When to Close Your Pool in Colorado

The rule of thumb

Close when your water temperature settles below about 65°F and stays there. On the Front Range that usually means the last week of September through the middle of October. Below 65°F, algae goes dormant and your winterizing chemicals last the whole winter; above it, they burn off while the pool sits under the cover, and you open to a green surprise in spring.

The other half of the rule: be closed before the first hard freeze. Denver's first freeze typically lands in early-to-mid October, but this is Colorado — it has come far earlier. In September 2020 the metro went from triple-digit heat to snow in a matter of days. The calendar is a guideline; the overnight forecast is the deadline.

Why the closing date matters so much

Because of what happens in the plumbing, not the pool. Water expands about 9% when it freezes, and water left sitting in your lines, pump, filter, or heater will split them open from the inside. You won't find out in October — you'll find out at opening in spring, when the system won't hold pressure and the repair bill runs to several times what the closing would have cost.

An early-season cold snap is the classic way this happens: the pool is "still open," a hard freeze rolls through overnight, and the damage is done before breakfast. If you take one thing from this page, take this — book the closing before you think you need it.

Can you close too early?

Yes, and it's the mistake people make when they're trying to be responsible. Close in August or early September, while the water is still warm, and you've sealed a warm, chemically-fading pond under a cover for eight months. Algae loves nothing more. That's why the sweet spot is a window, not a date: late enough that the water has cooled for good, early enough to beat the first freeze.

Swimming season doesn't have to end early either — a pool holding at 60°F isn't much fun anyway. When the afternoons stop being warm enough to swim, the water is usually telling you it's time.

What a proper closing involves

Winterizing is mostly plumbing work — the cover is the last step, not the point of the job. A full closing means:

• Balancing the water so it sits neutral against your liner or plaster all winter.
• Deep cleaning — anything left in the pool in October is stuck to it by April.
• Dropping the water level below the returns and skimmer.
• Blowing out every line with compressed air and plugging them — this is the step that actually prevents freeze damage, and the one that goes wrong in do-it-yourself closings.
• Adding pool antifreeze to the lines as insurance against the water that always finds its way back.
• Winterizing chemicals so the water you're leaving behind stays clear.
• Pulling and storing the equipment that shouldn't spend winter outside.
• Fitting the cover properly, so it survives the wind and holds the snow load.

If you're comfortable with a compressor and know where every line on your system runs, closing your own pool is doable. The honest risk isn't the chemicals — it's a return line that didn't get fully cleared and quietly held a cup of water into January. That's the part people pay us for.

A simple timeline for the Front Range

• Late August – early September: book your closing slot. The two weeks before the first freeze are the busiest of our year, and closings booked after the forecast turns are the ones that end up rushed.
• Mid-September: keep an eye on the water temperature and the overnight lows.
• Late September – mid-October: close. This is the window.
• After the first hard freeze: if the pool is still open, don't run the season longer — call us straight away and we'll get to you as fast as we can.

What it costs

Our pool closing covers everything above, with the exact price depending on the size of your pool and where you are in the Denver metro area and the Front Range — current pricing is on the pool closing page, and you'll always be told the exact figure before we start. We close pools everywhere we work, as far north as Erie and as far south as Highlands Ranch.