I've often heard it said that parsnips and some brassicas respond to freezing temperatures by increasing sugar levels inside the plant, thereby reducing the temperature at which they will freeze.
I can't help wondering if there isn't a bit more to it than that though. Mainly because if the sugars aren't already present in the plant presumably it needs to photosynthesise to produce them, and at such low temperatures and short day lengths is that even realistic? And if they are present, why don't we eat the bit they're already in?
Perhaps this is knowledge that comes from a time when frosts generally arrived earlier when the days were a little longer and the Sun rose higher? I have no idea. In any case, ours get eaten when we want to eat them. If we'd waited for a frost in some recent winters it could have been late January before we got to taste a parsnip.
James