Here in Colorado it is important to winterize your sprinkler system in the month of October. Some early birds will winterize at the end of September. That does not hurt anything but may have you doing some watering by hand. Which completely eliminates the risk of freezing. Just make sure you disconnect the garden hose.
During the month of October your lawn typically will require less water for many reasons but mainly because we have less heat and evaporation. We still can have some hot days and that is why so many people want to leave their system on until the end of the month. The rule of thumb is that every day you wait to winterize in October is an increase in the risk of freezing. If the temperature drops down to 32 degrees, there is a chance you will have a break in your vacuum breaker. Every November I have a few stragglers that put off winterizing and got away with it. At the same time I’ve done many freeze repairs in Octobers past.
How do I get my system to make it through October and into November? First of all grass is going dormant by then so it may not matter. If you cover you Vacuum Breaker with insulation and a plastic bag it will help keep it warm. Keeping the air and moisture off the unit will help prevent freezing but is not guaranteed. An over night freeze is not going to freeze valves that are below ground. Some systems have a curb stop valve and a double check vacuum breaker which may also be below ground in a box. These systems are protected from over night freezes but very expensive to install.
Some systems can be drained while other systems require the water to be evacuated from the pipes to prevent freezing. This creates a lot of confusion for people especially when someone tells me they never do a blow out. Fact is that the ground will develop frost 4 feet deep in a cold winter. Water expands by about 11 percent. In order for a sprinkler system to be safe the pipes can not be near full of water over the winter. Auto draining systems tend to have problems with the drains; then I repair them all summer long as well as pipes that have probably frozen in the past. If you have a self draining system make sure you drain your tap, manifold and run the valves for a minute or two with the water off to activate the drains in your zones.
A big cause of freeze damage occurs when the main shut off valve leaks. The valve may leak one drip at a time right back into your system after you have drained it. The only way to know about this is to have a freeze or by double checking your tap drain 30 days or so after you have drained it. If water has magically filled the pipe you know your valve is not fully off or it is leaking. If you had a freeze last year after having a proper winterization, check your tap drain in November this year. If there is water in the pipe replace your tap valve.