Pool closing · Colorado
When to Close Your Pool in Fort Collins, CO: Deadline, Window & Checklist
Last updated: July 15, 2026 · Model window year: 2026 · dates · checklist · FAQ
Circle September 30 on the Fort Collins 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 7. The window opens September 20 — the live widget below shows how this year is actually tracking against it.
Fort Collins closing dates at a glance
| Closing window | September 20 – September 30 |
|---|---|
| Close by (deadline) | September 30 |
| First freeze, 50% probability | October 7 |
| Open by (recommended) | May 12 |
| Opening window | May 5 – May 26 |
| 61°F crossing (7-day mean) | May 26 |
| Swim-season length (80°F+ days) | 92 days |
| NOAA normals station | Ft Collins · 2.2 mi · 5004 ft |
A classic four-season pool calendar: open early into cold water, close late into cold water, and Fort Collins's 92 days of prime swimming sit safely in between.
Elevation caveat: Fort Collins's station sits near 5004 ft, where clear-night cooling outpaces valley forecasts; the local normals above already reflect that.
Four water checkpoints anchor Fort Collins's year in the model: mid-April at about 48°F, mid-June at 66°F, mid-August near the 72°F peak zone, and mid-October easing through 53°F. The dates in the table are where those numbers cross the thresholds that matter.
The 12-step Fort Collins winterizing checklist
Sequenced against Fort Collins's September 20–September 30 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
Start midweek for a weekend close: bring alkalinity and pH into their label ranges and let the water settle. What you seal under the cover is what the pool soaks in until spring.
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Deep-clean the pool
Leave nothing organic behind: skim the surface, brush every wall and step, vacuum the floor slowly. What goes under the cover dirty comes out worse — winter only ever compounds what it's given.
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Service the filter one last time
Send the filter into winter clean: backwash the sand or DE, rinse and dry the cartridges indoors. Media stored dirty over winter hardens into a spring problem no backwash fixes.
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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
Air through every line — skimmer, returns, cleaner — until each blows dry mist, plugging returns while the air still pushes. Nothing else on this list protects as much plumbing per minute.
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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
If any line can't be verified dry, add pool-grade antifreeze per its label. Use only pool antifreeze — automotive products don't belong in pool plumbing.
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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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Remove and store ladders and rails
Pull ladders, rails, and the diving-board hardware; rinse, dry, and store them out of the weather. Anchor sockets get a dab of protectant so spring bolts turn freely.
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Shut down the heater carefully
Follow the manufacturer's winterizing sequence for your heater — drain it fully and, for gas units, close the supply valve. Heat exchangers are the most expensive freeze casualty on the pad.
What to buy before the rush
The September crowd empties shelves in a predictable order. This is the short list worth owning before Fort Collins's window opens — nothing exotic, just the stuff everyone needs the same weekend.
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Air pillow
A soft target for the ice sheet, centered under the cover.
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Winter cover
Measure with overlap; the winter workhorse over everything else.
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Cover pump
Standing water is a cover killer; this is the counter.
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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.
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Winter closing kit
The under-the-cover chemistry, measured for your gallons.
How Fort Collins compares locally
Two nearby data points to triangulate with: Loveland, 9 miles from Fort Collins, models its close at September 24 (about a week earlier); Greeley, 18 miles out, at October 1. Fort Collins's own window ends September 30. For the other half of the calendar see when to open in Fort Collins, or scan the full year on the season page.
Every number on this page traces to one instrument cluster: Ft Collins, 2.2 miles northwest of Fort Collins's center at an elevation near 5004 feet. NOAA computed its 1991–2020 normals from roughly three decades of daily readings — long enough that one strange spring in Larimer County barely moves the dates.
Field notes for Fort Collins owners
The skimmer is the most breakable part you own
Skimmer bodies crack because water freezes inside the throat with nowhere to push. A sacrificial bottle or spring-loaded guard absorbs that expansion for a few dollars. It's the highest-return item in the entire closing kit relative to what it protects.
The warm spell after you closed
A 78°F week in October doesn't mean reopening. Water under an opaque cover warms far less than air suggests, and a closed, balanced pool tolerates a warm stretch fine. Check the cover pump has somewhere to send rain, enjoy the weather, and leave the plumbing sealed.
Match the drainage plan to the cover
Solid covers shed nothing — they need a cover pump staged before the first storm and checked after each one. Mesh covers drain themselves but pass silt that settles all winter. Either way, the plan is decided in October, not improvised in January when the cover is an ice sheet.
Closing for a real winter
A Fort Collins closing has to hold for months of freeze-thaw, not a few frosty mornings. Spend the effort where winters bite: prove every line dry, drain every vessel on the pad, guard the skimmer, and tension the cover for wind that will actually come. The reward is a spring opening that's a rinse, not a rebuild.
Altitude closing notes
Elevation compresses Fort Collins's closing window: at about 5004 ft, radiational cooling can drop a clear night below freezing while afternoons still feel like pool weather. Trust the first-freeze normal (October 7) over the vibe, stage the blowout gear early, and treat any clear-sky cold front in September as your cue.
Fort Collins 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 Fort Collins, the 7-day mean drops through the threshold around September 20, so the window between then and September 30 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 Fort Collins, hold off until the cool-down near September 20 before covering.
Do I need antifreeze in pool lines?
It depends entirely on your confidence in the blowout. Lines that blew fully dry need nothing; anything uncertain — low runs, water features, a stubborn cleaner line — gets pool-grade antifreeze at the label's rate per foot. With Fort Collins's freeze clock starting near October 7, uncertainty is the thing to eliminate.
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?
The repair list writes itself in order of cost: heater heat exchanger, pump housing, filter tank, then every fitting the ice reached — discovered one leak at a time in spring. Around Fort Collins the exposure begins near October 7, and each skipped checklist step above is one more candidate for that list.
When is the last safe date to close in Fort Collins?
Treat September 30 as the deadline in Fort Collins. It's derived from the 1991–2020 normals: a week of margin before the October 7 first-freeze normal. Weather varies year to year, so watch the 10-day forecast in late September — an early cold snap moves the real deadline, and the widget above flags exactly that.
Email me when Fort Collins hits the closing window
Data: NOAA 1991–2020 climate normals via Ft Collins (2.2 mi); live outlook by Open-Meteo. Full attribution on the sources page. Model assumptions and error bars on methodology.