How Do I Prevent Algae During a Pool Closing?

Balance your water chemistry, shock the pool, and use an algaecide like our AquaDoc Pool Algaecide before covering the pool.

Why Algae Is a Problem During Pool Closings

When it’s time to close your pool for the season, algae isn’t just an eyesore, it’s a sign of trouble. Left unchecked, algae can stain surfaces, clog filters, and turn your first spring swim into a cleaning nightmare. It thrives in stagnant water, loves debris, and multiplies in dark, warm environments. A poorly closed pool is basically a five-star hotel for algae.

More importantly, algae growth is not just about visual clarity. It can damage pool finishes, make surfaces slippery, and throw off your water chemistry in a way that leads to more expensive maintenance later. If you’ve ever opened a pool in the spring to cloudy green water, you know how frustrating and time-consuming it is to clean up.

What Causes Algae to Grow Over the Winter?

Algae doesn’t care if your pool is in use or not. If it has nutrients, water, and a little warmth, it can bloom; even under a cover. Common causes of winter algae include:

• Unbalanced water chemistry before closing

• Debris left in the pool or on the cover

• Inadequate chlorine or sanitizer levels

• Poor circulation before shutting down the pump

• Leaks in the cover letting in sunlight and contaminants

Even small oversights can lead to algae outbreaks. A loose cover that lets sunlight hit the water, for example, creates the perfect environment for algae to flourish. Combine that with falling sanitizer levels over the winter and organic material trapped in the water, and it’s a recipe for green sludge.

Step-by-Step: How to Close Your Pool Without Inviting Algae

Let’s break down a pool closing plan that actually works and keeps algae out.

1. Deep Clean the Pool

Start with a full clean. This is your chance to remove anything that algae can feed on over the winter. If you leave behind leaves, bugs, or dirt, they’ll decompose and release phosphates; a key food source for algae.

• Vacuum the pool floor thoroughly.

• Brush all pool surfaces including walls, steps, and corners.

• Empty skimmer baskets and pump baskets.

• Skim the surface of the pool and remove any visible debris.

If your pool has had any signs of algae before closing, this step is even more important. Dead algae can fuel new blooms, so clean like your spring self depends on it.

2. Balance Your Water Chemistry

Proper water balance helps your chemicals work better and prevents corrosion or scaling during the off-season.

Here’s what to aim for:

• pH: 7.2 to 7.6

• Total Alkalinity: 80–120 ppm

• Calcium Hardness: 180–220 ppm

• Chlorine: 1–3 ppm

• Phosphates: Ideally 0

Phosphates act as fertilizer for algae, so it’s worth using a phosphate remover if your levels are high. You can test for this with a strip or kit, or have it checked at a pool store.

Give the water a few days to stabilize after balancing. Recheck the levels before you move to the next step.

3. Shock the Pool (But Time It Right)

Shocking the pool is your chemical deep clean. It burns off organic contaminants and wipes out any microscopic algae spores that might still be hanging around.

Key tips:

1. Use a chlorine-based shock product for most pools.

2. Add the shock in the evening to avoid sun breakdown.

3. Run the filter overnight to circulate.

4. Wait 24–48 hours before adding algaecide.

If you add algaecide too soon after shocking, the high chlorine can render it ineffective. Always allow the chlorine to return to normal levels first.

4. Add a Long-Lasting Algaecide

Not all algaecides are created equal. For winter protection, use a product labeled as a winterizing or long-lasting algaecide. These are formulated to stay active for 3–6 months and slowly release over time.

Look for:

• Non-foaming formulas

• Copper-free options for vinyl pools

• Algaecides rated for green and yellow algae prevention

Follow the manufacturer’s dosage instructions based on your pool size. Pour it evenly around the perimeter of the pool.

5. Lower the Water Level (But Not Too Much)

In colder climates, you’ll want to lower the water below the skimmer and return lines to prevent freeze damage. But draining too much water can cause the liner to shrink or the pool structure to shift.

• For mesh covers: Lower the water 12–18 inches below the skimmer.

• For solid covers with a pump: 3–6 inches below the skimmer is usually enough.

Use a submersible pump or your pool pump’s waste line to lower the water gradually.

6. Drain and Store Pool Equipment

Leaving water in pipes or equipment is a major freeze risk. Water expands as it freezes, and even a small amount left behind can crack expensive parts.

Here’s what to do:

• Drain the filter, pump, heater, and chlorinator.

• Use a shop vac or air compressor to blow out the return lines.

• Plug the lines with winterizing plugs.

• Add pool antifreeze to any lines that can’t be fully drained.

Store accessories like ladders, diving boards, and skimmer baskets in a dry area. Label parts for easy reassembly in spring.

7. Cover It Tight

A good cover is your first line of defense against debris and sunlight; both of which contribute to algae growth.

To get it right:

• Choose a cover that fits your pool snugly.

• Patch any holes or tears before installing.

• Use water bags or cover clips to secure the edges.

• Remove leaves and standing water regularly during the winter.

If you’re using a mesh cover, consider adding a floating chlorinator under the surface to help maintain sanitizer levels over time.

Common Questions About Algae and Pool Closings

Can I close my pool without using algaecide?
Technically yes, but it’s risky. Even with perfect chemistry, a good algaecide adds a layer of protection that’s worth it over the long haul.

How soon should I close my pool?
Wait until the water temperature consistently stays below 65°F. Algae becomes less active in cooler water, so you’ll have fewer issues when you close late in the season.

What happens if algae forms during the winter?
You’ll likely open the pool to cloudy green water and need to scrub, shock, and rebalance everything. It’s fixable, but a pain. Prevention saves time and money.

Is it okay to open the pool early?
Yes, in fact, it’s a good idea. Opening earlier (when temps are still cool) gives you a jump on maintenance and makes algae less likely to bloom.

Do saltwater pools need algaecide too?
Yes. Salt systems stop producing chlorine when the pump is off for the season, so a winter algaecide is still a smart move.

Is there a difference between green, yellow, and black algae?
Absolutely. Green algae is the most common and easiest to kill. Yellow algae (or mustard algae) is more stubborn and sticks to walls, while black algae has deep roots that are difficult to scrub out. Preventing them all starts the same way: keeping water balanced, surfaces clean, and light out.

What if I’m in a warm climate and don’t fully close the pool?
You still need to be algae-aware. Keep your pool running on a reduced schedule, brush weekly, and check chlorine and pH levels at least once a week. Even in mild winters, stagnant water and lack of maintenance invite algae in.

Keep Algae Out and Make Spring Opening Easy

Algae prevention during pool closing isn’t complicated. Clean thoroughly, balance your water, shock properly, and use a trusted winter algaecide. Lower your water, store your gear, and seal it tight. With a little planning, you’ll save yourself hours of work when warm weather returns and your pool will be ready to go.