When Should You Close Your Pool in Ontario? The 15°C Rule

One of the most common questions we hear every fall is: When should I close my pool?

The answer isn't a specific date on the calendar. In Ontario, a better way to determine when it's time to close is to watch your pool water temperature.

The target is 15°C (60°F)

For most Ontario pools, you want the water temperature to be consistently at or below 15°C (60°F) before closing.

This temperature is important because algae becomes much less active as the water gets colder. Closing while the water is still warm gives algae more opportunity to grow under the winter cover, which can leave you with green or cloudy water when you open the pool in the spring.

Don't confuse the air temperature with the water temperature. A cool September evening does not necessarily mean your pool is ready to close. A large volume of pool water holds heat and can remain much warmer than the surrounding air.

Don't Close Because It's September

There is a tendency to think that September means pool-closing time.

It doesn't.

Some years, Ontario has warm weather well into October. Other years, temperatures can drop quickly.

That's why we recommend watching the water temperature rather than choosing a closing date simply because the calendar says September or October.

For Southern Ontario, the closing window is generally late September through October, but the actual timing varies from year to year.

If your pool is still sitting at 20°C (68°F), for example, you're probably better off continuing normal maintenance rather than immediately putting the winter cover on.

What Does "Consistently Below 15°C" Mean?

You don't want to see the thermometer hit 15°C one morning and immediately close the pool.

The goal is for the water to have cooled and stay around or below 15°C as the weather continues to get colder.

Keep checking the water as fall progresses.

Once the pool is consistently around 15°C or lower, you're entering the ideal closing window.

At that point, you also need to look at the weather forecast. You want enough time to complete the closing before freezing conditions arrive.

Don't Wait for the First Freeze

Waiting for the water to reach the right temperature doesn't mean waiting until freezing weather is here.

Your pool equipment and plumbing need to be properly winterized before freezing temperatures can cause damage.

Water left inside plumbing, pumps, filters or other equipment can freeze and expand, potentially cracking components.

So the goal is to find the window between:

Warm water → too much algae risk

and

Freezing weather → equipment and plumbing risk

The sweet spot is when the pool water has consistently cooled to approximately 15°C (60°F) and you still have enough time to complete the winterization process properly.

What Should You Do While You're Waiting to Close?

If your water is still too warm, don't simply stop maintaining the pool.

Continue your normal maintenance:

• Test the water
• Maintain sanitizer levels
• Run the filtration system
• Remove leaves and debris
• Brush and vacuum the pool
• Keep the water clear

Fall debris can build up quickly, particularly once the leaves start coming down.

A pool that is clean and properly maintained when you close it is much easier to deal with in the spring.

September Is the Time to Get Ready

Even if your pool isn't ready to close yet, September is the right time to prepare.

Check your winter cover for tears or deterioration.

Make sure you have your closing chemicals.

Inspect your air pillow if you use one.

Check your winterizing equipment and make sure you have everything required to properly close your particular pool.

This is especially important if you need a replacement cover or a custom inground cover. You don't want to discover a problem with your cover when the water has already reached 15°C and you're ready to close.

What Happens If You Close Too Early?

Closing too early can create a different kind of problem.

If the water is still warm, algae can remain active after the pool is covered. Your pool is now dark, covered and no longer receiving the normal circulation and maintenance it receives during the swimming season.

That can result in:

• Green water
• Cloudy water
• Algae growth
• Staining
• More work when opening the pool

The colder the water is at closing, the less favourable the conditions are for algae growth.

What Happens If You Close Too Late?

Waiting too long creates a different risk.

Once freezing weather becomes a possibility, you don't want an open circulation system containing water to be caught unprepared.

Proper winterization takes time. Depending on the pool and equipment, this can include lowering the water as required, draining equipment, removing water from plumbing, winterizing the pump and filter, and installing the appropriate plugs and cover.

Don't wait until temperatures are already below freezing to start.

A Simple Rule to Remember

If you want one number to remember, remember this:

15°C (60°F)

When your pool water is consistently at or below 15°C (60°F) and the weather is moving toward colder conditions, it's time to plan your closing.

Don't base the decision solely on September 30, Thanksgiving weekend or the first cold night.

Watch the water temperature. Watch the forecast. Then close the pool before freezing weather arrives.

Get Your Pool Closing Supplies Ready

At PTL Direct, we carry the products you need to prepare your pool for an Ontario winter, including pool closing chemicals, winter covers, air pillows and winterizing supplies.

September is the time to inspect your equipment and order anything you need. That way, when your pool reaches 15°C (60°F), you're ready to close it properly instead of scrambling for supplies.

PTL Direct — Canadian pool products for getting your pool ready for the winter.