You may believe algae season is over once the temperature cools—but that green fuzz around your pool walls has other plans. Algae can grow in your pool water even during winter, particularly in areas such as Las Vegas where it hardly freezes.
Algae thrives in hot water but doesn’t totally shut off in cold temps. Algae can still grow at 50°F, so your pool is never really in the clear—particularly if you reduce chemicals or cut down on filtration during the off-season.
The good news? Algae in cold weather is avoidable with a few clever steps. With decades of assisting Las Vegas pool owners in overcoming this challenge, we have easy tips to ensure your water remains clean during the entire winter season—so that you’re not facing a green mess in the spring.
Can Algae Grow In Cold Water?
Yes, algae do grow in cold water. Indeed, we’ve seen our fair share of green, slimy affairs in pools with water temperatures of around 50°F. Although algae love the warm, toasty pool conditions of summer, they’re surprisingly tough little plants that are capable of surviving cooler temperatures.
Here’s the thing that most Las Vegas pool owners do not know: when we’re discussing “cold” pool water in our desert environment, we’re generally seeing 55-70°F water temperatures during the winter. That’s warm enough for some types of algae to not only survive, but to flourish.
What Temperature Does Algae Grow In A Pool? The Science Behind the Slime
Knowing what temperature algae can thrive in a pool takes a bit of a biology lesson, but we’ll make it short and to the point.
Most pool algae organisms can live and reproduce in temperatures between 40°F to 110°F. But their growth rates differ drastically:
- 85°F and above: Algae reproduction goes into overdrive, doubling every 3-6 hours under ideal conditions
- 70-84°F: Moderate growth rates, typically doubling every 12-24 hours
- 55-69°F: Slower but steady growth, doubling every 2-3 days
- Below 55°F: Growth slows significantly but doesn’t stop entirely
Winter algae is especially difficult to detect since it develops so slowly that many pool owners do not see it until it has become established. Unlike summer algae blooms in pools, which appear overnight, winter algae grows slowly, making it easy to dismiss the first green spots as “just some debris.”
The Different Faces of Cold-Water Algae
Not all algae are created equal, particularly when it comes to surviving in cold water. From our experience maintaining pools across Las Vegas winters, these are the most prevalent types we find:
Green Algae: The Winter Opportunist
Green algae are the most prevalent type we find in cold months. It usually begins as a few patches on pool walls, especially in poorly circulated areas such as steps, corners, and around lights. Can algae develop in cold water? Yes, and green algae verifies this each winter season in Las Vegas pools.
The problem with winter green algae is that it usually presents itself as a pale film instead of the thick, mucky mess we encounter in summer. Pool owners may confuse it with mineral staining or dirt and end up treating it late and creating larger issues later on.
Mustard Algae: The Cold-Weather Champion
Mustard algae (or yellow algae) are especially suited for cooler temperatures. We notice it most frequently on pools neglected during the switch from summer to winter maintenance schedules. It is seen in yellow or brown patches on pool bottoms and low-circulation zones.
What is particularly frustrating about mustard algae is that it can live on pool equipment and accessories. We’ve had instances where pool owners treated their water successfully, only to find the issue recurring because the algae was lodged in their pool cleaner or floating devices.
Black Algae: The Winter Survivor
Although less frequent in cold water, black algae can certainly tolerate cooler temperatures. Its extensive root system enables it to hold on tightly to pool surfaces and be resistant to treatment. When we do see black algae on winter service calls, it’s typically in those pools with untreated infections from last summer.
Seasonal Pool Care: Winter vs. Summer Algae Risks
| Season | Common Issues | Key Actions |
| Summer | Rapid algae growth, high phosphates, heat stress | Test 2×/week, shock after storms |
| Fall | Leaves & debris, rising phosphates | Cover pool, remove debris daily |
| Winter | Cold‑tolerant algae, reduced filtration | Maintain minimal filtration & chlorine |
| Spring | Warmth triggers dormant spores | Pre‑season shock & brush walls |
Cold water doesn’t give you a free pass—if you cut filter run times below 6 hours/day or let chlorine dip under 1 ppm, you’ll invite algae to take hold.
Winter Algae Prevention: What Works
After servicing thousands of Las Vegas pools over countless winters, here’s what we’ve discovered that keeps cold-water algae issues at bay:
Maintain Proper Circulation
The 75/25 rule is still valid in winter: 75% of your circulation goes through your skimmer, 25% through your main drain. This keeps your pool cleaner running effectively even with limited pump runtime.
For most residential pools in Las Vegas, we recommend minimum filtration times of:
- 6-8 hours when water temperature is 60-70°F
- 4-6 hours when water temperature is 50-60°F
- 3-4 hours when water temperature drops below 50°F
Chemical Balance Isn’t Optional
Even though algae develop more slowly in cold water doesn’t mean you can neglect water chemistry. We suggest testing 2-3 weeks at a time during winter, keeping:
- Free chlorine: 1-3 ppm (yes, even in winter)
- pH: 7.2-7.6
- Total alkalinity: 80-120 ppm
- Calcium hardness: 200-400 ppm
Regular Shocking Schedule
Cold shock treatments are not the same as summer procedures. At a 50-70°F water temperature, shocking every 3-4 weeks is typically adequate. At temperatures above 70°F, increase frequency to every 2 weeks.
The trick is operating your filter for a minimum of 4 hours following shocking to guarantee even distribution – something often omitted during wintertime by pool owners.
Physical Maintenance Matters
Even if you’re not swimming, your pool will need regular brushing and debris removal. Algae blooms in pool conditions tend to begin with organic material that supplies food for algae development. Brushing your pool every week and emptying skimmer baskets can avoid such issues before they begin.
When to Call the Experts
If you’ve tried everything and still see algae bloom in the pool, it’s time for a pro:
- Persistent Stains: Mustard or black algae that resist home remedies.
- Low Circulation Areas: Pools with dead spots around steps or waterfalls.
- Equipment Check: Aging filters or pumps can’t keep up with the cold‑water flow.
Nationwide Pool provides winter tune-ups—filter cleaning, chemical programming, and algae inspections—to make your oasis swim-ready throughout the year.
Yes, Algae Can Bloom, But You Can Stop It
So, do algae grow in cold water? Absolutely, but at a slower pace. Understanding the temperature at which algae grow in a pool, identifying early blooms, and applying targeted treatments will help you get rid of winter algae for good.
No matter if you’re fighting green, mustard, or black, consistency beats crisis: keep your filter running, water in balance, and cover in place. Too much of an algae battle? Nationwide Pool is just a phone call away—ending winter spores so you can bask in sparkling water, regardless of the season.
Ready to keep your cold water crystal clear?
Contact Nationwide Pool at (702) 435‑6060 for expert algae removal and year‑round care in Las Vegas.