This is an inexpensive fix for cold weather, as a simple aquarium heater will do the trick. Configure the heater so that it maintains a water temperature in the 65–85˚F range.
FREESEA 25/50/100/200/300 Watt Aquarium Heater with Aquarium Submersible Thermometer
(Affiliate Link)
For more suggestions to extend your growing season, check out this Tower Garden article: