Hardie board earned its reputation: fiber cement takes a Chicago winter better than almost anything you can side a house with. But what you look at is not the cement, it is the finish, and the same algae that colonizes vinyl grows on fiber cement just as happily.
What shows up on it
The usual tenants: a green haze on the north wall, black dots along the trim, streaks under the eaves, anywhere the wall stays damp. It is growth, not dirt, which is why a hose and a brush buy you a few weeks and not a season.
Why we keep the pressure wand away from it
Force is the wrong tool on any siding. It can push water up behind the boards and into the joints, chew a painted edge, and smear the living growth around instead of killing it. A tough substrate does not make that a good trade. It just means the damage shows up in the caulk lines and the finish first.
The gentle version
Hardie board gets the same soft wash as vinyl and cedar: the solution goes on low and slow, the algae and mildew die at the root, and the siding rinses clean with the paint, caulk, and seals left the way we found them. It is on the short list of sidings we get asked about most, and the answer is always the same: washed the gentle way.
If your fiber cement siding is showing green, send a photo of the worst wall. You get a straight answer and a real number back.