Why does everything in Naples grow mildew?

Start with the word. Cambridge Dictionary defines mildew as a black, green or whitish area caused by a fungus that grows on things such as plants, paper, cloth or buildings, usually when conditions are warm and wet. That is the everyday sense, and it is the sense most people mean when they say a Naples house grows mildew.

The roofing documents take that word apart. ARMA writes: “Often mistaken as mold or mildew, the black discoloration and streaks visible on many roofs across the United States and Canada are actually algae.” UF/IFAS Extension, in a household guide, puts mildew inside a larger group: “Mildew is mold that can attack cellulose and lignin.” So the dark streak on a roof and the mildew the dictionary describes on paper or cloth are not the same organism, even when one word gets used for both. The dictionary sense is set out under mildew, defined, and the roof version of the distinction under the difference between algae and mold on a roof.

TRI Alliance names three kinds of growth together on tile roofs: “Mildew, algae and moss do not grow directly on tile but on the accumulation of dust and other organic materials that settle on tile.” And on hard surfaces lower down, a UF/IFAS bulletin on Nostoc describes cyanobacterial mats on gravel and cement in nurseries that turn flaky in dry spells and swell again with water. That bulletin is about production surfaces, not any particular walkway, and what it can and cannot say about a green paver joint is set out under why the sand in paver joints turns green.

The organisms differ. What the documents say they need does not.

EPA's home guide says: “There are many types of mold, and none of them will grow without water or moisture.” It also says spores are everywhere to begin with: “Molds reproduce by means of tiny spores; the spores are invisible to the naked eye and float through outdoor and indoor air.” UF/IFAS Extension writes that mildew “requires moist, warm air.”

The roofing documents say the same about algae. ARMA writes that algae species are transported through the air and tend to collect and grow on roofing structures and other outdoor surfaces, that algae can form where moisture tends to dwell on roof surfaces, and that “Coastal or humid climates create the most conducive conditions for algae growth.” TRI Alliance writes that with enough cycles of heat, moisture and light, spores start to grow into the mildew, algae and moss it names.

Put plainly: the spores exist almost everywhere, as UF/IFAS puts it, and they grow where water stays. That is the whole mechanism behind a place that seems to grow mildew on everything.

If water is the condition, the Naples record shows how much of it arrives and when.

NOAA's 1991 to 2020 normals for Naples Municipal Airport make August, at 8.89 inches, and September, at 8.60, the two wettest months, and by simple arithmetic the four months from June to September hold about 65.9 percent of the year's normal rain. NWS Miami adds that showers and thunderstorms are almost a daily occurrence in a southeast Florida summer. No humidity percentage for Naples appears in any source read, so none is printed here; dew points and what they measure are taken up under whether Naples humidity is why exteriors get dirty.

The setting adds to it. TRI Alliance lists the environmental variables that affect organic growth on a roof: “location of the structure, the direction the roof is oriented to the sun and the quantity of trees, canals and lakes in the area.” The roof page shows the full table of normals and how they meet the roof, under why Naples roofs stain so quickly.

Knowing the mechanism does not identify what is on a given roof or driveway, and the documents show how easily one growth is mistaken for another.

ARMA writes that algae discoloration “may be mistaken for fungus growth, soot, dirt, moss, or tree droppings.” A dark patch can be any of those, and a page cannot tell which. The roof-specific answer to the most common version of the question is under whether the black streaks are mold or algae.

What 239 Exteriors itself says about growth is printed in the company's own words for each surface it names, starting with roof cleaning at 239 Exteriors.