I am raising lettuce in my green house. It works, the lettuce is tasty, however it is not nearly as crisp as the lettuce I grow outside the greenhouse (same variety). What can I do to make my green house lettuce more crisp?
@Ecnerwal answer is correct, the book "The Winter Harvest" by Eliot Coleman [speaking of some vegetables including Lettuce] states "No only do many of them tolerate cold conditions and even temperatures well below freezing (as long as they are spared the desiccating effects of cold winter winds) they actually thrive and are sweeter, tenderer and more flavourful.
I believe he also explains that in response to cold temperatures many vegetables produce more sugars as a type of anti-freeze to help the plants cope, which makes them taste better.