iberis sempervirens

The low evergreen foliage of this great rock garden plant erupts in snow white flowers in April. The one pictured here is over 30 years old and never gets food, mulch, or water during droughts, yet it's extremely happy. Iberis needs full sun and good soil that's well-drained. Wet soil and shade will make Iberis unhappy. 

So if you have a wall or raised rock garden and need something white (the last color to go to sleep at night), try Iberis, also called "candytuft." it's not always easy to get, so if you see it available, get it while you can because it might be years before you see it again.