Winterized Pool Turning Green: Salt Water Pool Problem Solved

Winterized Pool Turning Green: Salt Water Pool Problem (Solved)

Have you taken your winter pool cover off for spring opening and found yourself staring at a giant bowl of pea soup? Instead of seeing crystal clear water, you’re staring at dark green water that looks like a swamp.

Been there, done that.

For several years, I opened our salt water pool every spring to pea green soup and assumed that’s just how it was. I mean, you go an entire winter with no chlorine in the water so you must get algae as a result, right? That’s just normal, I figured. I was a new pool owner! Never had one before this.

It wasn’t until I did some research and learned a bit more about how I can prevent this from happening that I realized that my lazy pool company was improperly closing the pool each autumn. Plus I had a couple other issues most likely contributing that I had to deal with.

The best way to avoid having your salt water pool turn green during the winter after closing is by doing the following:

  • Properly balance water chemistry prior to closing.
  • Don’t close the pool if the water is green.
  • Properly winterize the pool.
  • Use a good quality winter pool cover.
  • Don’t close your pool too early in the autumn.
  • Don’t open your pool too late in the spring.

How to prevent your salt water pool from turning green over winter

Here are the things you need to do before closing your pool and considerations before you open it to eliminate the chance you end up with green algae.

Properly balance water chemistry prior to closing

Properly balance water chemistry prior to closing. Ensuring that all the chemicals are in line are important to give your pool water the chance to adjust as the pump and chlorinator is shut off and circulation stops.

Sure you’re not swimming in the pool but as long as the temperature is over 50°F you can develop algae in the water. So by having properly balanced water at closing you can help to prevent any larger issues over winter and again in spring when you’re ready to open the pool.

You can also shock the pool with chlorine at pool closing to give the water one last chlorine boost as part of the winterizing process. Speaking of which…

Don’t close the pool if the water is green

Don’t close the pool if the water is green with algae. Obviously if you close a pool with visible algae in the autumn, it can only get worse during the winter when the water is still (i.e. the pump is not running so there is no circulation) and no chlorine being produced since the chlorinator is shut off for.

It’s important to properly balance water chemistry prior to closing as mentioned above but also vacuum it clean and also ensure there is no visible algae. Brush the walls and floor one last time and remove any visible leaves or other organic material that will rot over the winter while sitting in the water.

Properly winterize the pool

Properly winterize the pool including use of chlorine shock as the pool is being closed along with an algaecide. This will help to keep chlorine levels in the water high enough while the autumn sun might keep the water temperature high enough that algae may develop.

If you live in an environment with a cold winter, you will need to pump water out of the pool below the water returns, cap the returns and do other winterizing tasks to protect the pool and plumbing from freezing temperatures.

As mentioned above, I didn’t realize about adding chlorine in at closing because my first pool company never did it. I learned when I switched pool companies that a closing should typically also include adding chlorine to the water just to keep the chlorine level high while the water temperature might still make the water susceptible to algae.

Use a good quality winter pool cover

This is part of the winterizing process but not everyone does it. Use a good quality winter pool cover that bolts to the ground and completely covers the water surface to keep all debris out.

This is an important consideration because the tighter the pool cover, the less debris can possibly make its way into the water over the winter. This includes rodents falling in and drowning and then rotting in the water.

I found that in our case, the shape of our pool – it’s an unusual one with a rock waterfall in one corner – prevents the pool cover from completely covering the pool surface. This means that over the winter, leaves, tree twigs, dirt and other debris falls into the water and rots. This makes spring opening more work intensive since I have to vacuum the pool very well.

Don’t close your pool too early in the autumn

Don’t close your pool too early in the autumn when you still have warm weather. When you close your pool, you can’t swim in it anymore but it also means it can’t produce chlorine. When the strong sun rays heat the water – more so if you have a pool cover! – the water gets heated and the conditions for algae increase.

The warmer the water temperature, the greater the chance of algae.

Algae can grow in any pool with no chlorine in it but once the water temperature goes above 50°F, the chances increase. When the water temperature goes above 60°F, algae will bloom i.e. multiply quickly.

Closing your pool later in the season even when you’re not using it costs money in terms of the electricity required for the pump but can benefit you by reducing the chance of green algae growing if you close the pool too early.

Keep an eye on autumn temperatures and try to coincide your pool closing as the temperature drops close to or below the green algae cut off mentioned above.

Don’t open your pool too late in the spring

Don’t open your pool too late in the spring when the warm weather has already returned. Green algae is typically caused by the presence of three things:

  • Lack of water circulation
  • Lack of sanitation i.e. no chlorine
  • Lack of filtration

When your pool is still closed, all three of these factors are in play. There is no circulation, no chlorine present and no filtering of the water taking place.

And then the spring sun comes out, guess what happens? The water heats up and the environment for green algae skyrockets.

Remember that once water temperature goes above 50°F, green algae can start to develop.


I’ve read a few times online about pool “experts” saying that a pea green pool isn’t a big deal and you can typically get it right in a day or so. Not in my experience! If your pool is dark green, it will take one of two bottles of algaecide and at least a few days to clear it.

Then you might be surprised to find what’s at the bottom of the pool that needs to be vacuumed (rotting leaves, twigs, etc) plus you’ll also undoubtedly be dealing with algae growing on the walls and pool floor too. That takes time to clean up! It could be 4-5 days all in before the water is sparkling clean.

Then you need to back wash or otherwise clean your filtration system to get rid of the collected algae. Again, this takes time.

The pool season is often short especially for those of us who live in northern parts of the world. You want to get your pool open and usable as quickly as possible and staying away from green algae is your best bet.

Carl Mueller

I bought a home with a salt water pool in 2006 and soon realized the benefits over traditional chlorinated pools. On this website I'll discuss all the tips and tricks I've learned over the years. I'll also help you troubleshoot various problems with pools in general and ones specific to salt water pools that I've experienced personally!

Recent Posts