Before the first hard freeze
What Happens If You Don't Winterize Your Sprinkler System
Everything on an irrigation system that holds water is a sealed container, and water expands about 9% when it freezes. That is the entire mechanism. What makes it costly rather than merely annoying is the delay: a pipe that split in December looks completely normal in January, because there is no pressure behind it. You find out in April, when the system is turned back on and a lawn-wide sprinkler system becomes a lawn-wide leak.
What to do
- Step 1
The backflow preventer goes first
It is above ground, it is brass, and it is full. It has no soil insulating it and no way to drain itself, so it is damaged by freezes that never reach the buried pipe — often the first real cold snap of the season. It is also the most expensive single component on most residential systems.
- Step 2
Then the low spots, the valve bodies and the heads
Buried poly lateral lines split where water pooled at a low point. Valve bodies crack at the diaphragm chamber. Sprinkler head cans split along the barrel and the internal assemblies bind. Below-grade components survive longer than the backflow device because soil buys time, but a sustained hard freeze with no snow cover reaches them.
- Step 3
On a pump or well system, the stakes go up
A pump, its pressure tank and the above-ground plumbing around it are all exposed and all hold water, and they are considerably more expensive than a sprinkler head. If the system draws from a well or a pond rather than the municipal main, winterizing is not optional maintenance.
- Step 4
Why nothing looks wrong until you turn it on
A depressurised split leaks nothing. The damage only announces itself when the system is pressurised again — which is why spring startup should be treated as an inspection rather than a flip of a switch, and why you re-pressurise slowly and watch the backflow device while the mains fill.
- Step 5
If it already froze: leave it off
Do not turn the system on to check. Shut it down, drain what you can reach, leave it depressurised through the rest of winter, and plan a careful startup in spring. Pressurising a cracked line finds the damage by flooding something.
A cracked backflow assembly can also be a cross-connection risk on a potable supply. If the device is visibly damaged, leave the irrigation main closed and have it replaced before the system is used.
When is your first freeze?
The steps are the same everywhere — the deadline isn't. Enter your ZIP for the NOAA freeze normals at the station nearest you — free, no signup required.
Free · no signup · NOAA 1991–2020 normals
Frequently asked questions
- What happens if you don't blow out your sprinkler system?
- Water left in the system freezes and expands about 9%, splitting whatever contains it — the backflow preventer first, then low spots in the laterals, valve bodies and heads. Typical repairs run $170–$2,500+.
- Will one freeze ruin a sprinkler system?
- One sustained hard freeze is enough to destroy an above-ground backflow preventer. Buried pipe usually needs a longer or deeper freeze, especially with snow cover — but the expensive component is the exposed one.
- How do I know if my sprinklers froze?
- Usually you cannot, until spring. Visible cracks or a split body on the backflow device are the one obvious tell. Everything else shows up on startup as a zone that will not pressurise, a soggy patch over a lateral, weeping valves, or heads that gurgle instead of popping up.
- Does it matter in a mild climate?
- It depends on whether you get real freezes rather than how often. Somewhere that freezes twice a winter still needs winterizing; somewhere that genuinely never drops below freezing does not. The ZIP frost tool answers which of those you are.