When to Open Your Pool: Estimated Dates for Your City and State

Open your pool before the water warms past about 60°F, when algae starts to grow faster, and after the freezing nights that can crack pipes and equipment are over. Most guides shorten this to “when daytime highs stay above 70°F.” In NOAA climate normals, that falls in March in much of Texas, in April across the Southeast and Mid-Atlantic, and from mid-May into early June in New England, the Great Lakes and the Pacific Northwest.

We added dates. Using NOAA’s 1991–2020 climate normals and the same method as our pool-closing guide, we found three dates for 49 US cities: when freezing nights usually end, when the typical daily mean air temperature reaches 60°F, and when the typical high reaches 70°F. These are air-temperature estimates, so treat each date as the day to start reading your pool thermometer.

The opening temperature: water below 60°F, air at 70°F

The rule that matters is about the water. Trouble Free Pool’s Pool School opening guide puts it in one line:

Open your pool before the water temperature reaches 60°F.

Its reason: algae grows much slower in cold water, so an early opening greatly reduces the risk of a green pool.² It is the mirror of the closing rule, where Trouble Free Pool waits until the water is solidly below 60°F.

Most pool brands give an air-temperature rule instead. Clorox Pool&Spa writes:

It’s time to open your pool when daytime temperatures consistently rise above 70 degrees.

River Pools and Pool Operation Management, a pool-operator training company, use the same 70°F figure.^4,5 Other guides use lower triggers, and Table 1 shows how they differ.

The water rule decides when to open; the freeze rule decides how early you can. Opening late costs time and money: Hometown Pools says green water from a delayed opening adds at least a week to the job.¹² Build in lead time too: Swim University suggests opening 2–3 weeks before you plan to swim, so there is time to fix equipment and balance the water.¹¹

When to open your pool: estimated dates by city

We placed each NOAA monthly normal at the middle of its month, joined the months day by day, and found the first spring day the typical daily mean air reaches 60°F and the typical high reaches 70°F.¹ The frost columns come from NOAA’s frost normals for the nearest station: the median last 32°F night, and the date after which only 1 year in 10 still sees one.

Seattle is the clearest case against a calendar rule. Its median last frost is Mar 13, but the typical high doesn’t reach 70°F until Jun 9, so freezing risk is long gone by the time the 70°F rule says to open. Denver is the reverse: highs reach 70°F on May 6, a week before its 1-in-10 frost date. For any other ZIP code, look up your last-frost date and its odds.

Why your water and the air don’t agree

The USGS explains that water takes longer to heat up and longer to cool down than land, which is also why cities near oceans and lakes warm more slowly in spring.¹⁵ That shows in the table: near the water, highs are slow to reach 70°F, 16 days after the 60°F mean in Atlantic City and 9 in Cleveland, against 2 in Newark. Sunshine, especially through a mesh cover, also warms a pool, so the water can run ahead of or behind the air.

Plan with the table, then let a thermometer decide. Once your last-frost date has passed, check the water every few days and open before it reaches 60°F.

The other limit: late freezes

Pool Operation Management advises against opening while temperatures can still freeze, because freezing water in the lines can crack them.⁵ Hayward’s salt-cell manual adds that freezing water damages the cell and flow switch just as it does pool plumbing.⁶ Our guide to frost-date odds explains the 1-in-10 date. If temperatures drop sharply after you open, Swim University’s advice is to keep the pump running and put the cover back on until it warms up.¹¹ Pentair’s manual also says continuous pumping prevents freeze damage to its cell.⁷

Does the 70°F rule wait out the frost?

In about half the country, no. We ran the same comparison for every ZIP code with both dates, 38,480 in all. In 53.0% of them, the typical high reaches 70°F before the 1-in-10 last-frost date. The median ZIP code reaches 70°F two days before that date and 15 days after its median last frost.

When to open a pool in NJ, PA, OH, TX and six more states

These ten states lead Google’s suggestions and related searches for this question. Inland and higher ground usually runs later than a coastal or big-city station.

New Jersey

Newark and Cherry Hill reach the 60°F air mark around May 8, Morristown around May 12, after a 1-in-10 frost date of May 6. Atlantic City is the odd one: its frost risk ends by early April, but the ocean holds back the air, and highs reach 70°F only on May 31.

Pennsylvania

Philadelphia is earliest, reaching 60°F around Apr 28. Harrisburg and Pittsburgh follow about two weeks later and Erie around May 20. Pittsburgh’s highs reach 70°F on May 5, before its 1-in-10 frost date of May 11.

