Algae doesn’t take the winter off — and neither should your pool care. The right closing products, a secure cover, and a little off-season maintenance make the difference between a swampy surprise and a crystal-clear pool come spring.

1. Algae Doesn’t Fully “Sleep” in Winter

While algae growth slows in cold temperatures, it doesn’t completely stop. Algae spores can survive under the cover, waiting for the right conditions to bloom. If your pool water chemistry isn’t balanced at closing, or chlorine levels fade too quickly, algae can start multiplying even in chilly water.

2. Cover Gaps Let in Light and Debris

If your pool cover isn’t secured tightly, small gaps can allow sunlight, leaves, and organic debris into the pool. Algae thrive on sunlight and food sources like decaying leaves. That’s why investing in a well-fitted safety cover, like Loop-Loc, makes such a big difference.

3. Water Chemistry Changes Over Time

Once your pump and filter are shut down, water circulation stops. Without movement, chlorine or algaecides can break down unevenly, leaving some areas of the pool unprotected. Cold weather also doesn’t prevent pH or alkalinity from drifting out of balance, which weakens your chemical defenses against algae.

4. Warm Spells During Winter

In Maryland and the Mid-Atlantic, winter isn’t always consistently cold. Warm snaps can temporarily raise pool water temperatures, giving algae the jump-start it needs. If there’s little to no sanitizer left in the water, a short thaw can trigger algae growth under the cover.

5. How to Prevent Algae All Winter

• Use BioGuard closing chemicals like Arctic Blue® Shock and Arctic Blue® Algae Protector for long-lasting protection.
• Balance your water before closing to protect surfaces and maintain sanitizer effectiveness.
• Add a scale or stain inhibitor like BioGuard Scale Inhibitor or Pool Closing Complete™ to guard against mineral buildup.
• Invest in a safety cover that blocks sunlight and debris.
• Schedule mid-winter service checks with Watercrafters to keep an eye on your pool during the off-season.