65-70 days to harvest. The Parris Island Cos lettuce is a Romaine lettuce, which means it is among the most nutritious of lettuce varieties. These non-GMO seeds grow upright into tall heads of lettuce that are ready to harvest in 65 to 70 days. Midribs are crunchy and juicy. Because of their higher chlorophyll content, romaine lettuces are among the most nutritious of all lettuces.