My DH told me that the regular fluorescent bulbs are not made for extreme cold and can shatter. He also said that the new twisty bulbs have mercury in them and are dangerous if broken. I would have to research this to verify, but I think I am going to use a rope light. Still in the build stage and not there yet.
Fluorescent bulbs including the twisted Compact designs contain small amounts of mercury, your husband is correct. I haven't had a flourescent bulb shatter in the cold in our area, they are commonly used in outdoor signs but our temperature rarely drops below 15-20 degrees F.
Read the box for the bulb you buy. My 60 watt Sylvania Instant-On Daylight twisty bulb is rated for use down to 0F degrees, but only uses 13 actual watts. However, I have switched to a 40 Watt LED bulb that uses only 7 actual watts and is not temperature affected and will have a 12 year life. The LED is not a daylight color, but egg production has stayed up so it's working