FrostList

When to Winterize Sprinklers in Sammamish, WA

ON TRACK141 days until first 28°F freezeNov 29

Aim to winterize your sprinkler system in Sammamish by November 19, roughly a week and a half before the median first 28°F freeze of November 29, which one fall in ten shows up by November 6. The early-odds date runs roughly 23 days ahead of the median, so build in that buffer.

OUTLOOK

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

Local freeze dates for Sammamish

ThresholdEarly (1-in-10)MedianLate (9-in-10)
32°F (light freeze)Oct 22Nov 7Dec 1
28°F (hard freeze)Nov 6Nov 29Dec 31
24°F (severe)Nov 21Dec 18Feb 6

NOAA station: Snoqualmie Falls · 10.6 mi away · 440 ft elevation.

For Sammamish, the nearest NOAA station with freeze data is Snoqualmie Falls, 10.6 miles out at 440 feet. Median first-freeze dates there run 32°F by Nov 7, 28°F by Nov 29, 24°F by Dec 18. The 28°F freeze has come as early as Nov 6 and as late as Dec 31, a 55-day spread. Spring's last 32°F freeze clears around Mar 30. Snowfall averages 4 inches a year, first reaching an inch near December.

The freeze arrives in two steps in Sammamish: 32°F around Nov 7, then a hard 28°F near Nov 29. The 32°F date swings from Oct 22 at its earliest to Dec 1 at its latest, near 40 days. The last spring freeze averages Mar 30 and as late as Apr 21, which sets the safe window for reopening outdoor water and de-winterizing gear. At about 4 inches of snow a year, the freeze — not snow load — is the thing to plan around.

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, Sammamish's first-freeze date near Nov 29 sits about a week ahead of Redmond (Dec 11) and about a week ahead of Bellevue (Dec 11). Washington's deadlines span Oct 6 to Dec 1 statewide — one date for all of Washington would be off by weeks for Sammamish. Once you know Sammamish's freeze date, use it to protect your indoor pipes and winterize an RV if you own one too.

Other winter jobs in Sammamish

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

Frozen pipes → Oct 22Ice dams → Nov 15Car battery → Nov 29Snow blower → Nov 24Sump pump → Mar 30RV → Nov 19Boat → Nov 22Motorcycle → Nov 7

See the full Sammamish 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 Sammamish?
Wait until the last spring freeze has passed, which averages around Mar 30 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 Snoqualmie Falls, live outlook by Open-Meteo. Sources · Methodology. Last updated: July 11, 2026.