Why do my pavers turn black in summer?

NOAA's thirty-year normals for Naples Municipal Airport draw the line clearly.

June, July, August and September average 7.41, 7.77, 8.89 and 8.60 inches of rain, at average temperatures of 82.1, 83.1, 83.3 and 82.4 degrees F. January and December sit at the other end: 1.68 and 1.32 inches, at 65.3 and 68.2 degrees. The rainy season months carry several times the rain of those dry season months, while the average temperature moves by less than twenty degrees between them. The full twelve-month table of Naples normals lays out every month the same way.

The EPA says there are many types of mold and none of them will grow without water or moisture, and that the key to mold control is moisture control. UF/IFAS, writing about nurseries and greenhouses, says Nostoc-like cyanobacteria, a blue-green algae, grow on the surface of soil, gravel, cement and even plastic containers as mats that dry and flake in dry periods but swell with any moisture into thick, dark green, gelatinous masses. UF/IFAS does not write about pavers; cement is the closest surface it names. No document cited here names the organism on a particular Naples paver, so the honest description is dark growth, with mold, mildew, algae and cyanobacteria among the candidates.

A seasonal resident who is away from June to September misses all four of the wettest months in the normals, so a whole rainy season passes between one look at the driveway and the next.

UF/IFAS Extension says mildew requires moist, warm air. In NOAA's normals the four wettest months, at 82.1 to 83.3 degrees, are also warmer than January and December, and even January averages 65.3, so the season that brings the water keeps the warmth too. No document cited here publishes the temperature the dark growth on a paver needs, so those averages describe the Naples year, not a growth threshold.

UF/IFAS adds that in a greenhouse the cyanobacteria mats grow quickly with light, high humidity, warm temperatures and nutrients from fertilizers and irrigation, and that increasing drainage and reducing overhead irrigation can decrease them. Irrigation that wets a driveway in the dry months supplies moisture the sky does not. Pavers beside a pool add moisture of their own, a question taken up at whether pavers grow mold faster near the pool.

The EPA says mold spores float through outdoor and indoor air, and UF/IFAS says physical removal of the cyanobacteria mats gives only short-term control because these algae will eventually return. A cleaning takes off what is on the paver; it does not take the spores out of the air or the rain out of the summer.

How the growth comes off, material by material, is covered at whether mold can be removed from pavers. Whether a sealer changes the picture is answered at whether algae comes back after sealing, and what September does to a Naples paver driveway covers the second-wettest month on its own.