Why are there black or green stains growing on my concrete patio?

You’re seeing black or green stains because your concrete stays damp long enough for something to take hold. Most of the time, that “growth” is algae or mildew living in a thin surface film; it can also be black-spot lichen that anchors into porous concrete. Sometimes it isn’t growth at all—mineral deposits (efflorescence) or leaf tannins can discolor the surface.

The frustrating part is that the same patch usually comes back. The same conditions keep feeding it like a wet terrarium: shade and ponding. In the sections below, you’ll use quick, reliable clues to figure out what you’re looking at, then match it to a cleaning approach that’s safe for concrete and helps slow regrowth instead of just “power-wash it and call it a day.”

Quick Stain ID by Clues, Not Color

Your neighbor calls it “mold,” the internet calls it “algae,” and the first cleaner you grab for mold on concrete patio might make zero difference. Two quick touch-and-scrape checks can save you from treating the wrong problem.

Black-spot lichen on concrete can look like “stuck-on” peppery dots that don’t lift with normal scrubbing. Read more in our article: Black Spots Concrete

Why It Keeps Coming Back Here

You scrub it clean, it looks fine for a week, and then the outline reappears as if you never touched it. When the outline repeats, the water source is usually still doing its job.

When staining keeps reappearing in the same footprint, it tends to come back quickly—especially in shaded areas. You’re usually tracking a repeat moisture source plus a place that traps grit and organics. Low spots and edges that stay dark after rain point to ponding; joints and cracks act like tiny gutters that hold debris.

The trap is thinking you just need a stronger wash. That is a bad idea. Instead, treat the stain map like a leak map: fix runoff, redirect spray, clear leaf litter from seams, and you cut regrowth dramatically.

If water is pooling or repeatedly soaking the same area, correcting the slope or drainage usually matters more than switching to a harsher cleaner. Read more in our article: Standing Water Driveway Issue

Clean Safely and Stop Regrowth

You can get a lasting clean without turning it into a chemical experiment or chewing up the slab. The win is removing the film and its food, then dialing back the daily re-wetting that rebuilds it.

You’ll get better results if you clean what’s feeding the stain before you try to “blast it off.” Case in point: a shady slab with a thin layer of leaf grit will look clean after a rinse, then turn green again fast because the biofilm never lost its food source.

Start with dry removal (sweep and scrape joints), then wash with a stiff brush and a mild detergent. If you’re dealing with obvious organic growth (slick green film, mildew-like darkening), step up to a sodium hypochlorite-based cleaner for sodium hypochlorite concrete cleaning, but don’t fall into the “stronger is always better” mindset. Aim for a controlled working strength (many disinfection guides land roughly in the 0.5%–2% range), follow the label, and don’t mix bleach with ammonia-containing cleaners.

Use pressure washing as a rinse step to help lift algae off concrete, not the whole plan with a Ryobi or SIMPSON unit. Keep the pressure moderate, use a wider fan tip or surface cleaner, and avoid digging into the paste.

To slow regrowth, reduce daily re-wetting—how to prevent algae on concrete: fix downspout splash and stop sprinkler overspray.

Using too much pressure or the wrong tip can etch concrete and create a rougher surface that holds onto biofilm even faster. Read more in our article: Pressure Washing Concrete Safe