When to Winterize Your Sprinkler System: Freeze Dates Where You Live

Winterize your sprinkler system before the first fall freeze, and no later than the first hard freeze of 28°F. In a normal year the first freeze comes about October 12 in Denver, October 16 in Minneapolis, October 31 in Detroit, November 5 in Chicago and November 9 in Boston, and in half of years it comes earlier.

Most guides give one month for a whole region. Below are freeze dates for 31 US cities from NOAA’s 1991–2020 climate normals, checked against local advice, plus the three ways to drain a system and how to protect the backflow preventer.

First freeze or hard freeze: which date to plan around

Hunter Industries says the blow-out comes “before the first freeze”⁹ and Rain Bird’s homeowner guide says water should be removed “before freezing occurs”.⁶ Salt Lake City Public Utilities says before the first frost,¹⁴ and Broomfield, Colorado, says before the first hard frost.¹³ The National Weather Service defines a hard freeze as 28°F or lower for an extended time,² the colder night that separates a hard freeze from an ordinary frost or freeze.

Across 37,570 ZIP codes in the lower 48 states that get both in most years, the median hard freeze comes a median 11 days after the median first freeze.¹ A median is a coin flip, though. The 1-in-10 early date, when there is already a 10% chance of a first freeze, falls a median 14 days before the median date.¹ So a practical plan has three dates:

• Book the job by the 1-in-10 early freeze date. Rain Bird notes that many irrigation specialists offer winterization services at this time of year.⁷
• Finish by the median first freeze, the deadline most of the sources use.
• Treat the median hard freeze as the last call. By then, half of years have already had one.

Exposed parts are most at risk. The University of Minnesota flags above-ground pipes around backflow preventers,⁴ and Hunter warns that ice in a backflow assembly can crack its brass body.⁹

When to winterize sprinklers in your city

Each city uses the weather station nearest a downtown ZIP code, sorted by median first freeze.¹

Denver, Boise and Colorado Springs reach their median first freeze in the first half of October, New York and Philadelphia not until mid- to late November, and Dallas and Houston later still. For any other place, look up your ZIP code’s first frost and hard-freeze dates.

Do the dates match local utility advice?

Four Colorado and Utah cities and utilities publish their own timing. We compared it with the computed dates:

Greeley’s mid-October advice lands between its median first freeze and median hard freeze.¹² Colorado Springs Utilities’ “in October” covers both medians,¹¹ though one year in ten the first freeze comes in late September. Broomfield’s October 30 date is a watering cutoff, 9 days after the area’s median hard freeze, not a winterizing date.¹³

Colorado Springs Utilities also says that the higher the elevation, the earlier you should winterize.¹¹ The data agrees. Across 645 Colorado ZIP codes, the median hard freeze comes about October 18 where the nearest station is below 6,000 feet, September 30 at 7,000 to 8,000 feet and September 23 above 9,000 feet.¹

Why waiting for freezing nights is too late

A common rule says to winterize once nighttime temperatures stay below 32°F. By then the first freeze has usually come and gone. In the 30,031 lower-48 ZIP codes where the normal nightly low does drop below 32°F, it does so a median 29 days after the median first freeze.¹ In 8,992 more, a freeze still comes in most years, but the normal low never reaches 32°F at all.

Do you need to winterize where you live?

Almost everyone gets a hard freeze. In 92.6% of lower-48 ZIP codes a 28°F night comes in at least half of years, including 94.8% of Texas ZIP codes.¹ Another 3.6% get a 32°F freeze most years but a hard freeze less often, and 3.8% rarely freeze at all, mostly in California and Florida, plus the Phoenix area.

A cold night on the thermometer is not the same as frozen ground at pipe depth, and that decides how far you go. Hunter describes the blow-out as the priority where the frost level extends below the depth of the pipes.⁹ The Texas A&M School of Irrigation says the method is most common in the northern US, where the ground freezes down to buried pipelines, and adds:⁵

Most landscape irrigation systems in Texas are not designed for a blow out.

