FrostList

When to Winterize Sprinklers in Rohnert Park, CA

ON TRACK151 days until first 28°F freezeDec 9

Rohnert Park's deadline to winterize your sprinkler system is November 29: the local first 28°F freeze runs December 9 on average and November 20 at its earliest (1991–2020 normals). The early-to-late range spans roughly 53 days, so treat the median as a midpoint, not a promise.

OUTLOOK

Typical first first 28°F freeze near Dec 9; local deadline about Nov 29. The live 10-day outlook loads here.

Local freeze dates for Rohnert Park

ThresholdEarly (1-in-10)MedianLate (9-in-10)
32°F (light freeze)Nov 9Nov 25Dec 16
28°F (hard freeze)Nov 20Dec 9Jan 12
24°F (severe)Dec 1Dec 21Jan 12

NOAA station: Petaluma AP · 7.6 mi away · 20 ft elevation.

The reference station for Rohnert Park is Petaluma AP (7.6 mi, 20 ft). First freeze there: 32°F by Nov 25, 28°F by Dec 9, 24°F by Dec 21. That hard freeze has landed anywhere from Nov 20 to Jan 12, a swing of roughly 53 days. Spring's last 32°F freeze clears around Feb 28.

Expect the first frost near Nov 25 in Rohnert Park and the first hard freeze by about Dec 9. The 32°F date swings from Nov 9 at its earliest to Dec 16 at its latest, near 37 days. The last spring freeze averages Feb 28 and as late as Apr 6, which sets the safe window for reopening outdoor water and de-winterizing gear. Snow barely registers here; the freeze date, around Feb 28, is the one that matters.

Your sprinklers checklist

  1. Shut off the irrigation water supply at the main valve and, if you have one, the dedicated sprinkler shutoff inside the house.
  2. Turn off the controller or set it to the "rain" mode so valves do not open while the system is dry.
  3. Drain the mainline using the manual, automatic, or blow-out method your system was built for; most pros prefer a blow-out.
  4. Connect a compressor to the blow-out port through a proper adapter and run 40–80 psi, one zone at a time, until the heads mist and clear.
    Helpful gear: Air compressor blow-out adapter — Recommended pick
  5. Insulate the backflow preventer and any above-ground valves; this brass assembly is usually the first part to crack.
    Helpful gear: Insulated backflow preventer cover — Recommended pick
  6. Cap outdoor hose bibs with foam covers after the hoses come off so the last exposed fittings stay protected.
    Helpful gear: Foam outdoor faucet covers — Recommended pick
  7. Open the backflow test cocks a quarter turn so any trapped water has room to expand.
  8. Log the date and the psi you used; you will want the reference next fall.

What to have on hand

Air compressor blow-out adapter
Quick-connect fitting that mates a shop compressor to your irrigation line.
Recommended pick
Insulated backflow preventer cover
Pouch that shields the above-ground backflow valve from early frost.
Recommended pick
Foam outdoor faucet covers
Slip-on domes that protect hose bibs after you drain them.
Recommended pick
Portable air compressor
A modest pancake unit moves enough air to clear most home zones.
Recommended pick

What this means locally

Compared with nearby cities, Rohnert Park's first-freeze date near Dec 9 sits about a week ahead of Santa Rosa (Dec 16) and close to Petaluma (Dec 9). California's deadlines span Jan 6 to Dec 27 statewide — one date for all of California would be off by weeks for Rohnert Park. Once you know Rohnert Park's freeze date, use it to protect your indoor pipes and winterize an RV if you own one too.

Other winter jobs in Rohnert Park

Every task below is dated to Rohnert Park's own freeze and snow normals.

Frozen pipes → Nov 9Car battery → Dec 9Sump pump → Feb 28RV → Nov 29Boat → Dec 2Motorcycle → Nov 25

See the full Rohnert Park winter checklist, in order →

Frequently asked questions

What temperature freezes sprinkler pipes?
Standing water in shallow lines and the backflow assembly can start to freeze once air temperatures drop into the mid-20s for several hours. Buried mains are slower, but the exposed brass backflow preventer and above-ground valves are the parts that crack first, which is why the deadline is tied to the first 28°F freeze, not the first frost.
Do I need to blow out my sprinklers or just drain them?
It depends on how the system was installed. Manual or automatic drains work if the lines were pitched to low points. Most irrigation pros recommend a compressed-air blow-out because it clears water that drains miss, especially in flat yards. If you are unsure how your system drains, a blow-out is the safer choice.
What happens if I don't winterize my sprinkler system?
Water left in the lines expands as it freezes and can split the backflow preventer, crack valves, or break fittings. Repairs to a backflow assembly commonly run more than the cost of a blow-out. The damage often is not visible until you turn the system on in spring and find leaks.
How much does a sprinkler blowout cost?
A professional blow-out typically costs somewhere in the range of a modest service call, depending on the number of zones and your region. Buying an adapter and using your own compressor costs less over time, but you need enough air volume to clear each zone. Prices rise once the first freeze warnings appear, so book early.
When should I turn my sprinklers back on in Rohnert Park?
Wait until the last spring freeze has passed, which averages around Feb 28 here in the 1991–2020 normals. Turning the system on too early risks refreezing a charged line. Open the main slowly to avoid a pressure surge, then walk each zone to check for winter damage before you rely on the schedule.
Can I winterize sprinklers myself?
Draining and insulating are within reach for most homeowners. A blow-out is doable if you own a compressor that moves enough air and you use a proper adapter, working one zone at a time at safe pressure. If your system lacks a clear blow-out port or you are not confident, hiring a pro once is a reasonable call.

Data: NOAA 1991–2020 normals via Petaluma AP, live outlook by Open-Meteo. Sources · Methodology. Last updated: July 11, 2026.