Green and Black Growth on Fences

Why the north side is always worse

The shaded face of a fence dries slowest and stays damp longest, so algae establishes there first and heaviest. Panels near sprinklers, in tree shade, or backing onto irrigated lawn get the same treatment. It is a moisture map more than a dirt map.

What pressure does to wood

High pressure erodes the soft spring growth between the harder grain lines, raising the grain and leaving a fuzzy, rougher surface. That rougher surface then holds more moisture and more spores, so a pressure-washed fence often regrows faster than one cleaned gently. It also drives water deep into the timber.

The gentler approach works better here

Organic growth on wood responds to appropriate cleaning solution and dwell time at low pressure, which kills the growth rather than eroding the surface it lives on. The fence looks less dramatically transformed on the day and holds its result considerably longer.

Vinyl and metal fencing

Vinyl fence panels take cleaning easily but oxidise like siding under sun, so a chalky white residue may be degraded surface rather than dirt. Metal fencing near sprinklers develops rust staining that is a chemical rather than pressure problem.