The greatest virtue of a white garden is the way that, towards evening, its flowers seem to gather and hold the light. In a white garden, trees and shrubs and even hard landscaping slowly sink from sight as darkness falls, leaving the white flowers afloat on the surface of the night.

The flowers of the Gothic garden, although not truly black but the deepest shades of purple, magenta and chocolate, melt quickly into the dusk. But when morning comes they are at their most mysterious, stubbornly drawing the light, slow to show themselves, carrying forward the intensity of night into the new day.

The petunias, 'Black Velvet' are particularly secretive. Slow to wake, they do not open until touched by the sun. But by late morning they are shaking out their skirts, some of which have a thin yellow stripe. They love the sun, but being highly strung they will throw themselves down on the very hottest days, which is why they do best in a pot that can be moved into filtered light or even into shade.

With their trailing habit, they look best in a tall urn or a pot on a pedestal where, raised towards eye-level, their trumpets of intense colour can be inspected, peered into, meditated on. So much more theatrical than ordinary petunias, which can be garish, or insipid, amid the razzle-dazzle of summer the velvet darkness of these beautiful plants soothes both eyes and imagination.

Years ago now, I lived in a house on the Isle of Man with a high, pointed roof. It backed onto a pine plantation, and soon after we moved in we discovered we had bats living in the ceiling. One morning at first light I glanced out of the bedroom window and saw them swooping down the hill out of the pines, and I ran downstairs and sat quietly on the back doorstep.

It was one of the most amazing and beautiful sights I've ever witnessed - the erratic flight paths that ended so smoothly in a perfect dive into an opening underneath the eaves that was no bigger than my hand. The last of them, momentarily lost, zigzagged madly until it too found its way home.

With the looming pines, and the bats whirling, and the sound of the nearby Sulby River flowing over its ancient stones it would have been the perfect place for a Gothic garden. Alas, I did not think of it, although the many white David Austin roses I planted almost qualified because of their eerie glow under the moonlight.

Fritillaria meleagris, the beautiful Snake's Head Fritillary, too, would have counted towards a Gothic garden. We planted them there beneath a strip of grass and looked forward to them raising their beautiful chequered heads each spring. I often wonder if they are still there, multiplying; it pleases me to think so.

At present, in the Southern Hemisphere, my Gothic-influenced part of the garden is restricted to a corner at the front where a dark angel watches over the few 'black' plants I grow. But as I gather more - from seed merchants and the generous donations of cuttings from friends - plans for the Gothic garden are beginning to expand.

The bearded iris, 'Midnight Oil' is already in the soil. Black tulips, 'Queen of the Night' are in the crisper of the refrigerator awaiting cooler weather at the beginning of May. Black Arum lilies, and Black Parrot tulips should be arriving soon. Black cornflowers, hollyhocks and poppies are waiting to be sown.

If all of this sounds a little gloomy, in reality dark flowers are at their most dramatic when played off against light. I like to put them with the palest of pink roses, and with herbs that flower more modestly. It's a plan, anyway, although it will be next spring before I know how well it has worked.