Collect rain water in an old barrel or other large container that is outfitted with a spigot and a suitable cover, and use the water on your garden. Use this water as an alternative to turning on the hose for newly transplanted material, window boxes, flower pots and container gardens. Rain water is actually better for your plants as it does not contain any chlorine and is at ambient temperature. Keep your rain barrel covered to prevent mosquitos breeding and for safety reasons.

Source:http://www.on.ec.gc.ca/reseau/watertips_e.html