When to Winterize Sprinklers in the Pacific Northwest

There is one date on the irrigation calendar that costs real money if you miss it, and it is not the one most people watch. Homeowners wait for a frost warning to show up on the ten-day forecast, then start calling. By that point every tow-behind compressor in South King County is booked a week and a half out and the cold is already sitting on top of the region. Winterizing a sprinkler system around Puget Sound is a scheduling problem more than a weather problem, and the schedule starts earlier than our mild reputation suggests.

The window is mid-October to mid-November

Sea-Tac normally logs its first 32 degree night somewhere in the first half of November. A genuine hard freeze, meaning 28 degrees or colder held for several hours, tends to arrive later in November or into December. Those are averages, and averages are not what split brass. What splits brass is the November that decides to behave like January, which happens here often enough that we start booking blowouts the first week of October. The goal is to have plateau systems empty by roughly Halloween and shoreline systems empty by mid-November.

Elevation is only half the story. The Green River valley floor is a cold-air drainage basin, so on a clear calm night in late October a thermometer in a Kent backyard can read three or four degrees lower than one on the Covington plateau six hundred feet above it. Valley lots also carry a high winter water table that keeps valve boxes wet. Both facts push the valley blowout earlier than a topographic map alone would suggest.

What actually freezes, in the order it fails

1. The backflow assembly. A Febco 765 pressure vacuum breaker or a Watts 007 double check sits above grade, holds water, and has the least material between that water and the air. It breaks first and it is the most expensive single component on the system.
2. Exposed risers and the copper at the point of connection. Anything between the meter and the first buried fitting is on its own once the temperature drops.
3. Spray head canisters at the high point of a zone. A four-inch pop-up body holds a few ounces of water in the barrel. When that freezes the body splits along the seam, and you find out in April.
4. Valve manifolds sitting in flooded boxes. On the Kent and Auburn valley floor a box set without a gravel sump fills with groundwater by November. Freezing rarely cracks the valve body, but it corrodes solenoid leads and destroys any splice that was not made waterproof.
5. Buried mainline and laterals, last and least often. The exceptions are the runs that never got real cover: a lateral crossing under a paver driveway, a line threaded through a rockery, or a pipe backfilled at four inches because the trench hit hardpan and nobody wanted to argue with it.

A blowout is about volume, not pressure

This is the most misunderstood thing about winterization. Pressure does not push water out of a hundred feet of lateral. Air volume does, measured in cubic feet per minute. A tow-behind rotary screw compressor moving 185 CFM at 50 psi will lift standing water up through every head on a zone and leave the pipe misting. A small compressor at 120 psi makes an impressive amount of noise, clears the first twenty feet, and cooks a rotor gear drive while the far end of the line stays full.

The sequence we run

1. Close the isolation valve upstream of the backflow assembly and relieve the pressure downstream of it.
2. Connect at the blowout port on the downstream side of the assembly, never through the assembly. Air forced backward through a check assembly wrecks the poppet and the relief.
3. Open one zone at the controller before introducing any air, so the compressor is never pushing against a closed system.
4. Work zone by zone in short bursts, starting with the zone farthest from the compressor. Watch the heads. When the spray thins to a dry mist, that zone is done.
5. Make a second pass on every zone. Water settles back into low spots after the first pass, and the second pass is where most people would have stopped too early.
6. Finish with the drip zones. Pull the filter screen, drain the pressure regulator, and open the flush end of the dripline by hand.

The half hour after the compressor leaves

The blowout is most of the job and not all of it. Plenty of systems still fail after a competent blowout, because the last few steps got skipped while everyone was coiling hose in the driveway.

• Shut the isolation valve, then crack both test cocks on the backflow assembly to roughly 45 degrees. A half-open ball valve is exactly where water hides and where a body cracks.
• Fit an insulated cover over the assembly. A rigid foam enclosure or a wrapped insulation blanket both work. A plastic bag that traps condensation against the brass does not.
• Set the controller to OFF rather than pulling the plug. Off keeps the clock and the programs in memory; unplugged means reprogramming from scratch in April on plenty of older units.
• Leave the rain sensor connected and put a fresh backup battery in the controller so it wakes up on the right date.
• Disconnect every garden hose from the sillcocks. A hose left threaded onto a frost-free sillcock defeats it completely, because the valve seat can no longer drain past the hose.
• Tape a note inside the controller door with the date, the pressure used, and anything that looked wrong. That note is worth an hour of diagnosis at spring start-up.

What the blowout costs, and what skipping it costs

Those are typical service ranges for the Puget Sound market in 2026, not quotes. The arithmetic is still hard to argue with. A single blowout costs less than the cheapest freeze repair on the list, and a freeze almost never breaks only one thing. When an assembly splits above grade with the supply still open, water keeps running until somebody notices, and the bill stops being about irrigation.

If the cold got here first

1. Shut the isolation valve at the point of connection right away, before anything thaws and starts running.
2. Leave the system alone. Do not charge it to hunt for leaks while the ground is still frozen, because a split you pressurize becomes a flood.
3. Get the blowout done on the next dry day above freezing. Clearing the system after one cold snap still protects it from the next one, and there is usually more than one.
4. Book the repair for spring start-up rather than February. Diagnosing a split lateral costs far less when the system can be run and watched and the ground is workable.
5. Photograph any visible damage before anything gets repaired, in case there is an insurance conversation later.