Trough Gardens-Purple Rock Cress Purple rock cress is a creeping groundcover that looks lovely trailing over rocks or the edges of a trough garden. Most varieties have pink or purple flowers in spring. Name: Aubrieta deltoidea Size: 2-8 inches tall; 4-24 inches wide Zones: 5-7 Top Picks: 'Bressingham Pink' has double pink flowers. 'Royal Velvet' bears large purple blooms. 'Hartswood Purple' produces violet flowers.