A sprinkler blowout in Bozeman should be completed before the first stretch of consecutive nights below freezing, and that stretch begins Friday, October 23. Overnight lows are forecast between 29 and 31 degrees every night for roughly a week, with daytime highs in the upper 40s that will not fully warm the ground back up.
Lows near 30 for a week starting Friday. This is the blowout deadline.
Three weeks ago, people in the valley were asking whether an early frost had already ruined their irrigation. The honest answer then was no, and the reason was that a single cold night cannot do the damage people fear.
What can is a sustained cold stretch. That is now on the forecast.
What changes on Friday
Through Thursday the valley stays in the mid to upper 60s. Friday the high drops to the upper 40s, and from Friday night onward overnight lows sit between 29 and 31 degrees every single night for about a week.
That combination is the one that matters. It is not the low on its own. It is a low below freezing followed by a daytime high that only reaches the upper 40s, repeated night after night. The ground does not get a chance to recover between freezes, and the cold starts working downward instead of burning off by mid morning.
Why consecutive nights are different from one cold night
Water in a pipe does not usually split the pipe by expanding against the wall. An ice plug forms somewhere in the run while water is still liquid behind it. As the plug grows, the trapped water between it and the nearest closed valve or fitting has nowhere to go, and pressure builds until a component fails.
Forming a plug like that takes more than one night. It takes repeated freezing without a full thaw in between, which is precisely the pattern starting Friday. This is the week the theoretical risk becomes a real one.
The backflow preventer is still first
It sits above grade with nothing but air around it, usually brass, and it is the most expensive single component on a residential system. It has no soil protecting it and no thermal lag helping it. When people say a freeze destroyed their sprinklers, this is almost always the part they mean.
The other deadline nobody mentions
Crew availability. Every system in the valley gets blown out inside a window of roughly four weeks, and we are now in the last stretch of it. The calendar fills from the front, and the week before a forecast cold snap is the week everyone calls at once.
If you can only do one thing today
Turn your controller off. Not down, not to every third day. Off.
A controller still running will pressurize the system and push water through an exposed backflow preventer at five in the morning, which is the coldest hour of the coldest part of the day. You would be actively feeding water to the one component that cannot survive a hard freeze. Shutting it off takes thirty seconds and costs nothing.
Then shut the irrigation supply off at the isolation valve, which on most homes here is in the basement or crawl space where the line leaves the house. If you are comfortable, relieve the trapped pressure at the backflow assembly and insulate it. That is a stopgap to get you through the weekend, not a winterization.
Then get it on the schedule this week.
There is no fixed calendar date. The practical deadline is the first stretch of consecutive nights below freezing without a full daytime thaw, which in 2026 begins Friday, October 23, with overnight lows forecast between 29 and 31 degrees for roughly a week.
Not yet, but the window is closing. Work can be performed until the ground begins to freeze. The constraint that bites first is usually scheduling, since most systems in the valley are blown out within a four week window that is now nearly over.
Pipe damage comes from an ice plug forming while water remains liquid behind it, trapping pressure until a component fails. Forming that plug requires repeated freezing without a full thaw in between, which a single cold night followed by a warm day does not produce.
Turn the irrigation controller off entirely, shut the supply off at the isolation valve, and if you are comfortable doing it, relieve pressure at the backflow assembly and insulate it. This is a stopgap that protects the most vulnerable component, not a substitute for a blowout.