FrostList

When to Winterize Sprinklers in Longmont, CO

ON TRACK93 days until first 28°F freezeOct 12

Aim to winterize your sprinkler system in Longmont by October 2, roughly a week and a half before the median first 28°F freeze of October 12, which one fall in ten shows up by September 30. The early-to-late range spans roughly 25 days, so treat the median as a midpoint, not a promise.

OUTLOOK

Typical first first 28°F freeze near Oct 12; local deadline about Oct 2. The live 10-day outlook loads here.

Local freeze dates for Longmont

ThresholdEarly (1-in-10)MedianLate (9-in-10)
32°F (light freeze)Sep 23Oct 5Oct 16
28°F (hard freeze)Sep 30Oct 12Oct 25
24°F (severe)Oct 6Oct 21Nov 5

NOAA station: Longmont 2 Ese · 1.3 mi away · 4,950 ft elevation.

Longmont draws its numbers from Longmont 2 Ese, 4,950 feet up and 1.3 miles away. Its median first-freeze dates are 32°F by Oct 5, 28°F by Oct 12, 24°F by Oct 21. The 28°F freeze has come as early as Sep 30 and as late as Oct 25, a 25-day spread. Spring's last 32°F freeze clears around May 7.

Longmont usually sees its first 32°F night about Oct 5, with the first 28°F hard freeze close behind near Oct 12. That first freezing night has ranged from Sep 23 to Oct 16, roughly a 23-day spread. On the spring side, the last 32°F freeze clears around May 7 and as late as May 22 — the green light for reopening water and de-winterizing. With almost no snow in a normal year, cold — not plowing — sets the calendar, and it centers on May 7.

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

Against its neighbors, Longmont (first freeze Oct 12) runs close to Boulder (Oct 15) and close to Loveland (Oct 10). Across Colorado, local prep deadlines in our data range from Sep 30 to Oct 23, so a statewide rule of thumb would miss Longmont by weeks. In Longmont, that same cold is your cue to protect your indoor pipes and winterize an RV if you own one.

Other winter jobs in Longmont

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

Frozen pipes → Sep 23Car battery → Oct 12Sump pump → May 7RV → Oct 2Boat → Oct 5Motorcycle → Oct 5

See the full Longmont 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 Longmont?
Wait until the last spring freeze has passed, which averages around May 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 Longmont 2 Ese, live outlook by Open-Meteo. Sources · Methodology. Last updated: July 11, 2026.