My pool turns green every spring - how do I stop it this year?
The pool went under the cover blue in October and came out a swamp in April. Again. It is tempting to blame the cover, the winter, or bad luck. It is none of those. A pool that greens over the winter almost always lost the fight at two specific moments: the week it was closed and the weeks before it was opened. Both are decisions, and both are fixable this year without buying anything new.
Why a blue pool opens green
Winter greening is a race between two curves: how fast the chlorine under the cover runs out, and how long the water stays warm enough for algae to grow. Below roughly 60°F (15.5°C), algae growth nearly stops and chlorine decays slowly. Above it, algae divide comfortably while the chlorine, cut off from any replenishment for six months, gets used up.
So the green does not happen in January. Cold, dark, covered water in midwinter is remarkably stable. It happens in the warm shoulder weeks: water at 70°F (21°C) in late September under a cover with a fading shock dose, or water back at 65°F (18°C) in April with nothing left to defend it. Stop the warm, unchlorinated weeks and you stop the green.
When should you close your pool?
The honest answer is a thermometer, not a calendar. Close when the water holds at or below 60°F (15.5°C) - "holds" meaning several readings in a row across a week or two, not one cold morning after a front comes through. Air temperature is nearly useless here; a 20,000 gallon pool changes temperature slowly and lags the weather by weeks.
The consensus window opens around mid-September in the northern hemisphere (mid-March in the southern), but that date is only when to start watching. What the water reads at the moment the cover goes on is what decides your spring:
| Water temperature at closing | What the winter looks like |
|---|---|
| 75°F (24°C) | The shock is spent in a couple of weeks; algae gets a month of warm water to itself. Green by Thanksgiving, discovered in April. |
| 65°F (18°C) | Borderline. Fine after a fast, cold fall; green after a mild one. |
| 60°F (15.5°C) or below | Chlorine outlasts the remaining algae season. This is the target. |
The common failure is closing on Labor Day weekend because the schedule is convenient or the service company's calendar fills up. In most of the US that means covering 75°F (24°C) water, which is the single most reliable way to open a swamp. If a booking genuinely forces a warm closing, peel the cover back once in late fall, test, and hand-dose chlorine to buy the water another month. It is a chore, which is exactly why closing cold is the better answer.
Balance the water before the cover goes on, in this order
Whatever chemistry the water has on closing day is the chemistry it lives with until spring. The order matters as much as the numbers.
- Test everything a few days ahead, and fix alkalinity before pH. Total alkalinity is the water's shock absorber; bring it into its band first and read pH again, because half the time pH settles on its own once alkalinity is right. Small steps, one full turnover with the pump running between additions.
- Check CYA, because this is probably why last year's closing failed. Stabilizer at 30 to 50 ppm is where an outdoor pool belongs; a summer of trichlor tabs can leave it at 80 or 100, where it binds chlorine so tightly that a normal-looking dose does far less work. Nothing uses CYA up - the only fix is dilution, and fall is the right time: swapping out a quarter of the water takes roughly a quarter off the reading. Do it on a cool, overcast day, never all at once where the water table is high, and retest about 24 hours after the refill.
- The evening before the cover goes on, raise free chlorine to shock level. Shock level scales with stabilizer: a widely used rule of thumb is free chlorine around 40 percent of your CYA reading, so about 12 ppm at 30 ppm CYA and about 20 ppm at 50. In the evening, so the sun does not burn most of it off before it has done any work. For scale, the normal operating range at that CYA is only 2 to 4 ppm of free chlorine - a closing dose has to last.
- Add a winterizing algaecide. A polyquat algaecide rather than a copper-based one, which can stain plaster. This is the backstop for the shoulder weeks, not a substitute for the chlorine.
- Then do the mechanical close: lower the water below the skimmer, blow out and plug the lines, pull the ladder, fit and strap the cover. Chemistry first, hardware second - once the lines are plugged you cannot circulate anything you add.
The cover decides what kind of spring you get
Neither cover type is wrong, but they fail differently, and knowing which one you own changes your opening date.
A mesh safety cover is the safer choice and never needs pumping off, but it passes light, fine debris, and every rain and snowmelt straight into the pool. By March the water under mesh is diluted, nutrient-fed, and lit - three of the four things algae wants. A solid cover keeps the water dark and chemically close to how you left it, at the cost of a cover pump and a heavier lift twice a year.
The rule that follows: with a mesh cover, plan to open earlier. The dilution has a silver lining - it pulls CYA down over the winter - but light plus warmth plus no chlorine is not a combination to leave sitting into May.
The reopen-early rule
The spring mistake is the mirror image of the fall one: waiting for swimsuit weather. Open while the water is still below 60°F (15.5°C). You are not opening the swim season; you are opening the chemistry, so that circulating, filtered, chlorinated water is in place before the water crosses the algae threshold.
In much of the US that means April, not Memorial Day. Consensus guidance calls the water swimmable once it holds around 70°F (21°C) - which is precisely why the cover should come off well before then. An early opening on cold, clear water is an afternoon: cover off, refill to the normal level, unplug the lines and prime the pump, run the filter around the clock for a day, shock and balance. A late opening on 68°F (20°C) green water is a week of continuous filtering and pounds of chlorine. The cheapest algaecide you will ever buy is an early opening date.
If it happens anyway
Sometimes you do the close right and lose anyway - a freak warm winter, a cover that shifted in February. At that point prevention is over and recovery is a known, mechanical process: chlorine dosed in proportion to your CYA, the filter running continuously, daily brushing, patience. That full process is its own post: my pool is a green swamp - how do I clear it? The thing to salvage for next year is data: note the water temperature the day you closed and the day you opened, and move both toward 60°F.
The short version
Close when the water holds at or below 60°F (15.5°C), not when the calendar says so. Before the cover goes on: alkalinity first, then pH, CYA at 30 to 50 ppm (dilute it now if it is not), free chlorine to shock level the evening before, polyquat algaecide in. Know your cover: mesh means open earlier. And open before the water climbs back through 60°F, because the race restarts the moment it does.
Basinly is the pool app we built to keep this rhythm from depending on memory. The Care tab carries opening and closing checklists in working order - "Balance water before covering" is deliberately the first closing step - with windows that follow your hemisphere, and if you log water temperature the calendar line gives way to the water's own verdict: "Water at 58°F · the closing window is here." The Season panel flags the window and task reminders do the nudging, and every test and dose stays on file, so next spring you can see what temperature you closed at and prove it worked. It lives at the Basinly page.