Most people assume dirt runs down a house. On a lot of Central Texas homes the black growth does the opposite. It starts in the bottom two or three courses of stone and climbs, and by the time anyone notices it there is a dark band running along the base of the whole elevation.
The reason is moisture, not dirt. Algae needs damp more than it needs anything else, and the base of a wall is the dampest part of the house. Sprinkler heads throw water directly onto it twice a day. Rain splashes back up off concrete and paving. Ground cover planted tight against the foundation holds humidity against the stone long after the surface of the wall looks dry.
Staining concentrated around an air conditioning condensate line. Anything that keeps one patch of stone permanently damp will grow the same band.
It is not only irrigation. Air conditioning condensate lines run for months in a Texas summer, and a downspout that empties against the wall does the same thing. Wherever water is delivered to the same spot day after day, the growth follows it.
Limestone and stucco make it worse because they are porous. They hold that moisture in rather than shedding it, which gives the growth somewhere to live rather than just something to sit on. The same band is almost always sharpest on north and east elevations, because those never get enough direct sun to dry out properly.
◆ Same Corner, Before & After
The same garage door corner, photographed before and after. Same hose, same conduit, same planting.
This is why washing alone does not solve it. If the sprinkler head is still hitting the wall every morning, the growth comes back on the same schedule it did before. Part of walking a property is pointing out what is feeding it.