"In the North, Mount Stewart." Situated on the edge of Strangford Lough on the western shore of the Ards Peninsula in Co Down, the 80-acre site is largely the creation of Edith, Lady Londonderry, who arrived there in 1919 and between then and her death in 1959 created a garden which is also a world. The broad, thickly planted borders flanking Dillon's lawn reflect her regard for the colour combinations of Mount Stewart.

Asked to name her favourite garden in the South causes her to pause, flick a crumb of earth back into place and announce: "It would have to be Altamont, yes Altamont." As with Mount Stewart, Altamont, well hidden in the Co Carlow countryside, is associated with one individual, the legendary Corona North, who died last year having spent her life continuing the work of her father, an accomplished plantsman, Fielding Lecky Watson. On his death in 1943, Corona, then newly married, returned home, and devoted her energies to Altamont.

Snowdrops are already out at Sandford Road. Dillon has about 50 varieties of them. "But there are about 200. People who are really red hot would have them all. If I could grow only one snowdrop, it would be Galanthus `S. Arnott'." The secret is to transplant them in growth, either in flower or just after, and then they settle in immediately. As she says, they do a lot of growing between flowering and dying down. Green is the dominant winter colour of the Dillon garden; the pyramid box and yew currently enjoy centre stage. It is also pleasing to hear her praise the virtues of an ordinary worker plant such as Viburnum tinus. But there are flashes of colour. Delighted by the response to the bright red provided by her rare quince, tree peonies, evergreen ferns, she mentions another rare fern which has yet to make its appearance, but she is less pleased to be asked about herself.

Although by now resident in Ireland for almost 30 years and certainly a national institution in Irish horticulture, the nervy, flamboyant Dillon appears quite paranoid about being an outsider and blames it on her accent. The eldest of three and the only girl, she is a Scot and grew up in Perthshire. Her family belong to the Scottish peerage and she is, by right, the Hon Mrs Val Dillon. An ancestor, Sir William Rollo, was beheaded in 1645 by Cromwell's men. Make no mistake about it, the accent is upper class and intensified by having attended Heathfield school near Ascot.

There she says, "gels" learnt more about being prepared for "good" marriages than about academic pursuits. "I began gardening as a child. I was always interested. I gardened at school. It was always the thing I most loved. It was also a good way of getting out of sport, which I hated." She was born into privilege and openly says as much. But she also stresses she wanted to get out and fend for herself.

Having left school at 16, she went to London and tried a number of jobs. "I always wanted to be a journalist," she says. She has also resented that after school she did not go to university, a place reserved for her brothers. "It's a thing I've always been chippy about. I wanted to go. I did when I was 23 or 24; I decided to study Russian but . . . " she shrugs and explains that she realised then that "it wasn't really for me" and left after six weeks.