Winter Density romaine lettuce seeds are really a butterhead/romaine cross. The heavy, compact, and extra-dark green heads stand about 20cm (8″) tall and display upright but tender leaves. Grow this unique, flavourful English variety in spring, summer, or fall; as it is frost tolerant and quite bolt resistant. On the coast, an August planting will overwinter as full-sized plants if given some protection from rain. Started as late as September, they may overwinter as small plants to head up in March.