About 90% of the United States' supply of lettuce in the winter comes from Yuma, Arizona [0].

I think it more appropriate for most people to eat seasonally-appropriate vegetables during the winter months, than to get a little lettuce from thousands of miles away. Winter squash, carrots, apples, and many other food products store very well in root cellars [1].

[0] http://www.visityuma.com/agritourism.html [1] https://en.wikipedia.org/wiki/Root_cellar