FrostList

When to Winterize Sprinklers in Boulder, CO

ON TRACK96 days until first 28°F freezeOct 15

Boulder's deadline to winterize your sprinkler system is October 5: the local first 28°F freeze runs October 15 on average and September 30 at its earliest (1991–2020 normals). Year to year the date swings about 30 days, which is why the live outlook beats the calendar.

OUTLOOK

Typical first first 28°F freeze near Oct 15; local deadline about Oct 5. The live 10-day outlook loads here.

Local freeze dates for Boulder

ThresholdEarly (1-in-10)MedianLate (9-in-10)
32°F (light freeze)Sep 21Oct 5Oct 18
28°F (hard freeze)Sep 30Oct 15Oct 30
24°F (severe)Oct 7Oct 25Nov 9

NOAA station: Boulder · 1.6 mi away · 5,484 ft elevation.

For Boulder, the nearest NOAA station with freeze data is Boulder, 1.6 miles out at 5,484 feet. Median first-freeze dates there run 32°F by Oct 5, 28°F by Oct 15, 24°F by Oct 25. That hard freeze has landed anywhere from Sep 30 to Oct 30, a swing of roughly 30 days. Spring's last 32°F freeze clears around May 6. Snowfall averages 93 inches a year, first reaching an inch near September.

The freeze arrives in two steps in Boulder: 32°F around Oct 5, then a hard 28°F near Oct 15. Year to year, the first 32°F night has fallen anywhere from Sep 21 to Oct 18 — about 27 days apart. Spring's final freeze lands near May 6 and as late as May 18, so that is when outdoor water and stored gear can safely come back online. With about 93 inches of snow a year, roof, snow-blower, and ice-dam prep all belong on the same calendar.

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

Boulder freezes about a week ahead of Broomfield (Oct 21) and close to Longmont (Oct 12) — a reminder that even nearby towns differ by days. Statewide, Colorado prep dates run Sep 30 through Oct 23, which is why Boulder gets its own number rather than a Colorado-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 Boulder

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

Frozen pipes → Sep 21Ice dams → Aug 16Car battery → Oct 15Snow blower → Aug 25Sump pump → May 6RV → Oct 5Boat → Oct 8Motorcycle → Oct 5

See the full Boulder 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 Boulder?
Wait until the last spring freeze has passed, which averages around May 6 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 Boulder, live outlook by Open-Meteo. Sources · Methodology. Last updated: July 11, 2026.