When should you open your pool in Northeast Ohio?

Two things pull in opposite directions. Open too early and a hard freeze can crack plumbing or equipment that is full of water again. Open too late and the water under the cover warms up, algae gets a head start and the first week of the season goes to cleanup instead of swimming. The sweet spot sits between the last hard freeze and the point where the water starts to warm.

The calendar is only a guide. What you are really watching is the forecast for overnight lows and the temperature of the water under the cover.

NOAA's 1991 to 2020 climate normals give the median date of the last spring low of 32 degrees for stations across the region:

• Cleveland Hopkins: April 20
• Ravenna: April 21
• Akron-Canton Airport: April 24
• Hiram, in northern Portage County: May 1
• Chardon, in the Geauga County snowbelt: May 5

A median means that in about half of years the last freeze comes later. NOAA also publishes the tail: in roughly one year in ten, a freeze still arrives after May 11 at Akron-Canton and after May 21 in Chardon. Snow is possible late, too. The National Weather Service's review of the 2022 to 2023 winter notes 1 to 2 inches of snow in the higher elevations of Geauga and southern Lake counties on May 2.

That does not mean waiting until late May. A pool can be opened before the last freeze as long as you keep an eye on the forecast and keep the pump running through any cold night so water keeps moving in the lines and equipment.

Algae. Under a winter cover the water sits still, unfiltered, with its chlorine long gone. As long as it stays cold, very little grows. Once it warms, algae and bacteria start to multiply. Latham's opening guide says to open the pool in any month before the water temperature reaches 70 degrees for most of the day, because algae can grow fast at those temperatures and the sooner you shock the pool, the better.

Orenda, a pool chemistry company, makes a similar point: pools closed properly in fall should open clean and clear if they are opened before the water reaches about 65 degrees. It also notes that many test kits read inaccurately in water at 60 degrees or colder, so warm the sample indoors before testing on an early opening.

A simple floating thermometer under the edge of the cover tells you more than the calendar. When the water starts climbing into the 60s, it is time.

• Akron, Canton, Kent, Ravenna and the Cleveland south suburbs: late April to mid-May. A warm spell in late April is a good trigger, with a watch on overnight lows.
• Near the lakeshore: similar, sometimes a little earlier, because Lake Erie keeps spring nights near the shore from dropping as low.
• Geauga County, inland Lake County and higher ground in northern Portage: early to mid-May, later after a cold spring.

Whichever group you are in, work backward from when you want to swim. If Memorial Day weekend is the target, open two to three weeks ahead. That leaves a week to get the water clear and balanced and, if you have a heater, time to bring the water up without running the heater flat out.

A lot. Liberty's cover page notes that a solid safety cover blocks 100 percent of sunlight. With no light reaching the water, algae has a much harder time getting started, so a pool under a solid cover can usually wait a little longer into May and still open clean. The trade-off is the water that collects on top of a solid cover, which you need to pump off before removal, and a heavier cover to handle.

A mesh cover lets rain, snowmelt and sunlight through. The water level rises over the winter, fine dirt settles on the floor and the sun warms the water sooner. Pools under mesh covers tend to need opening earlier, and they usually need more cleaning and a stronger first shock when they are opened. If you have a mesh cover and a warm spell arrives in late April, that is your signal.

An automatic cover used through the off-season, where its maker allows, keeps the pool clean and lets you open whenever the weather turns, since the cover is easy to roll back. That makes an early opening far less work.

The most common problem is a green pool. By early June, even an unheated pool under a cover has usually warmed well past the point where algae takes off. A solid cover limits sunlight, which helps, but a mesh cover lets sun and debris in and the water can turn quickly. A green pool takes heavy shocking, extra filter cleaning and several days of running the pump before it is swimmable, and it costs more in chemicals than an early opening would have.

The other problem is scheduling. Pool service calendars across the region can fill up in May. If you use a service for openings, book in March or early April for a slot that suits the weather rather than the service calendar.

Freeze damage. Once the lines are filled and the plugs are out, a hard freeze with the pump off can crack a pipe, a pump housing or a heater exchanger. Heating costs also climb, because warming a pool in April nights near freezing burns a lot of energy for very little swimming.

If you open in April, run the pump continuously on any night forecast to go below freezing, keep the heater off unless you have a reason to use it and put a cover back on at night. A solar or automatic cover makes this easy. The winter safety cover can go back on if a real cold snap arrives.

Latham's opening guide lists them as: remove the cover, fill, reconnect, add solution, test, clean and filter, then shock. In a little more detail:

1. Pump standing water off a solid cover, then remove, clean and dry the cover before storing it.
2. Take out the winterizing plugs and skimmer protection, and reinstall return fittings, ladders and rails.
3. Refill to the normal level, about the middle of the skimmer opening.
4. Reinstall pump, filter and heater drain plugs, reconnect equipment, prime the pump and check for leaks.
5. Brush, skim and vacuum.
6. Test and balance the water, then shock and run the filter continuously until the water is clear.

On a fiberglass pool, use the opening to look for anything unusual: a waterline that is no longer level, a gap between coping and deck or a wall that looks bowed. Those can be signs that groundwater moved the shell over the winter, and they are worth a call before you refill heavily.

• Order chemicals and test supplies a week ahead, including enough shock for a stronger first dose.
• Check the equipment pad for anything the winter did to it: cracked fittings, animal nests in the heater, loose wires or a tripped breaker.
• Trim branches that hang over the pool before the leaves come out fully, while the cover is still on.
• Find the plugs, fittings and parts stored at closing. If whoever closed the pool left a list, now is when it pays off.
• If you have a heater, have it checked before you need it rather than on the first warm Saturday.
• Look over the deck and coping for freeze-thaw damage, such as heaved joints or spalled concrete, and note anything that should be repaired before the season starts.

An hour of preparation makes opening day a morning job rather than a weekend project, and it catches small problems before they turn into a service call in the middle of the season.

It lets you swim sooner, but it does not change the logic about algae or freezes. Open on the same timeline, get the water clean and balanced first, then heat it. Heating dirty, unbalanced water wastes energy and can scale the heater. A cover on every night matters more in May than at any other time of year, because nights near 50 degrees pull heat out of an uncovered pool quickly.

If you want help, we do spring openings, water testing and balancing and equipment checks as part of our pool service, and we can pair the opening with a look at anything that needs repair before the season starts.