 Due to its short lifespan after harvest , lettuce was originally sold relatively close to where it was grown . The early 20th century saw the development of new packing , storage and shipping technologies that improved the lifespan and <unk> of lettuce and resulted in a significant increase in availability . During the 1950s , lettuce production was revolutionized with the development of vacuum cooling , which allowed field cooling and packing of lettuce , replacing the previously used method of ice @-@ cooling in packing houses outside the fields . 
