I've ridden down to about -35F and lived to tell about it. At -20 you'll probably want something covering your mouth, whether it's a scarf, balaclava, or something else so you breathe in warmer and more humid air.

Your definition of riding may change slightly in the winter as well. Around here some folks will just throw studded tires on a CX bike, some will ride a MTB with studded tires, and some of us will break out the fat bikes. A good excuse for N+1.