I believe it freezes at -58F.
Propane will not freeze at temperatures possible for local ambinent conditions.
It will stop boil off (producing gas) below -44°F. It will slow down as temperatures approach that point. As the propane in the tank is used, it boils off more gas. The boil lowers the temperature so the propane in the tank will be colder than the outside temperatures.
http://www.propane101.com/aboutpropane.htm