Kale is a hardy biennial plant grown as an annual. It’s a member of the cabbage family and looks like cabbage with a permanent wave. Scotch kale has gray-green leaves that are extremely crumpled and curly; Siberian or blue kale usually is less curly and is a bluer shade of green. There are also decorative forms with lavender and silver variegated leaves.Kale is a cool-weather crop that grows best in the fall and will last through the winter as far north as Maryland and central Indiana. Frost even improves the flavor, and kale is better adapted for fall planting throughout a wide area of the United States than any other vegetable. Kale doesn’t tolerate heat as well as the collard — which it resembles in being one of the oldest members of the cabbage or cole family. All cole crops are frost-hardy and can tolerate low 20°F temperatures.