For most of Texas, its steps for light to mild freezes are to turn off the controller and the irrigation water supply, insulate PVC pipe above ground, and drain above-ground parts such as the backflow assembly.⁵

Manual drain, automatic drain or blow-out

Rain Bird’s homeowner guide names three ways to get the water out:⁶

In freezing Colorado, CSU Extension’s fact sheet on preparing a home sprinkler system for winter doesn’t rely on drains at all:³

Do not trust manual or automatic drain valves.

Buried pipe shifts over time, the lines can’t be inspected afterward, and electric valves can only be emptied with air, it explains.³ If you don’t know how your system was built, Hunter says to use the blow-out method.⁹

Blow-outs: what the manufacturers’ safety notes say

Rain Bird and Hunter warn that compressed air can cause injury from flying debris. Their safety notes, with CSU’s and Toro’s:

• Wear eye protection and never stand over pipes, sprinklers or valves during a blow-out.^6,9
• Pressure limits differ by source. Rain Bird’s guide and Toro’s installation guide say not to exceed 50 psi.^6,10 Hunter and CSU allow up to 80 psi for rigid PVC but 50 psi for flexible polyethylene pipe.^9,3 A 2025 Rain Bird blog post gives both 50 psi and 75–85 psi.⁸ Hunter adds that pressure must stay below the rating of the lowest-rated part in the zone.⁹
• Rain Bird warns that air heats the pipes, so no zone should be blown for more than 2 minutes at a time.⁶
• Never blow air through the backflow preventer; Hunter and CSU say to blow out the lines downstream of it.^9,3

Hunter says it is best for a qualified licensed contractor to do it,⁹ Toro recommends hiring a professional for any compressed-air winterizing,¹⁰ and Rain Bird recommends a local irrigation specialist because of the safety risks.⁷ The University of Minnesota is the most direct:⁴

we strongly recommend you leave this step in the hands of a professional.

It suggests a WaterSense or Irrigation Association certified professional;⁴ the EPA keeps a directory of WaterSense certified professionals.¹⁶

Protecting the backflow preventer

The sources agree on the winter position for the backflow preventer’s valves:

• Leave the ball valves half open, at about 45° to the pipe. CSU says fully open or fully closed valves tend to trap water.^3,4,9
• Open the test cocks. The University of Minnesota says to turn their screws a quarter turn, to 45°.^4,9
• Insulate it if it is above ground, without blocking its air vents and drain outlets, Rain Bird says.⁷
• Don’t use its valves as the system’s main shutoff; Medford Water says they need to stay partly open so they can drain.¹⁵

CSU gives separate steps for pressure vacuum breakers and reduced pressure assemblies, and Rain Bird says to check with your device’s manufacturer.^3,6

The controller, and your lawn after shutdown

Rain Bird and Hunter describe leaving the controller powered but set to off or rain mode, which keeps the program and keeps the box warm enough to avoid condensation; CSU also says not to unplug it.^7,9,3 Systems with a pump need extra steps, so check the manual.⁶

Your lawn usually keeps growing after the sprinkler is drained. By the 50°F daytime-high rule in our guide to when lawns stop growing in fall, normal highs stay above 50°F for a median 22 more days after the hard freeze in 96.7% of the ZIP codes where both happen.¹ If you shut the system off early, Colorado Springs Utilities says to water with a hose rather than stop for the rest of the year.¹¹ It suggests fertilizing just before winterizing, or watering it in with a hose-end sprinkler;¹¹ see when to apply winterizer fertilizer and how much to water your lawn.

Next steps

Look up your dates, book the work for the 1-in-10 early freeze date, and watch the forecast. Pool plumbing needs its own freeze protection; see when to close your pool. For the house’s own pipes and outdoor faucets, see at what temperature pipes freeze.

Related guideHow Much to Water Your Lawn, and How Much Your Rain Already Covers

How we got the dates

All dates come from NOAA NCEI’s 1991–2020 U.S. Climate Normals for the station nearest each ZIP code.¹ Freeze dates are NOAA’s 10% and 50% probability dates for the first fall night at 32°F and 28°F in the air, not at pipe depth. We show a hard-freeze date only where a 28°F night comes in at least half of years. For normal lows and highs, we placed each monthly normal at mid-month and joined them day by day, as our mowing guide does. Real years vary.