Pool closing · Minnesota
When to Close Your Pool in Plymouth, MN: Deadline, Window & Checklist
Last updated: July 15, 2026 · Model window year: 2026 · dates · checklist · FAQ
Circle September 28 on the Plymouth 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 12. The window opens September 18 — the live widget below shows how this year is actually tracking against it.
Plymouth closing dates at a glance
| Closing window | September 18 – September 28 |
|---|---|
| Close by (deadline) | September 28 |
| First freeze, 50% probability | October 12 |
| Open by (recommended) | May 10 |
| Opening window | May 3 – May 24 |
| 61°F crossing (7-day mean) | May 24 |
| Swim-season length (80°F+ days) | 55 days |
| NOAA normals station | New Hope · 4.1 mi · 910 ft |
A short season cuts both ways: every week opened before May 24 is a real slice of Plymouth's roughly 55-day warm-swim budget.
Four water checkpoints anchor Plymouth's year in the model: mid-April at about 43°F, mid-June at 66°F, mid-August near the 71°F peak zone, and mid-October easing through 52°F. The dates in the table are where those numbers cross the thresholds that matter.
The 12-step Plymouth winterizing checklist
A closing is a plumbing project with a chemistry warm-up. Start a few days ahead of your target date, keep every dose per its product label, and don't skip the photographs — spring-you reassembles from them.
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Balance the water a few days ahead
Give the chemistry a head start — balance to label ranges several days out, while circulation can still mix corrections evenly. Closing-day dosing never distributes as well.
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Deep-clean the pool
Skim, brush walls and steps, and vacuum carefully. Any leaves or algae you seal under the cover become spring's chemistry problem, so closing day cleanliness pays twice.
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Service the filter one last time
Clean media goes into storage, dirty media comes out worse: backwash the sand or DE, rinse the cartridges, all per the manual, before anything drains.
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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
Your cover's manual sets the number — commonly a few inches under the skimmer for solid covers, barely below normal for mesh. Stop there. The remaining water isn't laziness; it's ballast holding the shell in the ground.
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Blow out the lines and plug returns
Work line by line: push air until the return spits dry mist, plug it against the flowing air, move on. Skimmer, returns, cleaner line, in whatever order your plumbing prefers — dry pipes are the entire point of closing.
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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
Any line you can't prove is dry gets pool-grade antifreeze at the label's rate per foot. Automotive antifreeze is toxic in this context — pool-rated only, always.
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Drain the equipment
Open every drain plug on the pump, filter, heater, and chlorinator, and store the plugs in the pump basket so spring reassembly is a one-stop hunt. Move what you can 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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Winterize the water features
Waterfalls, slides, and spillover spas hold water in places gravity won't clear — blow those lines separately and plug them, or they'll be the one crack you find in spring.
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Note this year's dates
Jot down when Plymouth's water actually cooled and when you closed. A two-line note beats memory next September 18 — your own yard runs a few days off any model, including ours.
What to buy before the rush
A small stack of supplies turns the checklist above into a single-weekend job. Buy before Plymouth's September rush and the whole list ships at leisure instead of sprinting.
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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.
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Air pillow
A soft target for the ice sheet, centered under the cover.
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Winter cover
The one purchase every other closing step depends on.
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Cover pump
Solid covers collect rain all winter; this is the drainage plan.
How Plymouth compares locally
Plymouth closes in the earlier half of Minnesota's calendar. Neighbors run close: Brooklyn Park (8 mi away) models its deadline at September 29 (1 day later vs Plymouth's September 28), while Minneapolis (10 mi) shows September 30. The spring mirror of this page is the Plymouth opening guide, and the season overview draws both windows on a single bar.
The instrument behind this page is New Hope, 4.1 miles east of Plymouth — the closest station publishing complete 1991–2020 daily temperature normals. Thirty years of its readings set every date above; your own yard adds or subtracts a degree with shade, wind, and pavement, which is what the window's width is for.
Field notes for Plymouth owners
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.
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.
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.
Hard-winter homework
Where winter is long — Plymouth banks only about 55 warm-swim days — the closing carries months of load. Bury the effort where it counts: verified-dry lines, fully drained equipment, a skimmer guard, and a cover secured for real wind. A short season forgives a late opening; it never forgives a cracked pump.
Plymouth 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 Plymouth, the 7-day mean drops through the threshold around September 18, so the window between then and September 28 is the sweet spot for a clean, stable close.
Can you close a pool too early?
Early closing is the mistake the whole model is built to prevent from the other direction. A cover installed over 70°F water is a terrarium: sanitizer decays, algae compound, nobody looks for months. Plymouth's water isn't reliably out of that zone until about September 18 — the calendar's first cold weekend doesn't change that.
Do I need antifreeze in pool lines?
Only where water might remain. If every line is properly blown out and plugged, air is the antifreeze. Lines you can't verify dry — long runs, low spots, water features — get pool-grade antifreeze dosed per its label. With Plymouth's first freeze normal near October 12, don't leave that question open.
How far should I drain my pool for winter?
Follow the cover's instructions first: solid covers usually want water a few inches below the skimmer; some mesh setups run higher with the skimmer sealed. The hard rule is never empty — hydrostatic pressure can lift or crack an empty pool, a far worse outcome than any freeze.
What happens if you don't winterize a pool?
The freeze finds every shortcut. Ice in an unprotected pump or heater cracks castings from the inside; ice in underground lines splits fittings you can't see until spring. Plymouth reaches freeze territory around October 12 — the checklist above is cheaper than any one of those repairs.
When is the last safe date to close in Plymouth?
Our model's practical deadline is September 28 — set by the cool-down plus ten days (the freeze normal, October 12, leaves room to spare). 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 18–September 28 window, none of that drama applies.
Email me when Plymouth hits the closing window
Data: NOAA 1991–2020 climate normals via New Hope (4.1 mi); live outlook by Open-Meteo. Full attribution on the sources page. Model assumptions and error bars on methodology.