Remember that Coleman's work is called "Four Season's HARVEST", he doesn't
actually grow things all four seasons but rather HOLDS them for harvest
during the coldest months.
Exactly. Timing plants so they are near harvest as winter approaches allows
holding them over much or all of the winter. And, as he and many of us
learned from the Nearings, many salad plants can freeze during the night
but will thaw and be healthfully crisp when the sun warms the space,
presumably with no cellular damage.
Don't recall if I have mentioned it--friends with a serious hoop house use
a crock pot to avoid frost. Fill with water and leave on Low as I recall.