Why Alabama Humidity Turns Everything Green
The reason your north wall and your shaded patio go green here and would not in Arizona, and what that means for how often anything outside needs cleaning.
This is not a microbiology paper. But the pattern here is not subtle.
Surfaces in East Alabama go green. Fast. Faster than the same surface would in most of the country. There is a reason.
Three things have to line up
Algae needs moisture, something to hold onto, and light. Take away any one of them and it does not establish.
In this part of the state, all three are available nearly year round.
Moisture. Humidity here means a surface can stay damp without it having rained. Morning dew sits. Something in shade may not dry out fully between one night and the next.
Texture. Concrete is porous. Vinyl siding has a grain. Wood has grain and open pores. All of it gives algae something to grip.
Light without heat. Shade is not darkness. A patio under a tree is getting plenty of ambient light. What it is not getting is the direct sun that would dry it out.
Put those together and a shaded concrete patio in Lee County is close to ideal growing conditions from about March through November.
Why the shade line is so sharp
If you have ever looked at a driveway where half is clean and half is green, and the line between them is almost straight, you have seen this in action.
That line is the edge of the shade. One side dries out every day. The other does not. Same concrete, same age, same weather, completely different result.
The same is true on a house. The north side is the one that goes first, every single time, and it is not because that wall gets dirtier.
Trees make it worse in more than one way
Tree cover gives you the shade. It also drops leaves, pollen, sap, and berries onto the surface, and all of that is organic material sitting on a damp surface.
Auburn in particular has a lot of mature tree cover, and a house under it will not hold a wash as long as one on an open lot ten minutes away.
What it means practically
Nothing you clean stays clean permanently. Anybody who tells you otherwise is selling something. In this climate, expect a cleaned house to hold for roughly a year and a cleaned driveway for one to two, depending on shade.
Killing it beats rinsing it. This is the whole argument for using a cleaning solution instead of just pressure. Pressure removes what you can see. A solution kills what is growing. The difference in how long the result lasts is enormous.
Drainage and airflow are worth more than cleaning frequency. If a shrub is holding moisture against a wall, trimming it back will do more for that wall than an extra wash. If water pools at a low spot on the drive, fixing the drainage fixes the stain.
Pressure washing your patio in February is fine. People assume this is warm weather work. It is not. Cleaning in the cooler months works, and it is usually easier to get on a calendar.
The honest version
You cannot beat the climate. You can stay ahead of it. Growth that has been on a surface for one season lifts easily. Growth that has been there for five has worked into the material and sometimes leaves a shadow behind even after a thorough cleaning.
That is the real argument for maintaining on a cycle instead of waiting until it looks bad from the street.
Have a surface you are not sure about?
Send a photo and tell us what you are dealing with. You will get a straight answer, including if the answer is that cleaning will not fix it.
