Our recent frosts have put an end to all of the warm season vegetable crops like tomatoes, peppers and squash.
The cool weather crops on the other hand are still hanging in there, even though the colder temperatures have slowed down their growth rate.
The flavor of leafy vegetables, like lettuce, cabbage, Brussels sprouts, kale and the like, is enhanced by light to moderate frosts. Root crops like carrots, and especially parsnips, sweeten up after exposure to cold temperatures.
The mechanism behind this phenomenon involves plant carbohydrates. Starches and sugars are different types of carbohydrates that are present in plants. When the plant is exposed to cold temperatures the starches get converted into various sugars that sweetens the flavor. This is the main reason why you should never keep potatoes in the refrigerator, it is the starches that give potatoes their distinct flavor.
Lettuce, cabbage and kale will eventually winter kill as the season progresses. It takes quite a bit to kill Brussels sprouts, they can survive well into November getting more flavorful with each frost but they too will eventually freeze and die back.