One can freeze lettuce if it is for cooking purposes or as a flavoring for a dish. However, if one wants to use it in a salad that is crispy, then freezing is not a suitable method to preserve this vegetable.
Although one can preserve many vegetable and foods by freezing them, lettuce does not freeze well. The reason for this is that the freezing process causes the formation of ice crystals in lettuce, which is a vegetable with a high-moisture content. This leads to damage of lettuce cell walls and lettuce that turns mushy.
If one has to freeze lettuce, then the thicker types of lettuce may be better suited for this purpose. Some of these lettuce types are romaine, Cos and Boston lettuces.