Purple rock cress is an old-fashioned little thing, certainly not on the perennial “A” list these days. It is not particularly long-flowering, useless as a cut flower and would never be considered a good foliage plant. But for a mighty jolt of warm spring color in the right setting, there’s nothing quite like it.

A member of the mustard or Cruciferae family, it bears cruciform (cross-shaped) four-petaled flowers. Sometimes called false rock cress, as differentiated from rock cress or Arabis, purple rock cress is the common name for the lovely May-blooming perennial Aubrieta x cultorum and its cultivated hybrids.

Although the plant was named to honour the French botanical artist Claude Aubriet and is pronounced “o-bree-ta”, it is often misspelled as “aubretia”. The origin of botanical names is something modern gardeners pay little attention to, but the plant explorers, botanists, professional gardeners and artists so honoured were intimately involved in the discovery and classification of many of the plants we take for granted today. In the The Art of Botanical Illustration, by Wilfrid Blunt and William Stearn, we read how Claude Aubriet, a young illustrator in the French court, travelled with the famous plant explorer Tournefort in 1700. “After spending three months in Crete, they proceeded to visit no less than 32 islands of the Archipelago, braving pirates and storm-tossed seas. Wherever they passed, Aubriet’s pencil was busy making records of plants, landscape, antiquities and even the dresses of the natives.”

Purple rock cress has mat-forming, silvery-green foliage and masses of purple, lilac, pink or white flowers in loose racemes 10-20 centimeters (4-8 inches) above the plant. Blooming in the second or third week in May, established plants in flower appear to be blanketed in colour.

Although it may be used as a front-of-border perennial, purple rock cress shows to best advantage in the sunny rock garden, trailing over stone walls, edging a path or acting as a spring ground cover. It is effective when planted with other spring-bloomers for the rock garden, including brilliant yellow basket-of-gold (Aurinia saxitilis), rockress (Arabis), white candytuft (Iberis sempervirens), biennial blue forget-me-not (Myosotis sylvatica) and brilliantly-coloured cultivars of moss pink (Phlox subulata).

The species itself is a wan lilac colour and seldom grown, but cultivars have much larger flowers (including some doubles) in a wide range of rich colours. Seedlings do not come true, so desirable plants should be propagated by division in early spring or by stem cuttings.

Purple rock cress enjoys full sun and sandy or gritty, moist but well-drained, neutral to slightly alkaline soil that is not overly rich. When it has finished blooming, the plant should be sheared right back to keep it from become straggly-looking.

Adapted from a column that appeared originally in Toronto Gardens

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