Pollen season and pool filter care

Why does the pool turn yellow in spring without being algae?

Because pollen is a fine, buoyant, organic dust and your pool is the largest flat collector in the yard. On a heavy release day a pool can go from clear to visibly yellow between morning and evening with nothing chemically wrong.

The two useful tells are location and behavior. Pollen collects on the surface first and along the downwind edge, where wind pushes it into a visible line or a raft. Algae starts on surfaces, in shade, and in the corners where circulation is worst. Pollen also brushes into a cloud and disperses. Mustard algae brushes off and leaves a clean patch behind that looks obviously different from the wall next to it.

The third tell is the skimmer. Open the lid. If the basket and the weir are coated in a uniform yellow dust, that is pollen doing exactly what it should be doing, which is getting caught.

How often should the filter be cleaned during pollen season?

More often than your normal schedule, and the number is set by pressure rather than by the calendar. Record the clean starting pressure for your system when the filter is genuinely clean. When it rises 8 to 10 psi above that, it is time, whether that is six weeks later or twelve days later.

During a heavy pollen run, a cartridge filter that would normally go three or four months can load enough to need attention in a few weeks. That is not a defect. It is the filter working. The mistake is holding to a quarterly schedule while the filter is choking, because a loaded filter stops catching fine particles and starts passing them straight back into the pool.

One nuance for cartridge owners: a hose rinse between full cleanings buys real time during pollen season. It will not deal with oils soaked into the pleats, and it is not a substitute for a proper soak, but knocking the surface layer off a loaded cartridge in spring is worth the twenty minutes.

Does pollen actually raise chlorine demand?

Yes, and this is the part people miss. Pollen is organic. Chlorine oxidizes organic material, so a pool taking a heavy pollen load is spending sanitizer on the pollen instead of holding a reserve against algae and bacteria. Your free chlorine reading falls faster than it did in February and you have not changed anything.

The practical consequence is a pool that goes green in April, which surprises people because the water is still cool. It is not a temperature failure. It is a demand failure. The chlorine was being consumed by pollen and nobody raised the dose to match.

So during the heavy weeks, test more often and hold the free chlorine target at the upper end of your normal range rather than the lower end. It is cheaper than the alternative, which is on this site under green pool cleanup at $450 to $1,250+.

Does the filter type change what you should do?

Cartridge filters catch fine pollen well and clog fastest. Plan on more frequent removal and rinsing, and own a second set of cartridges if pollen season is genuinely brutal at your house, so the pool is never down while a set soaks.

DE filters catch the finest particles of the three and handle pollen best, but they need backwashing and recharging more often in spring, and a spent grid or a torn fabric shows up faster under load. If you backwash a DE filter and the pressure does not drop back near its clean starting number, the grids need to come out and be looked at.

Sand filters are the weakest at fine pollen because the media simply passes particles below a certain size. If you are on sand and pollen keeps circulating back into the pool, a small dose of DE or a cellulose fiber additive through the skimmer will tighten the media considerably. That is a well established trick and it works.

How do you tell pollen from mustard algae?

Brush a patch and watch what happens. Pollen clouds up and disperses into the water column. Mustard algae comes off the wall in a sheet and settles again within a few hours, usually in the same shaded spot.

Check the chlorine. Mustard algae grows in the presence of low but detectable chlorine, which is exactly why it fools people, but it will not establish in water carrying a genuinely strong free chlorine residual. If your chlorine has been solid all week and you have a yellow film, it is very likely pollen.

Check the shade line. Mustard algae favors the north wall, the steps, behind the ladder and under the diving board. Pollen does not care about shade. It cares about wind.

If you are wrong in the direction of treating pollen as algae, you waste chemicals. If you are wrong the other way, mustard algae gets a two week head start and comes back all season. When in doubt, get the water read properly and look at the wall with somebody who has seen both.