Ohio

Cincinnati reaches 60°F around May 4, Columbus May 8 and Cleveland May 15. On the Cleveland lakefront the frost risk ends by Apr 20, but highs reach 70°F only on May 24.

Texas

Houston, Austin and San Antonio reach 60°F between Feb 27 and Mar 6, Dallas around Mar 18. The 1-in-10 frost dates run from Mar 5 in Houston to Mar 31 at Austin’s frost station, so early-March openings there need a freeze plan.

Connecticut

Hartford, New Haven and Stamford all have a 1-in-10 frost date of May 1 or 2. The air reaches 60°F between May 13 and May 21, latest at the New Haven shore.

North Carolina

Charlotte and Raleigh reach 60°F around Apr 12–14; Asheville, in the mountains, around Apr 27. In all three, highs reach 70°F before the 1-in-10 frost date.

Florida

In Orlando, Tampa and Miami the typical daily mean stays above 60°F all year and frost is rare, so the question there is less when to open than whether to close at all. Jacksonville reaches 60°F around Feb 18, with a 1-in-10 frost date of Mar 3.

Oklahoma

Tulsa reaches 60°F around Apr 14 and Oklahoma City around Apr 19. Both see 70°F highs about a week before their 1-in-10 frost dates of Apr 15 and Apr 20.

Georgia

Savannah reaches 60°F around Mar 16 and Atlanta around Apr 9, with 1-in-10 frost dates of Mar 25 and Apr 10.

Illinois

Springfield reaches 60°F around May 7, Chicago May 12 and Rockford May 15. Chicago’s temperatures come from Midway airport and its frost dates from a lakefront station, where frost ends earlier than in inland suburbs.

Do the dates match published regional advice?

Broadly, yes. The New Jersey and Virginia windows end close to our 60°F dates. The New York and Connecticut window starts in late March, before the frost risk ends there, and closes in early May, before the air reaches 60°F at most of its stations. None of these guides gives a date for a specific city.

Sources: Hometown Pools, Neave Outdoor, Perkinson Homes and Swim University’s survey of 1,900 owners.^12,13,14,11 The Virginia row uses Richmond and Virginia Beach; the New Jersey row uses Newark, Morristown and Cherry Hill.

Salt pools: the cell waits for warmer water

A salt chlorine generator won’t chlorinate cold water at opening. Hayward’s AquaRite manual says:

The AquaRite automatically stops generating when the pool water temperatures drops below 50°F.

Pentair’s IntelliChlor shuts off at 52°F, plus or minus 3°F.⁷ Hayward says not to turn the cell on until the water chemistry is in range. Pentair says to super-chlorinate from an outside source at start-up, then wait until chlorine is back to 2.0–4.0 ppm before switching the cell on.^6,7 Trouble Free Pool starts its cell once the water is above 60°F and the salt has circulated for a day.² Test salt at opening and work out any top-up with the pool salt calculator.

Opening steps at a glance

Every pool and equipment pad is different, so treat this as an overview and follow your equipment manuals and product labels.

1. 1

   Clear the cover before you lift it

   Pump standing water off a solid cover and remove leaves and debris, so they don’t fall into the pool when the cover comes off.^2,3

2. 2

   Inspect the equipment

   Clorox starts with a safety inspection of the equipment pad and says to call a service professional for loose wires or cracked lines.³

3. 3

   Reconnect, refill and circulate

   Remove winter plugs, reinstall fittings, fill to the middle of the skimmer opening, and start the pump as its manual directs. Trouble Free Pool circulates for 24 hours to mix the water.^2,3

4. 4

   Test the water

   CDC recommends at least 1 ppm free chlorine, or 2 ppm with stabilizer, and pH 7.0–7.8.⁸ Our ideal pool chemical levels guide covers the other ranges.

   

5. 5

   Add chlorine first

   Trouble Free Pool notes that most pools open with zero chlorine.² One 10% liquid chlorine label gives 64–128 fl oz per 10,000 gallons for start-up, with no swimming until chlorine is below 4 ppm.¹⁰ Size the dose with the pool volume calculator and our guide to how much shock to add, which also covers green water.

This guide is general information, not professional advice; your manuals and product labels come first. See our estimates and safety disclaimer.

Next steps

Set your ZIP code above to see your last-frost date, and put two reminders in your calendar: your 1-in-10 frost date, to start checking the water, and your 60°F date. Then see the opening and closing section and the rest of Pool & Spa; our methodology page explains the numbers.