Pool closing · Colorado
When to Close Your Pool in Longmont, CO: Deadline, Window & Checklist
Last updated: July 15, 2026 · Model window year: 2026 · dates · checklist · FAQ
Circle September 28 on the Longmont calendar. Closing earlier traps warm, algae-friendly water under the cover; closing later gambles the plumbing against the first freeze, which the 1991–2020 normals place near October 5. The window opens September 21 — the live widget below shows how this year is actually tracking against it.
Longmont closing dates at a glance
| Closing window | September 21 – September 28 |
|---|---|
| Close by (deadline) | September 28 |
| First freeze, 50% probability | October 5 |
| Open by (recommended) | May 13 |
| Opening window | May 6 – May 27 |
| 61°F crossing (7-day mean) | May 27 |
| Swim-season length (80°F+ days) | 105 days |
| NOAA normals station | Longmont 2 ESE · 1.2 mi · 4950 ft |
Longmont's 105-day warm season leaves comfortable margins on both ends — the windows above aim you at the cheap-chemistry versions of each transition.
At roughly 4950 ft, Longmont 2 ESE runs cooler than lowland forecasts for the same region — trust these local numbers over metro-wide weather graphics.
Put dates aside and follow the water: the Longmont curve says roughly 46°F by mid-April, 66°F by mid-June, 72°F in mid-August, then back down through 52°F in mid-October. The summer ceiling sits near 73°F, and every window above is just a line drawn on this curve.
The 12-step Longmont winterizing checklist
Sequenced against Longmont's September 21–September 28 window: chemistry while the pump still runs, blowouts before the equipment drains, cover last. Labels dictate every dose.
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Balance the water a few days ahead
Three or four days before closing, adjust alkalinity and pH into label ranges. Balanced water is gentler on the liner, plaster, and equipment through the long covered months ahead.
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Deep-clean the pool
Make the last cleaning the best one of the year: full skim, full brush, careful vacuum. Debris left behind steeps all winter and greets you as April's water problem.
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Service the filter one last time
Backwash sand or DE, or pull and rinse cartridges, per the manual. A filter stored dirty cakes over winter and starts spring half-clogged.
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Apply winter chemicals per label
Winter chemicals go in before shutdown, not after: label-dosed, circulated for a few hours, distributed evenly. A floater dropped on still water protects one corner.
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Lower the water level
Drop the level as your cover manufacturer specifies — typically below the skimmer mouth for solid covers. Never drain a pool fully; groundwater pressure can damage the shell.
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Blow out the lines and plug returns
Push air through skimmer, return, and cleaner lines until they run dry mist, then seat expansion plugs at each return while air still flows. This is the single most important freeze-protection step.
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Protect the skimmer
The skimmer throat is where trapped water has no escape — park a guard bottle or rated plug in it and let ice crush the cheap part.
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Add pool antifreeze where blowouts fall short
Antifreeze is the insurance policy for doubtful lines, not a replacement for the blowout: pool-grade product, label dosing, and only where air couldn't finish the job.
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Drain the equipment
Nothing on the pad should hold water overnight: pull the drain plugs from pump, filter, heater, and feeder, stash them all in the pump basket, and carry the portable pieces indoors.
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Set the air pillow and cover
Float a centered air pillow, then land the cover and secure it the way its design intends — bags, cable, or straps. Ice sheets need somewhere to collapse inward, and the pillow is that somewhere.
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Stage the cover pump
Solid covers need drainage all winter: set a cover pump or siphon before the first storm, not after. Standing water strains seams and invites a mid-winter emergency.
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Calendar the off-season checks
Set a monthly reminder from September 28 to spring: check cover tension, pump off standing water, and glance at the water level. Fifteen minutes a month protects the whole closing job.
What to buy before the rush
Every item below sells out somewhere in Colorado every September. Stocking the short list before the rush costs nothing extra and saves the mid-project store run — the chemicals guide explains what each category actually does.
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Winter closing kit
The under-the-cover chemistry, measured for your gallons.
