Don’t Wait for the First Freeze to Winterize Your Irrigation System
Why October 7 Should Drive Your Winterization Schedule, Not the Forecast
This article is intended for facilities directors, HOA boards, property managers, and grounds teams responsible for larger irrigation systems in Spokane and Eastern Washington.
Spokane’s average first fall freeze is around October 7.[^1] But that doesn’t mean October 7 is a safe deadline. Freezing temperatures can show up earlier, particularly in outlying or higher-elevation areas of the Inland Northwest.[^2]
For a small residential system, getting caught by an early freeze may mean dealing with a relatively limited repair. On a commercial property with 50, 80, or 100+ zones, it can become a much bigger problem. There are more valves, more pipe, more elevation changes, more exposed components, and a lot more places for water to remain trapped.
That’s why we recommend scheduling winterization with enough time before the typical first freeze rather than waiting for freezing temperatures to appear in the forecast.
Key Takeaways
- Spokane’s average first freeze is around October 7, but freezing temperatures can arrive earlier depending on location and elevation.[^1][^2]
- More pressure isn’t better. Air pressure should generally stay below 80 PSI for rigid PVC and 50 PSI for polyethylene pipe.[^3][^4]
- CFM is just as important as PSI. You need enough air volume to move the water through the system without compensating by increasing pressure.
- Backflow preventers need special attention. They’re commonly installed above grade and can retain water even after the irrigation mainline has been drained.[^3]
- Large systems need to be winterized methodically. A 50+ zone property shouldn’t be treated like a residential sprinkler system with more zones.
When Should You Winterize in Spokane?
October 7 is a useful planning date, but it shouldn’t be treated as a deadline.
National Weather Service data reported locally puts Spokane’s average first 32°F temperature around October 7.[^1] Across the Inland Northwest, however, colder locations can see freezing temperatures much earlier.[^2]
The important word here is average.
If you manage a large commercial property, waiting until the forecast shows the first hard freeze doesn’t leave much room for scheduling problems, compressor availability, unexpected repairs, or simply getting through a large number of zones.
A better approach is to schedule winterization with a reasonable buffer before the average freeze date.
Freeze vs. frost: A freeze generally refers to an air temperature of 32°F or below. Frost can develop when temperatures are somewhat warmer and depends on conditions such as humidity and radiational cooling near the ground.[^5]
What Can Freeze Damage?
Even after an irrigation system has been shut down or gravity-drained, water can remain in low sections of pipe, valve bodies, laterals, fittings, and backflow assemblies.
When that water freezes, it expands.
Depending on where the water is trapped, the result can be cracked PVC, damaged fittings, split components, valve damage, or problems inside the backflow preventer.[^3]
Backflow assemblies are especially vulnerable because they’re often above ground and exposed directly to cold air. They can also hold water in check valves, test cocks, and other internal areas after the rest of the system appears to be drained.
That’s one reason simply shutting the water off and opening a drain isn’t always enough on a large irrigation system.
How Much PSI and CFM Do You Need?
This is one of the areas where winterization can go wrong.
The goal isn’t to put as much air pressure into the system as possible. The goal is to move enough air through the piping to push the water out without exceeding what the system can safely handle.
Hunter Industries recommends staying below 50 PSI for polyethylene pipe and 80 PSI for PVC, which is also consistent with Colorado State University Extension guidance.[^3][^4]
CFM (cubic feet per minute) is equally important.
PSI tells you the pressure of the air. CFM tells you how much air is actually moving through the system.
A commonly used field calculation is:
Largest zone GPM ÷ 7.5 = minimum required CFM
For example, if the largest irrigation zone operates at 60 GPM:
60 ÷ 7.5 = 8 CFM
The compressor needs enough air volume to move that water out of the system.
Trying to make up for an undersized compressor by increasing the PSI is where problems start. You may still leave water in the piping while increasing the risk of damaged fittings, pipe, valves, or sprinkler heads.
| Pipe Material | Maximum Air Pressure | Risk if Pressure Is Too High |
|---|---|---|
| Rigid PVC | Approximately 80 PSI | Pipe or fitting damage |
| Polyethylene | Approximately 50 PSI | Ruptured pipe or couplings |
| Any material with insufficient CFM | — | Water may remain trapped in the system |
These limits are based on winterization guidance from Hunter Industries and Colorado State University Extension.[^3][^4]
Large Irrigation Systems Need a Different Approach
Winterizing a 50-zone commercial property isn’t the same as winterizing a five-zone residential yard.
Every zone can behave differently.
One zone might have a long pipe run and several feet of elevation change. Another may be short and flat. Some may use rotors, others sprays or drip. Pipe sizes and materials can also change across properties that have been expanded or renovated over the years.
That means there isn’t necessarily one compressor setting or one cycle time that works perfectly for every zone.
Zones should be cleared in a planned sequence, using short cycles rather than simply leaving compressed air running until everything looks dry. Sprinkler heads are normally cooled and lubricated by water, so running them dry for too long can cause damage of its own.
