Why it is always the north wall
North-facing walls in the northern hemisphere receive almost no direct sun. In Los Angeles, and especially within reach of the marine layer, they may not fully dry for weeks at a time. Permanently damp porous surface plus airborne spores is all algae needs.
Shade from other sources
It is not only compass direction. Walls shaded by a neighbouring building, a mature tree, a dense hedge or a deep overhang behave the same way. Side yards between closely spaced houses are frequently the worst surface on a property.
Why scrubbing fails
Algae in porous stucco is not sitting on top of the surface — it is rooted into it. Mechanical removal takes the visible layer and leaves the root structure, which regrows within a season. It comes back looking worse because the surface has been opened up.
What actually removes it
A cleaning solution applied at low pressure with enough dwell time to kill the organism at root level, then a gentle rinse. Done correctly, the wall stays clear far longer because you have killed it rather than shaved it.
Reducing how fast it returns
Cut back vegetation touching or shading the wall. Redirect sprinklers that spray the surface. Improve airflow where you can. None of these eliminate the problem in a marine-influenced climate, but they meaningfully extend the interval between cleanings.
Key points
Why it is always the north wall
North-facing walls in the northern hemisphere receive almost no direct sun. In Los Angeles, and especially within reach of the marine layer, they may not fully dry for weeks at a time. Permanently damp porous surface plus airborne spores is all algae needs.
Reducing how fast it returns
Cut back vegetation touching or shading the wall. Redirect sprinklers that spray the surface. Improve airflow where you can. None of these eliminate the problem in a marine-influenced climate, but they meaningfully extend the interval between cleanings.