AN herbal knot garden . . . a complicated maze . . . a colonial garden lush with ancient roses, larkspur, hollyhocks, and lemon lilies. These are theme gardens, designed around one central idea, adapted and embellished to fit their particular locales. They are the kind of garden that can start out as a casual experiment and become a lifetime obsession, as they did for Barbara Damrosch, author of ``Theme Gardens.'' And as they have for hundreds of gardeners who pattern their gardens after some of the great theme gardens.
Probably the most emulated theme garden in the world is the ``white'' garden at Sissinghurst, in England. Hundreds of American gardeners have been inspired by that small corner of the magnificent Sissinghurst gardens, created earlier in this century by Vita Sackville-West and her husband, Harold Nicolson.
I succumbed to its charm, so in one corner of our garden each spring a white clematis climbs the lamppost, a curved planting of white H. H. Hume azaleas shapes the path -- edged with iris Cristata alba -- and a drift of Thalia (a small white narcissus) decorates the driveway fringe, while phlox Divaricata alba covers the ground underneath a dogwood.
The clematis continues to produce an occasional flower during the rest of the garden season, and spikes of white astilbe pop up behind the azaleas in midsummer, followed by the lovely white shooting stars of cyclamen Hederifolium album, blooming where the daffodils had appeared in early spring -- my salute to Vita Sackville-West.
She in turn was probably inspired by similar ``moon'' gardens, planted centuries earlier in India.
In ``Theme Gardens,'' Ms. Damrosch explores the elements of a moon garden, one of 16 different theme gardens the book explains how to plan, plant, and grow.