What Causes Green Algae on House Siding?
The green growth on house siding is most commonly Cladosporium (and related algae and mold species) that colonize moist, shaded surfaces. North-facing walls in humid climates like West Michigan are hit first because they get the least UV light — which would otherwise suppress the growth — and stay damp the longest. It feeds on moisture, pollen, and organic film on the siding, not on the siding itself, so a soft wash that kills it at the root clears it completely.
Why the north side goes green first
It comes down to sun and moisture. South and west walls get enough direct sunlight to dry out and to suppress organic growth with UV. North-facing walls — and any wall shaded by trees, a fence, or a neighbor's house — stay damp and shaded, which is exactly what algae needs. That's why the green almost always shows up on the north side and the shaded corners first.
Is it hurting my siding?
Over time, yes. The growth holds moisture against the siding, and persistent moisture is what causes vinyl to warp and wood to rot. It also traps dirt and looks like the house is aging badly. None of that damage is immediate, but letting it run for years is harder on the house — and on resale appeal — than clearing it periodically.
How to remove it (and keep it off)
A soft-wash house washing kills the algae with a low-pressure cleaning solution and rinses it away — no high pressure that could force water behind the siding. To slow its return, trim back vegetation touching the walls to let sunlight and air reach the siding, and keep gutters and downspouts from dumping water down the face of the house. For how often, see how often to wash your house.
Common questions
Is the green stuff on my siding mold or algae?
It's usually algae (commonly Cladosporium) or a mix of algae and mildew. It's a different organism from the indoor mold people worry about health-wise, but it does hold moisture against the siding, so it's worth removing.
How do I stop my house from turning green again?
A soft wash kills it at the root so it stays gone for years. To slow regrowth, trim vegetation back from the walls to let in sunlight and air, and keep gutters and downspouts from running water down the siding.