Water from your irrigation system can keep your keep plants alive, but once it gets too cold, the frozen water can break even the toughest pipes. In areas where the outside temperature almost never dips below 39 degrees, you will never have to worry about winterizing the system, but in cooler to cold areas, it is an important fall project.
Drain the system of water. You can get most of the water out of the irrigation pipes simply by letting gravity pull the water from the pipes. Open the drain valves at the top and bottom of your system, making sure that you have turned off the water source first.
Determine the size of air compressor you need to blow out your lines. Using gravity alone can be risky since some of the pipes may have water puddling in them that could freeze. According to M. Higgins of Colorado State University, "the ideal pressures are in the range of 40 to 80 pounds per square inch (psi) for the air compressor with 80 psi being the maximum for rigid PVC pipe and 50 psi for polyethylene pipe." To determine the actual size of the compressor to use, he recommends you to "divide gallons per minute (gpm) by 7.5 to determine the cubic feet per minute (CFM) needed.
Remove the pump, if possible, from the system and bring into a storage area where it will not freeze. You don't want water sitting in it and then freezing. Keep the controller hooked up to the power source to keep the rising and falling winter temperatures from forming condensation in the electronics. Program it to turn on once a week and run through the shortest cycle.
Leave your valves halfway open to keep them from freezing. Just remember to close them before you start you system up in the spring.