Romaine

As an annual leaf vegetable, romaine forms a tall, upright, oblong head of elongated leaves with firm, dark green ribs running down their centers, giving it a looser shape than a head lettuce. The leaves are thick and crisp, qualities that hold up well after harvest, and texture shifts through the plant: the heart and ribs are the crispest part, with the surrounding leaves growing more tender toward the inner portion and only moderately tender near the outside. Those thick ribs, especially on older outer leaves, carry a milky fluid that gives romaine its characteristic bitter, herbal edge. Unlike most lettuces it tolerates heat well, though butterhead and leaf types still outlast it in the hottest conditions.

Romaine is best known as a fresh salad green and is the traditional lettuce used in Caesar salad, commonly serving as the base for green salads generally. It also carries roles beyond the produce aisle, appearing regularly in Middle Eastern cuisine and sometimes standing in as the bitter herb at the Passover Seder. Though most often eaten raw, it can be cooked as well, braised or simmered into soup.

Romaine's heat tolerance shows up as slow bolting, which keeps it productive through both spring and fall plantings rather than running quickly to seed. It generally withstands higher temperatures than head lettuce, extending the window in which it can be grown successfully across those cooler-season slots.

The alternate name "cos" is widely traced to the Greek island of Cos, from which the lettuce was presumably introduced, while "romaine" itself derives from the French word for Roman. Its earliest known appearance in English print comes from John Evelyn's 1699 work Acetaria: A Discourse of Sallets, marking its documented entry into English garden writing.