Mukdenia is a hosta-like perennial in it's height and width. Mine get to be about 12-16" tall and about 2' wide although they may spread slowly after a while. The one pictured has been there for about six years and I'd say its about 24" wide. Growing in zones 4-9, it needs either filtered light all day long or full sun for a little part of the day then more shaded. Indirect light will make it happy too, in nice organicky soil. White astilbe-like flowers appear in April with soft green maple tree-like leaves persisting through the summer on a mound seemingly unaffected by deer. The best part of my story occurs in the fall, however, when the foliage turns a deep shiny maroonish-reddish orange. after about a month of this nice transitional coloration, the leaves blow away and you wont know Mukdenia is there until spring when it reliably pops up again for a repeat performance. 

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.