nursery.
The gardens were started at the end of the 19th century by the Rev John Moore. He laid out the pleasure grounds and built a walled garden (its walls are striated with his own invention of glazed, pierced tiles through which plant-ties can be neatly threaded). His nephew, Hugh Armytage Moore, was a passionate plantsman and it was during his tenure that Rowallane became a repository of choice specimens from the Far East, and the birthplace of several new garden cultivars. Famous Rowallane plants that are still in cultivation today include Primula `Rowallane Rose', Hypericum `Rowallane' and Chaenomeles `Rowallane '. The "virtually-organic" gardens are still a fertile ground for chance seedlings: about eight years ago, a new Welsh poppy appeared at the base of the wall in the nursery. Mike Snowden has christened it Meconopsis `Rowallane Red' in honour of its fruity, icepop, orange-red flowers.
But Rowallane is not just a delight for plantspeople: its hilly acreage, with a fresh surprise around every copse of rhododendron and laurel, is so compelling that you don't need to know a single plant name to enjoy its riches. The garden, except for the walled-in part (where a 50-strong collection of large-flowered hybrid penstemons can be seen in late summer) is relaxingly informal. Old, stone field boundaries delineate its shape and occasional outcrops of whinstone knuckle up from the ground.
Mount Stewart, within easy driving distance, is an entirely different place. "The result of a society lady's exuberance," says Mike Snowden of the over-the-top, formal creation of Edith, Lady Londonderry. "It was developed at around the same time as Rowallane, but the two of them (Lady Londonderry and Hugh Armytage Moore) never met socially: they were from different strata."
After Edith came to Mount Stewart in 1919, she was assigned 20 ex-servicemen as part of the government's post-war demobilisation drive. This, she wrote, "was the opportunity to plan and make the grounds surrounding the house not only more cheerful and liveable, but beautiful as well." She started, safely enough, with an Italianate garden of neat parterres (formal beds), their shape copied from her mother's home in Dunrobin Castle in Scotland, and the stonework inspired by two Italian villas. As her confidence grew, her gardens became more extravagant: a Spanish garden with elongated arches of tightly-clipped Leyland's cypress (proving that in strict hands, this scourge of rural Ireland can be a real stunner); a trefoil-shaped Shamrock Garden with a Red Hand of Ulster depicted in red bedding plants, an outsize harp sculpted from yew and sundry other topiary figures including a ship, stag and crowns.
Her Dodo Terrace - with finely-moulded cement figures made by local artisan, Thomas Beattie - was a tribute to the recently-defunct Ark Club, a gathering of some of the major political and social figures of the time. Winston Churchill was immortalised as Winnie the Warlock, Edward Carson was Eddie the Eagle, while Edith herself was Circe. But, to quote Lady Londonderry: "The really exciting and important thing about Mount Stewart was discovering the climate." It is, she wrote, "a garden with possibilities such as exist in Devon and Cornwall". Encouraged by other, more experienced gardeners, she filled her formal spaces with a jungly concoction of halfhardy plants: tender rhododendrons, mimosa, Chilean fire bush, tree ferns, Chusan palms, broad-leaved cordylines (C. indivisa), olives (from the Mount of Olives) and kiwi fruit, now bearing its furry brown eggs.
The effect today, with many of the specimens in their peak of maturity, is extraordinary. Fantastic, subtropical plants give shade to cool-loving primulas and ferns, while strange beasties of green topiary and grey stone roost at strategic corners. The trees and shrubs are filled with real, warmblooded birds - whose calls seem louder than usual in this aristocratic, horticultural wonderland. And up above, several buzzards who have made Mount Stewart their home, wheel menacingly across the sky. The only thing that brings reality back into focus are the garden visitors (Northerners are dedicated weekend picnickers) with their wellbred dogs, well-trained children, tartan rugs and Thermos flasks.
For further information on Northern Irish gardens and for details of a Short Breaks in Northern Ireland punt-for-pound campaign, contact The Northern Ireland Tourist Board, 16 Nassau Street, Dublin 2. Tel 1850-230230.