Romaine or cos lettuce (Lactuca sativa L. var. longifolia) is a variety of lettuce that grows in a tall head of sturdy dark green leaves with firm ribs down their centers. Unlike most lettuces, it is tolerant of heat. In North America, romaine is sold as whole heads or as “hearts” that have had the outer leaves removed and are often packaged together.
Greenhouse romaine varieties are much smaller than the field grown - standard greenhouse romaine matures at about 8-10 inches but there are also mini romaines available can be 4-5 inch only.