Many people successfully harvest rainwater in freezing climates, though these systems require extra precautions so the pipes and components are not damaged from freezes. Systems can include whole house, indoor systems, with rainwater being the sole supply of water, to simple outdoor irrigation systems used only during the growing season. If you live in a freezing climate find installers in a similar climate to yours and ask them about their recommendations for freeze protection. Also consult resources such as the Alaskan Building Research Series HCM-01557, Water Cistern Construction for Small Houses and the Alberta Guidelines for Residential Rainwater Harvesting Systems (see the resource page for links).