Giant onion (Allium giganteum) forms a clump of 2-foot-long, strappy, gray-green leaves. In midspring, the foliage starts to wither, and 4-foot flowering stems rise up with large, 6-inch globes of purple (rarely white) flowers. It prefers well-drained soil in full sun in a site protected from the wind. Key to naturalizing the giant onion in the garden is allowing the soil to dry from midsummer to fall and applying an annual layer of mulch before the seeds are dispersed, not afterward.