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Air pillow
Centers under the cover so ice pushes inward, not outward.
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Winter cover
Sized to overlap; the cheapest insurance the pool wears all winter.
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Cover pump
Solid covers collect rain all winter; this is the drainage plan.
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Expansion plugs + skimmer guard
The blowout's finishing move — one per return, one for the skimmer.
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Pool antifreeze
For lines you can't verify dry — pool-grade only, per label.
How Longmont compares locally
Two nearby data points to triangulate with: Lafayette, 12 miles from Longmont, models its close at October 4 (about a week later); Boulder, 13 miles out, at September 28. Longmont's own window ends September 28. For the other half of the calendar see when to open in Longmont, or scan the full year on the season page.
The measuring stick here is Longmont 2 ESE — 1.2 miles to the east, elevation about 4950 feet. Its 1991–2020 record is what the model reads for Longmont; your backyard in Boulder County will run a touch warmer or cooler with shade, wind, and pavement, which is exactly the slack the two-week lead absorbs.
Field notes for Longmont owners
The mesh-cover spring surprise, prevented in fall
Mesh-covered pools green up early because late-winter sun plus nutrient-carrying meltwater reaches the water. The fall counter-moves: close late and cold, dose the winter kit exactly per label, and plan an early-spring peek under the cover rather than a Memorial Day reveal.
The fifteen-minute monthly walk-around
Once a month all winter: pump or siphon standing water off solid covers, re-tension straps or top up water bags, confirm the level hasn't dropped enough to strand the cover, and glance at the pad for critter nests. Every major cover failure starts as a skipped walk-around.
What comes indoors
Cartridges, the cover pump when idle, chemical containers, and anything with a small motor overwinter better in the garage. Cold cycling is hard on plastics and seals; shelf space is cheaper than replacements. Label a single bin now and spring assembly becomes a scavenger hunt with a map.
Altitude closing notes
Elevation compresses Longmont's closing window: at about 4950 ft, radiational cooling can drop a clear night below freezing while afternoons still feel like pool weather. Trust the first-freeze normal (October 5) over the vibe, stage the blowout gear early, and treat any clear-sky cold front in September as your cue.
Longmont pool closing FAQ
What temperature should water be to close a pool?
Close once water holds below about 65°F — the point where algae go mostly dormant — and before hard freezes. In Longmont, the 7-day mean drops through the threshold around September 21, so the window between then and September 28 is the sweet spot for a clean, stable close.
Can you close a pool too early?
Yes — it's the most common closing mistake. Seal 70°F water under a cover and algae keep growing in the dark all autumn; the spring opening turns green and expensive. In Longmont, hold off until the cool-down near September 21 before covering.
Do I need antifreeze in pool lines?
Treat antifreeze as a backup, not a substitute: the real protection is air in dry lines. Where a full blowout isn't possible, pool-grade antifreeze per label is cheap insurance against a cracked pipe — worth it anywhere freezes are routine, and Longmont sees them from about October 5.
How far should I drain my pool for winter?
Less than most people think. Below the skimmer opening is the classic solid-cover target; mesh covers often specify higher water. The cover's own manual wins every argument, and "drain it completely" is never the answer — empty shells pop out of wet ground.
What happens if you don't winterize a pool?
Expect equipment damage first — cracked pump volutes, split filter tanks, ruptured heater exchangers — then plumbing leaks that surface at startup. In Longmont, normals put the first freeze near October 5; everything after that date is borrowed time for an unwinterized system.
When is the last safe date to close in Longmont?
Our model's practical deadline is September 28 — set by a week of margin before the October 5 first-freeze normal. Push much past it and you're winterizing in freeze-warning weather, rushing the blowout, and hoping the cover goes on before the first hard night. Inside the September 21–September 28 window, none of that drama applies.
Email me when Longmont hits the closing window
Data: NOAA 1991–2020 climate normals via Longmont 2 ESE (1.2 mi); live outlook by Open-Meteo. Full attribution on the sources page. Model assumptions and error bars on methodology.