FrostList

When to Winterize Sprinklers in Lake Oswego, OR

ON TRACK150 days until first 28°F freezeDec 8

Aim to winterize your sprinkler system in Lake Oswego by November 28, roughly a week and a half before the median first 28°F freeze of December 8, which one fall in ten shows up by November 16. Cold deepens quickly here — only about 20 days separate the first frost from that hard freeze.

OUTLOOK

Typical first first 28°F freeze near Dec 8; local deadline about Nov 28. The live 10-day outlook loads here.

Local freeze dates for Lake Oswego

ThresholdEarly (1-in-10)MedianLate (9-in-10)
32°F (light freeze)Oct 30Nov 18Dec 12
28°F (hard freeze)Nov 16Dec 8Jan 5
24°F (severe)Nov 29Dec 26Feb 4

NOAA station: Oregon City · 5.5 mi away · 167 ft elevation.

The reference station for Lake Oswego is Oregon City (5.5 mi, 167 ft). First freeze there: 32°F by Nov 18, 28°F by Dec 8, 24°F by Dec 26. Year to year the 28°F date has ranged from Nov 16 to Jan 5 — about 50 days apart. Spring's last 32°F freeze clears around Mar 18. Snowfall averages 4 inches a year, first reaching an inch near January.

The freeze arrives in two steps in Lake Oswego: 32°F around Nov 18, then a hard 28°F near Dec 8. That first freezing night has ranged from Oct 30 to Dec 12, roughly a 43-day spread. On the spring side, the last 32°F freeze clears around Mar 18 and as late as Apr 11 — the green light for reopening water and de-winterizing. Only about 4 inches of snow falls in a typical year, so cold protection outranks snow removal.

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, Lake Oswego's first-freeze date near Dec 8 sits later than Tigard (Nov 30) and about a week ahead of Portland (Dec 20). Oregon's deadlines span Sep 24 to Dec 11 statewide — one date for all of Oregon would be off by weeks for Lake Oswego. Once you know Lake Oswego's freeze date, use it to protect your indoor pipes and winterize an RV if you own one too.

Other winter jobs in Lake Oswego

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

Frozen pipes → Oct 30Ice dams → Dec 16Car battery → Dec 8Snow blower → Dec 25Sump pump → Mar 18RV → Nov 28Boat → Dec 1Motorcycle → Nov 18

See the full Lake Oswego 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 Lake Oswego?
Wait until the last spring freeze has passed, which averages around Mar 18 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 Oregon City, live outlook by Open-Meteo. Sources · Methodology. Last updated: July 11, 2026.