Lettuce is an edible, leafy herbaceous biennial that is grown as an annual. This leafy annual, which is grown for its succulent crisp leaves, will produce globular heads which are pale green. The lettuce will mature very quickly and can be harvested by the leaf or by the head. The leaves - notched, scalloped, frilly or ruffled - are generally eaten raw in salads and are a rich source of vitamins K and A. Depending on the variety, lettuce plants can vary in size, shape and colour but generally, the leaves form a dense head or a loose rosette of green or red, or a mixture of the two. If allowed to bolt or go to seed, the flowers will be yellow, followed by hairy brown seed heads. Lettuce is a 'cool-season' crop that grows well in the spring, and autumn, as well as the summer and some lettuce seedlings will tolerate a light frost. It's a good idea to make regular sowings for a constant supply. They need a lot of water for a few weeks before harvesting. The loose-leaf varieties are harvested by the outer leaves so that the plant can grow new leaves to be harvested later. The crisphead varieties are very popular for their loosely formed heads of crisp, curling leaves. Butterhead types produce tender leaves that are very tasty. Grow in moist, fertile soil that will not dry out and give full sun or dappled shade.