Nodding Onion (Allium cernuum) Good for: Year-round color Zone: 4 to 8 Height: 1 to 3 feet Sun: Full Blooms: June to August For use in milder climates, this evergreen plant adds height and interest to roof gardens, Bousselot says. “It has grasslike but thick leaves with a beautiful flower. Its flower head dries and creates excellent winter interest.” Once abundant on the Chicago River banks, the plant features edible leaves, bulbs, and bulblets.