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Dolphin Pool & Spa

When Should You Close Your Pool in Minnesota? (It's Not "When It Gets Cold")

September 14, 2026  |  Your Happy Dolphin Pool & Spa Team

Backyard pool in early fall in Minnesota

Images are AI generated and do not depict an actual pool or Dolphin Pool & Spa location.

Every fall, we get the same question: is it time to close yet? Most people check the calendar, or feel one cold morning, and guess. But the calendar doesn't run your pool. Neither does the air. Your water temperature does. Here's how to pick the right day to close, not just a day that feels right.

1 Check the Water, Not the Air

The number that matters is water temperature, not air temperature. Once your water sits at a steady 65°F or below, algae and bacteria growth slow down a lot. A 50°F morning can fool you. Your water might still sit at 72°F after a warm September stretch. Use a pool thermometer. Don't trust your gut, or your jacket.

2 Closing Too Early Costs You

Close in early September while your water still sits warm, and you give up swim days for nothing. Worse, water above 65°F can still grow algae under a cover. Close too soon, and you may open a green pool in spring instead of a clear one.

3 Closing Too Late Costs You More

Wait past the first hard freeze, and water left in pipes, pumps, and heaters can freeze and crack. A split pipe or cracked pump housing costs far more than a week of lost pool time. In Minnesota, that first hard freeze can land anywhere from late September to late October. Watch the forecast this year, not last year's date on the calendar.

4 Signs It's Time to Close

Sign Why It Matters
Water temp steady at 65°F or below Algae growth slows sharply below this line
Overnight lows near 32°F in the forecast Freeze risk to pipes, pump, and heater
Leaves dropping fast, more debris than swims Sign the season is winding down
Family swim time has dropped off Little reason to keep running the pump and heater

5 Balance Your Water Before You Close

Before you close, run a final water test. Water testing runs $7, or free with a $50+ chemical purchase. Balanced water going into winter means less scale, less staining, and a clearer pool when you open in spring.

BioGuard Tip: The Balance Pak line fixes pH, alkalinity, and calcium fast before you close. BioGuard only needs 15 minutes between doses, where many generic chemicals need up to 4 hours. Finish your final balance and get the cover on the same afternoon.

Heads Up: Minnesota can throw a warm week at you in late September. A warm stretch does not mean summer is back. Once your water drops below 65°F and stays there, close on schedule, even if the sun comes back out.

Not sure when to close, or want a hand doing it right the first time? Our team handles closings across the Twin Cities every fall, so your pool comes back clean and ready next spring — we make pool ownership a joy!

Call Us Now — 763-542-9000

Your Happy Dolphin Pool & Spa Team