All these root vegetables, brassicas, and other leafy greens want to develop in cold weather. They are well adapted with a defense mechanism that not only keeps them from dying in winter, it makes them sweet and delicious.
When temperatures drop, root crops convert some of their starches into sugars. This keeps the water in their cells from freezing, and it works the same way as putting salt on a road to keep it from icing over.
A lower freezing point means that while the cells inside the plant might have icy cold water, that water won’t turn into ice. Think of it as nature’s antifreeze.
In the cabbage family, cold weather increases sugars and releases special proteins in the plants (called “antifreeze proteins”) that provide freeze tolerance, which subsequently alter—and in our view, improve—their flavor.
All of these changes take time, which is why many leafy greens (kale, for instance) turn sweeter when hit with frost a few times. It might be alarming the first time you see a dusting of snow on your crops, but it’s simply just amping up the sugar content in your food!