When is the Best Time to Start the System
The average last freeze for the metro area is around May 12. The problem with waiting until that date to activate the system is a lawn will require moisture in April and May because of warm and dry weather.
When a system is started prior to the final freeze of the season the exposed backflow pipes on the side of the house should be insulated/wrapped to prevent freezing and doing this will provide a measure of protection for temperatures at or slightly below freezing. If temperatures fall into the mid-twenties there will likely be damage despite the insulation.
Damage to the backflow device, caused by freezing, can be an expensive repair. If the damage is severe it often leads to a rupture and ten gallons of water per minute flowing from the device.
My best advice is to activate a sprinkler system after the last possibility of freezing weather. My next best advice would be to wait until mid April before activating a system and be sure the backflow is wrapped.
Finally, if a system has been activated a possible catastrophe can be averted by turning off the water if the overnight temperatures are projected into the mid twenties or lower. This action won’t prevent damage but it will remove the possibility of water gushing from a rupture once the device thaws.
During the springtime the only freezing concerns would be for the exposed pipes at the side of the house. The system components in the ground are perfectly safe from damage caused by freezing as the ground is warm.
This means the backflow can be drained on those anticipated cold nights and there will be no system damage. Those who know how to drain a backflow can open a system anytime in April and not be concerned about system damage.