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Well, you can, but you're not likely to be happy with the soggy slimy mess. Lettuce leaves are delicate and have a very high water content. Freezing the leaves destroys the molecular structure from the inside out. If you've got a bunch of lettuce that you don't know what to do with you can try braising it or making a soup. If you want to do a freezing experiment with lettuce you will have much more success if the lettuce is cooked in some sort of way first.
Yeah. Avoid freezing any and all greens. They are extremely delicate and the process of freezing will destroy what crispy crunch and leave you with a soggy slurp. So NO do not freeze heads of lettuce, unless you like soggy slurps of lettuce.
Lisanne is a trusted home cook.