FrostList

When to Winterize Sprinklers in Union City, CA

ON TRACK171 days until first 28°F freezeDec 29

Aim to winterize your sprinkler system in Union City by December 19, roughly a week and a half before the median first 28°F freeze of December 29, which one fall in ten shows up by December 11. Year to year the date swings about 37 days, which is why the live outlook beats the calendar.

OUTLOOK

Typical first first 28°F freeze near Dec 29; local deadline about Dec 19. The live 10-day outlook loads here.

Local freeze dates for Union City

ThresholdEarly (1-in-10)MedianLate (9-in-10)
32°F (light freeze)Dec 2Dec 22Jan 21
28°F (hard freeze)Dec 11Dec 29Jan 17

NOAA station: Fremont · 3.9 mi away · 38 ft elevation.

The reference station for Union City is Fremont (3.9 mi, 38 ft). First freeze there: 32°F by Dec 22, 28°F by Dec 29. The 28°F freeze has come as early as Dec 11 and as late as Jan 17, a 37-day spread. Spring's last 32°F freeze clears around Jan 7.

In Union City, freezing nights (32°F) typically begin around Dec 22 and the first hard freeze (28°F) follows near Dec 29. Year to year, the first 32°F night has fallen anywhere from Dec 2 to Jan 21 — about 50 days apart. Spring's final freeze lands near Jan 7 and as late as Feb 7, so that is when outdoor water and stored gear can safely come back online. Measurable snow is rare, so the freeze near Jan 7 — not snow load — drives the local prep list.

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

Union City freezes close to Fremont (Dec 29) and later than Dublin (Dec 18) — a reminder that even nearby towns differ by days. Statewide, California prep dates run Jan 6 through Dec 27, which is why Union City gets its own number rather than a California-wide average. The same freeze also decides when to protect your indoor pipes and winterize an RV if you own one.

Other winter jobs in Union City

Every task below is dated to Union City's own freeze and snow normals.

Frozen pipes → Dec 2Car battery → Dec 29Sump pump → Jan 7RV → Dec 19Boat → Dec 22Motorcycle → Dec 22

See the full Union City 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 Union City?
Wait until the last spring freeze has passed, which averages around Jan 7 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 Fremont, live outlook by Open-Meteo. Sources · Methodology. Last updated: July 11, 2026.