Lettuce (Lactuca sativa) is a cool-season annual vegetable in the Asteraceae family. It is cultivated for its leaves, which are consumed fresh in salads and sandwiches. Lettuce is the most widely consumed leafy green globally and is native to the Mediterranean region.
Lettuce has been cultivated since at least 500 BCE in ancient Egypt, where it was grown for both food and oil from the seeds. The plant spread to Greece and Rome, where it was consumed primarily as a cooked vegetable. Leaf lettuce varieties became popular in Europe during the Renaissance.
Lettuce is consumed almost exclusively fresh. The four main types are crisphead (iceberg), romaine, butterhead, and leaf lettuce. It is a low-calorie source of vitamin K and folate.
Leaf lettuce is harvested by taking the outer leaves, allowing the center to continue growing. Head lettuce is harvested when the head is firm. Cut-and-come-again harvesting is used for leaf varieties. Lettuce is stored in the refrigerator for 5 to 10 days.
Lettuce prefers cool temperatures between 7 and 18 degrees Celsius (45 to 65 degrees Fahrenheit). High temperatures and long days trigger bolting (flowering), which makes leaves bitter. Lettuce has a shallow root system and requires consistent moisture. Shade cloth extends the spring season.