· by Michael Blair, Nashville's Pressure Washing
Nashville humidity plus a shaded elevation grows a green film reliably. It is usually algae rather than mold, and the distinction matters mostly because the treatment is the same and the panic is not warranted.
Why the north side
It stays damp longest. Direct sun dries a wall and inhibits growth; a north elevation shaded by the house itself or by trees never gets that.
Add overhanging vegetation and poor airflow and you get the classic Middle Tennessee green wall.
Removing it properly
A dilute sodium hypochlorite solution with a surfactant, applied at low pressure, left to dwell, then rinsed. That kills the organism rather than removing its visible layer.
Pre-wet planting before and rinse it thoroughly afterwards, because the solution is effective and hard on foliage.
- Soft wash with a dwell time, not high pressure
- Pre-wet and rinse surrounding planting
- Cut back vegetation touching the wall
- Improve airflow where you can
- Expect to repeat every one to three years here
Preventing the return
Trim vegetation back from the wall to improve airflow and let some light in. Redirect any sprinkler that hits the siding.
Nothing prevents it permanently in this climate on a shaded wall — expect a repeat every one to three years.
When it is more than algae
Green on the wall is cosmetic. Black staining that follows a specific pattern — beneath a window, below a gutter joint, in a vertical stripe — points at water rather than at growth.
That is worth investigating, because the wall behind may be wet.