· Michael Blair
Nashville does not freeze the way the upper Midwest does, which is exactly why irrigation systems here get left charged over winter. It only takes one hard freeze to split a backflow assembly.
What actually breaks
The backflow preventer is first and most expensive — it sits above ground, holds water, and cracks when that water expands.
Then above-ground valves and any exposed piping. Buried lateral lines are usually below frost depth and rarely the problem.
Blowout, drain, or both
Compressed air blowout is the thorough method and is what most Middle Tennessee contractors do. Manual and automatic drains help but rarely clear everything, particularly on flat runs.
At minimum, shut off the supply to the system, drain and insulate the backflow assembly, and open a test cock.
- Shut off the irrigation supply before the first hard freeze
- Blow out the lines with compressed air
- Drain and insulate the backflow preventer
- Set the controller to off or rain mode
- Note the shut-off location for spring
When to do it
Late November into early December in most Middle Tennessee years, before the first sustained freeze rather than after the first cold night.
Waiting for the forecast to look serious is how systems get caught, because the damaging event is usually the first one.
Spring start-up
Open the supply slowly. Charging a dry system quickly produces a pressure surge that damages heads and fittings.
Then run each zone and walk it, checking for heads that were damaged over winter by mowers or frost heave.