On larger properties, a second pass can also help clear water that remained in low spots or sections that didn’t fully clear during the first cycle.
**Planning your fall shutdown? A Water Use Analysis gives you a baseline that can also be used when the system is started back up in the spring. [Request one at irrmgmt.com](https://irrmgmt.com).\*\*
A Practical Winterization Process
- Shut off the irrigation water supply and relieve pressure before connecting the compressor.
- Determine the required compressor capacity. Use the largest zone’s GPM to estimate the CFM needed rather than selecting a compressor based on PSI alone.
- Set the pressure regulator for the pipe material. Keep air pressure below approximately 80 PSI for PVC and 50 PSI for polyethylene.[^3][^4]
- Work through the zones in a planned sequence. Start with the zones farthest from the compressor connection and use short cycles rather than one long continuous run.
- Stop once the heads are producing mostly air or mist. Continuing to run sprinkler heads dry doesn’t improve the winterization and can damage internal components.
- Make another pass where needed. Large properties, elevation changes, and low points can leave water behind after the first cycle.
- Handle the backflow preventer separately. Drain and protect the assembly rather than assuming clearing the irrigation mainline also cleared the backflow.[^3]
- Leave appropriate drains and test cocks positioned for winter so residual moisture has room to expand.
- Put the controller into its seasonal shutdown state and document the shutdown date, compressor settings, and any zones that showed problems.
That last step is easy to overlook, but it can save time in the spring. If a valve didn’t clear properly or a zone behaved differently than expected, documenting it now gives the startup crew something specific to check.
What to Do Over the Next 30 Days
Before winterization season gets busy:
☐ Confirm the pipe material used throughout the property, especially if the system has been expanded in phases.
☐ Identify the GPM of the largest irrigation zone and calculate the compressor CFM requirement.
☐ Make sure the compressor you plan to use can provide the required air volume without exceeding safe pressure.
☐ Walk the property and identify exposed backflow assemblies, valves, hose bibs, and other components that may need additional freeze protection.
☐ Schedule the blow-out with enough time before Spokane’s average October 7 freeze date.
☐ Document any known problem zones so they receive extra attention during winterization.
☐ If next year’s irrigation work is part of the current HOA or commercial budgeting cycle, include winterization and spring startup in that discussion now.
How Irrigation Managers Handles Winterization
Winterization falls within the Manage portion of our AIM framework—Analyze, Implement, Manage.
For larger properties, we approach shutdown as part of the overall management of the irrigation system rather than as an isolated seasonal service.
That means working from the system layout, accounting for pipe material and zone requirements, sequencing zones appropriately, and making sure components such as backflow assemblies are addressed separately.
We also document what happens during shutdown. If a zone doesn’t clear normally, a valve behaves unexpectedly, or something needs to be checked during spring startup, that information shouldn’t disappear when the compressor leaves the property.
Full Irrigation Management, including monitoring and seasonal reporting, starts as low as $5 per zone per month. Team Support & Guidance starts as low as $1 per zone per month for properties that primarily need technical support and oversight.
We can also work behind the scenes with landscape maintenance contractors who want technical irrigation support while maintaining their existing relationship with the property.
Request a Water Use Analysis →
FAQ
Our landscape contractor already blows out the system every fall. Why would we need anything else?
If your landscape contractor is already handling the blow-out correctly, there’s no reason to change that relationship. The additional value comes from documenting how the system was shut down, confirming that pressure and compressor capacity were appropriate for the site, and identifying problem zones before they become spring repairs.
If a valve stays wet, a zone takes unusually long to clear, or a head doesn’t operate normally during the blow-out, that should be documented and checked during startup rather than rediscovered as a leak in April.
Does a large irrigation system need more than one pass?
Often, yes.
Large systems have different pipe lengths, elevations, head types, and flow requirements from one zone to another. A single cycle time and compressor setting may not clear every section equally well.
A second pass gives remaining water in low areas or difficult zones another opportunity to clear without extending the first cycle long enough to risk running sprinkler components dry.
Can drip zones be skipped?
No.
Drip tubing and emitters can still hold water, particularly in low spots. They may require a different approach than large rotor zones, but they should still be included in the winterization plan.
What if temperatures stay warm past October 7?
October 7 is an average, not a prediction for a particular year.
The reason to winterize ahead of that date is to leave some margin for weather changes and scheduling. Waiting until a freeze appears in the short-term forecast can leave very little time to get a large commercial system properly shut down.
For a 50+ zone property, it makes more sense to have the system winterized a little early than to be trying to find a compressor and an available crew the day before temperatures drop below freezing.
References
[^1]: KREM. Spokane weather: First frost of the season on Thursday. Reporting NWS Spokane climate data.
[^2]: KXLY. First freeze of the fall will be late again this year - Spokane. October 6, 2023.
[^3]: Hunter Industries. Winterizing Your Irrigation System.
[^4]: Colorado State University Extension. Home Sprinkler Systems: Preparing Your Sprinkler System for Winter.
[^5]: National Weather Service. “Frost” and Freeze Dates.