Just to meddle in the discussion a bit, both sides are probably correct. It is not the air temp that counts but it is the soil temp. There is a frost line that determines the temp of the soil. I once lived in Watseka, IL when winter temps hit -15 to -20 F and the frost line could be 3 feet deep. Sometimes we were told to keep a faucet turned to a trickle to prevent the water mains from freezing. Here in Salem, -10 is very cold and then only for a short period of time. Below 12 inches, the soil was usually +50 or so most of the time. Then there is permafrost which never thaws except for a shallow layer on top. I have read where it is safe to plant peonies until the ground freezes too deeply to dig the hole. Dryness is probably more harmful than cold.
