Cos lettuce is a type of edible lettuce that is very common in salads. The name “cos” is typically used in countries like the United Kingdom, though it’s not unusual to hear it mentioned in other places like the United States, where “romaine” is the more common name. “Romaine” is a French name, based on the word for “Roman,” while “cos” is based on the name of a Greek island.
The shape of cos lettuce is oblong, with the leaves extending up from the base instead of wrapping around themselves to form a ball or lying in a loose clump. The base is white to light green and becomes progressively darker toward the top of the leaves, though the inner leaves tend to remain much lighter in color and can take on a yellow hue. Cos lettuce is considered one of four major subgroups of lettuce, with head or crisphead, leaf and butterhead being the other three. The leaves in a head of cos can grow to as much as 10 inches (25.4 cm) in length.
Romaine or cos lettuce is relatively easy to grow in home gardens, and it can withstand more heat than other types of lettuce like iceberg. It still needs cool temperatures overall, however, with a growing season with temperatures in a range from about 45 degrees Fahrenheit (7.2 degrees Celsius) to 65 degrees Fahrenheit (18.3 degrees Celsius). Cos lettuce can grow in many types of soil, but the looser the soil, the better, and packed soils need amendments to loosen them up. Good drainage is necessary.
Like other lettuces, cos lettuce is often used as the base lettuce for salads, and it is the lettuce used in Caesar salads. It is edible raw, but cooks can also braise or grill it, although it doesn’t hold up well when a well-meaning cook tries to preserve or can it. The lettuce is available year-round in grocery stores in countries like the United States, both as a whole head and as a precut, bagged lettuce. Cos is low calorie and does contain a few nutrients including vitamins A and C, and there’s a small amount of fiber, too. The darker the leaves, the more vitamins they contain.