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http://www.growingformarket.co...
This was informative - the second paragraph includes:
"In warmer areas, the goal is to get enough top growth to get off to a roaring start in the spring, but not so much top growth that the leaves cannot endure the winter.
If garlic gets frozen back to the ground in the winter, it can re-grow,
and be fine.
If it dies back twice in the winter, the yield will be decreased from the theoretical possible amount if you had been luckier with the weather."
(I added bolding and whitespace.)
Later:
"It is unwise to over-fertilize in the fall or the growth will be too fast and tender to survive cold conditions,"
I tried to find what the mechanism is for garlic's cold-hardiness but didn't find anything. One abstract hinted around, but the full text cost $36 so I may never know! But it sounds like the cloves are very cold-hardy once they get some roots out, but the above-ground green parts may freeze. If they freeze TWICE in one winter, the bulbs will suffer somewhat.
http://worldwidescience.org/to...
"This work shows that peeled garlic cloves demonstrate significant supercooling during freezing under standard conditions and can be stored at temperatures well below their freezing point (-2.7 C) without freezing. The nucleation point or 'metastable limit temperature' (the point at which ice crystal nucleation is initiated) of peeled garlic cloves was found to be between -7.7 and -14.6 C. Peeled garlic cloves were stored under static air conditions at temperatures between -6 and -9 C for up to 69 h without freezing, and unpeeled whole garlic bulbs and cloves were stored for 1 week at -6 C without freezing. (author)"
