Go and look at the north side of your house, then the south side. On most Jacksonville properties there is a visible difference, and the north wall is the one with a greenish or grey-green cast to it.
That is not dirt settling unevenly. It is a living colony, and the reason it picked that wall tells you everything about how to deal with it.
Why the north wall, every time
Algae needs three things: moisture, warmth and light. Northeast Florida supplies warmth and light generously. Moisture is the limiting factor — and moisture is really a question of how long a surface stays wet.
A south-facing wall in Jacksonville gets direct sun for much of the day and dries within an hour of a rain shower. A north-facing wall may get no direct sun at all. After an afternoon storm it can still be damp the next morning.
That difference in drying time is the whole explanation. Same house, same air, same rain — different outcome, because one wall is wet for two hours and the other for fourteen.
It also explains the other patterns you will notice: heavier growth under eaves and soffits where water lingers, a band at the bottom of the wall where irrigation and splashback keep it damp, and dense growth on any elevation shaded by a fence, a neighbouring house or an oak.
Why stucco is worse than vinyl
Stucco is porous and textured. Water sits in the surface profile rather than running off it, and the organism gets down into the texture where it is physically sheltered.
Vinyl and Hardie board are comparatively smooth and non-porous, so growth sits on the surface and comes off more readily. Two identical houses on the same Southside street with different cladding will not green up at the same rate.
This is also why stucco needs dwell time rather than force. Pressure cannot reach into the texture at all — only a solution that sits there long enough to work will.
Why scrubbing or blasting makes it come back faster
Removing the visible growth is not the same as removing the organism. Blast the wall with a pressure washer and you take off the top layer while leaving an established, living colony in the pores.
Worse, on stucco or painted surfaces you have now roughened the finish slightly — which gives the regrowth more surface to anchor into. That is the mechanism behind the common complaint that a house "gets dirty faster every year".
Soft washing works the other way round: a dilute biocide with a surfactant kills the colony, dwells for five to ten minutes, and then rinses off gently. Because nothing is left alive, it stays clean two to three times longer.
What actually slows it down
- Redirect sprinklers off the wall. The single highest-impact change on most properties, and it costs nothing. A head wetting the lower wall every morning is running a continuous algae experiment.
- Cut back planting touching the house. Shrubs against a wall stop airflow and hold humidity exactly where you least want it. Twelve to eighteen inches of clearance makes a visible difference.
- Thin overhanging branches. Less shade means faster drying.
- Fix gutter overflow. A gutter spilling down a wall keeps a permanent damp stripe. See gutter cleaning.
- Wash annually rather than reactively. Growth removed early has not established root structure, so each clean is easier and cheaper.
None of that eliminates it. In a humid subtropical climate with 52 inches of rain a year, nothing will. What it does is turn a nine-month cycle into an eighteen-month one.