Depending on the type, the lettuce is ready to be harvested when leaf lettuce reaches the size indicated on the seed package, or when the leaves of head lettuce begin to cup inward to form a head. Gently remove the entire plant with its roots from the soil with a trowel. Be careful not to disturb the roots of neighbouring lettuce plants.
Store your lettuce in the refrigerator. To prevent it from drying out, wrap in a damp tea towel or kitchen paper. Leaf lettuce stays crisp for 2 to 3 days – the smaller the leaves, the quicker they perish. If left whole, firm head lettuces keep for up to 2 weeks.
Too much sunlight and high temperatures in the summer can cause bitterness in lettuce, as well as turning the plants into seed stalks when the lettuce leaves shrivel and seeds form (called 'bolting'). To prevent this, grow lettuce in a shaded place during the summer months, or select special slow-bolting or heat-resistant varieties.