Thu hoạch rau diếp vào buổi sáng cho lá tươi, giòn. Chọn 6–8 lá ngoài cùng để kéo dài vụ thu hoạch. Ngắt lá rau diếp một cách dứt khoát. Thu hoạch những chiếc lá bên trong khi chúng đạt được khoảng 10 cm. Rửa sạch lá thu hoạch được và bảo quản trong tủ lạnh đến 10 ngày.

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If you pick lettuce leaves early in the day, they won’t have been dried out by the sun. If you wait too long and pick your lettuce in the afternoon or evening, you may end up with slightly withered leaves.  If you lose track of time and forget to harvest in the morning, it’s best just to wait until the next morning and harvest then. Mature romaine leaves are typically dark green and 4–6 inches (10–15 cm) in height. If you’d like to have a harvest of only mature leaves, choose the outermost 6–8 leaves from the lettuce head. The advantage to this method of harvesting romaine is that you’ll be able to enjoy a prolonged harvest, as each set of inner leaves takes another week or so to mature. The disadvantage is that each harvest will be relatively small. To pluck individual romaine leaves, grasp each leaf firmly at its base, and tweak it sharply downward until the leaf snaps off. If you try to pull upwards on the leaves, you may start to uproot the whole plant. Keep an eye on the inner leaves of the romaine head and give them time to continue growing. Once they’ve opened up and matured, they’re ready to harvest. This process can happen quickly, so check on your garden daily. You’ll be able to get 3–4 additional harvests by picking only the mature, outermost leaves each time. Once you’ve harvested the outer leaves from each of your romaine plants, rinse dirt off of the leaves by running them under cool tap water. Pat them dry and store them in a plastic bag in your refrigerator. If kept dry in the fridge, the romaine leaves should keep for about 10 days.