National guide · Pool closing

When to Close Your Pool: After the Cool-Down, Before the Freeze

Last updated: July 15, 2026 · top-50 dates · why it works · FAQ

Close your pool after the water has cooled below 65°F — and before the first 32°F freeze threatens your plumbing. Between those two events sits every city's closing window. We compute it from NOAA 1991–2020 climate normals for 698 U.S. cities: the day the 7-day mean temperature falls back through 61°F opens the window, and the local first-freeze probability date caps it. Find your deadline below or in the calculator.

Closing deadlines for the 50 largest covered metros

Each date links to the full local winterizing guide — live water estimate, window, and the complete checklist. Year-round climates are marked; their guides cover winter care instead of a shutdown.

The two-clock problem every closing has to solve

Closing runs against two clocks ticking at different speeds. The first is biological: as long as water sits above roughly 65°F, algae keep growing — and they grow especially well in the warm, still, dark environment under a freshly installed cover. Close too early and you seal a functioning greenhouse; the classic result is lifting the cover in April onto water the color of a golf-course pond.

The second clock is physical: the first hard freeze. Water expands about nine percent as it freezes, and it does not negotiate with pump volutes, filter tanks, heater exchangers, or the fittings in underground lines. Everything in the winterizing checklist — blowouts, plugs, drained equipment, the skimmer guard — exists to make sure that when the expansion happens, it happens where you planned for it.

The window between the clocks is the answer. Our model opens it on the day the 7-day mean temperature falls back through 61°F (water goes algae-dormant shortly after) and closes it ten days later — or a week before the local 50% first-freeze probability date from the NOAA normals, whichever comes first. In most of the country that leaves a comfortable two-to-three-week working window; in mountain climates the freeze cap arrives first and the local guides say so explicitly.

Closing too early is the expensive mistake

It feels tidy to winterize on the first cold weekend of September. It's also the single most common cause of green spring openings: water sealed at 70°F keeps growing algae for weeks under the cover, with the sanitizer fading and nobody watching. The fix costs nothing — patience. Keep the pump on its fall schedule, keep the water balanced, and let the cool-down do its work. Every local guide shows the exact date that patience should end.

Late closing, by contrast, is mostly a comfort problem: you're blowing out lines in gloves. The genuine risk appears only when procrastination meets an early freeze — which is why the model caps every deadline a full week before the local 50% first-freeze date, and why the live widget on each city page flips to URGENT when a 32°F night shows up inside the 10-day forecast.

What closing actually involves

The local checklists run twelve steps, but the spine is: balance a few days ahead, deep-clean, service the filter, dose winter chemicals per their labels, lower the water only as far as the cover manufacturer specifies, blow out and plug every line, protect the skimmer, drain all equipment, set the air pillow, secure the cover, and stage the cover pump. The order matters — chemistry while the pump still runs, blowouts before the drain-down of equipment — and each city guide sequences it with local dates attached.

Two rules carry the whole project. Air is the only guaranteed antifreeze: a verifiably dry line cannot burst, and pool-grade antifreeze (dosed per its label) is the fallback for lines you can't verify — never the substitute for the blowout. And never fully drain the pool: an empty shell can shift or crack under groundwater pressure, an outcome worse than any freeze it was meant to prevent.

Pool closing FAQ

What temperature should the water be when I close?

Below about 65°F and trending down — low 60s or cooler is ideal. At that temperature algae are near-dormant, so the water you seal in October is essentially the water you meet in April. Every city guide shows the local date that cool-down typically arrives.

Is it bad to close a pool too early?

Yes — it's the classic mistake. Warm covered water grows algae in the dark all autumn, and spring opens green and expensive. If the weather still feels like pool weather, it's too early to close; wait for your city's window.

When is it too late to close?

Practically, a week before your area's first-freeze normal — that's the cap our model applies to every deadline. Past it you're gambling equipment against the forecast. If a freeze warning arrives before your planned date, protect the plumbing first (blowout, drains) and finish cosmetics later.

Does every pool need winterizing?

No. In year-round climates — South Florida, coastal Southern California, the mildest Sun Belt cities — many owners never close: circulation, sanitation, and a freeze-guard habit replace the whole ritual. Our model marks those cities explicitly and their guides cover winter care instead.

Should I blow out the lines or just add antifreeze?

Blow out first, always; antifreeze is the backup for lines you can't verify dry — long runs, low spots, water features. Use only antifreeze labeled for pools, at the label's rate, and never automotive product anywhere in pool plumbing.

Cover pump: really necessary?

For solid covers, yes — rain and snowmelt have nowhere else to go, and a few hundred gallons of standing water strains seams, stretches straps, and eventually ends up in the pool carrying everything it collected. Mesh covers drain themselves and skip this chore.

Mesh or solid winter cover?

Mesh drains itself and shrugs off snow load but passes fine silt and nutrient-rich meltwater, so springs open slightly greener. Solid seals everything out but demands drainage management all winter. Both work; pick the failure mode you'd rather manage and close late-and-cold either way.

What if I skip winterizing entirely?

In freeze country: cracked pump housings, split filter tanks, ruptured heater exchangers, and underground leaks that surface at startup — the freeze finds every unprotected water pocket. In mild climates the penalty is biological instead: an untended pool drifts green. Neither is cheaper than an afternoon of checklist.

Every state we cover

• Alabama
• Arizona
• Arkansas
• California
• Colorado
• Connecticut
• Delaware
• District of Columbia
• Florida
• Georgia
• Hawaii
• Idaho
• Illinois
• Indiana
• Iowa
• Kansas
• Kentucky
• Louisiana
• Maine
• Maryland
• Massachusetts
• Michigan
• Minnesota
• Mississippi
• Missouri
• Montana
• Nebraska
• Nevada
• New Hampshire
• New Jersey
• New Mexico
• New York
• North Carolina
• North Dakota
• Ohio
• Oklahoma
• Oregon
• Pennsylvania
• Rhode Island
• South Carolina
• South Dakota
• Tennessee
• Texas
• Utah
• Vermont
• Virginia
• Washington
• West Virginia
• Wisconsin
• Wyoming

Spring planning lives on the national opening guide. Formulas, thresholds, and honest error bars: methodology. Data credits: sources.