Direct answer
What should you know first?
Green or black staining on home siding is usually algae or mildew thriving in shaded, damp elevations. Rinsing with a garden hose or blasting with high pressure removes only the surface layer and can force water behind the siding. Soft washing applies a cleaning solution at low pressure, kills the growth at the root, and keeps the finish intact.
The stain is living growth, not dirt
Algae spores settle where surfaces stay damp: the north-facing side of a home, elevations shaded by trees, areas behind shrubs, and spots where irrigation or roof runoff keeps the wall wet. Once established, the growth holds moisture against the siding and slowly darkens the surface.
Because it is biological, the stain returns if only the visible layer is removed. Effective cleaning has to address the organism, not just its color.
- North-facing and shaded elevations stain first
- Areas under trees and near sprinklers follow
- Stains darken gradually over one or two seasons
Why rinsing alone doesn't work
A garden hose moves loose dirt but leaves the growth in place. High pressure can strip the surface layer, but on vinyl, wood, and painted siding it risks forcing water behind seams, lifting finish, and leaving the spores that start the next stain cycle.
Soft washing pairs low pressure with a cleaning solution and dwell time, so the treatment reaches the organism without driving water where it does not belong.
Keep the result longer
After a soft wash, small changes slow regrowth: trimming shrubs back from the wall, redirecting sprinkler overspray, and clearing gutters that drain onto siding. Most homes then stay clean for a season or two, depending on shade and exposure.
When the first faint tint reappears, an early cleaning is quick and inexpensive compared with waiting until the wall is fully stained again.