Black or Green Growth on a Concrete Patio or Sidewalk

Why the growth lives in the concrete, not just on it

Concrete looks solid, but it is full of tiny pores. PROSOCO, a company that makes masonry cleaners, puts it simply: mildew on concrete starts with trapped moisture, because the slab’s pores and texture hold water (PROSOCO mildew guide). A slab that dries quickly after rain stays fairly clean. A slab that is still damp the next morning becomes a place for growth to settle in.

The usual reasons a slab stays wet, according to the same guide:

• It is shaded or faces north, so the sun never dries it.
• It sits under dense plants or trees.
• A sprinkler head leaks or overspray hits it every day.
• It has poor pitch or a clogged drain, so water sits.

Our own driveway page describes the same thing on Louisville drives under heavy tree cover: shaded concrete does not stay gray, it turns dark.

Why it gets slippery

Some of what grows on wet concrete is slick. Purdue University’s extension service describes a ground algae called Nostoc that shows up on concrete in hot, rainy weather and calls it a slippery safety hazard (Purdue Extension). If a shaded walk or the steps off a patio feel greasy after rain, growth like this is a likely reason.

What not to do

PROSOCO warns that household recipes often reach for bleach, vinegar or aggressive pressure washing, and that harsh chemistry and too much pressure can damage masonry. It also warns that bleach can leave residue, stain masonry and lighten the color (PROSOCO mildew guide). Decorative, stamped and sealed concrete is the most at risk, so any cleaner should be tested on a small area first.

How we clean it

Plain concrete driveways, sidewalks and walkways are the one place we use high pressure: cold water at high pressure, with a degreaser added where the surface needs it. Patios and pool decks are matched to what the surface can take. Stamped or sealed concrete is not treated like a bare driveway.

PROSOCO’s guide also says to rinse thoroughly so no residue dries on the surface, and to let the slab dry before judging what still needs work.

Keeping it from coming back

PROSOCO’s advice is to fix the moisture first: correct the drainage, trim back growth that blocks air, and adjust the sprinklers. It says a water repellent applied to a fully dry slab can greatly reduce future growth (PROSOCO mildew guide). We do not seal concrete, so that step is yours to decide.