1. The 65 degree closing rule has a lot to do with algae growth. Algae thrives in warm conditions, and research has shown that it slows down considerably once temperatures drop below 65. So, by waiting until temperatures are at least below 65, you decrease the likelihood of algae growth when closing up your pool for the rest of the year. 2. If you close too early, your chances of algae growth are higher. It's definitely more common for people to discover significant algae problems when they take their pool cover off. If you close too late, there is a greater risk of damage due to water freezing. That damage is typically more significant and costly to fix than damage caused by algae. 3. I've seen everything from pipes, to pumps, to even the shell of pools themselves crack because of pools being closed too late, causing water to freeze and expand.
Hello, The 65degF rule isn't just a number it's biology and chemistry intersecting. Algae thrives in warmer water, and chlorine breaks down more quickly above that threshold. Once water consistently dips below 65degF, biological activity slows dramatically, meaning chemicals hold longer and you're not feeding a green pool all winter. In my experience, closing too early—say at 70-72degF—creates a spring nightmare of murky water and heavy clean-up, even in mild climates like Southern California. Conversely, closing too late in colder zones like the Midwest can mean catastrophic freeze damage. The most expensive case I've seen was a pool in Illinois where a delayed close led to cracked travertine coping and a burst circulation line beneath a stone patio. Repairing the structural stonework and plumbing cost upwards of $40,000, far more than a proper closing would have. The lesson: closing isn't about a date on the calendar, but about water temperature as a scientific benchmark that saves homeowners both time and money. Best regards, Erwin Gutenkust CEO, Neolithic Materials https://neolithicmaterials.com/