Why Is My Concrete Turning Green or Black?
The green or black film on a driveway, patio, sidewalk, or pool deck isn't dirt — it's living organic growth. The green is algae; the darker black-green patches are algae that has had longer to establish alongside mildew and mold. It thrives on concrete because the surface is porous, holds moisture, and stays damp in shade — which is why it shows up first on the north side, under trees, and along downspouts and lawn edges. It matters beyond looks: that biofilm gets dangerously slippery when wet. In a West Michigan summer of humidity and afternoon storms, a shaded slab can turn into a genuine slip hazard for kids and anyone in bare feet.
What the green and black actually are
Concrete looks solid, but it's porous and slightly alkaline — a comfortable home for organisms that just need moisture and a little shade. The green is algae, the same family of growth that greens up the shaded side of siding. The black-to-dark-green staining is usually that algae after it's had longer to dig in, mixed with mildew and mold feeding on the trapped organic film. It's not the same thing as the black streaks on a roof (that's a specific bacteria, Gloeocapsa magma), and it isn't simple dirt you can sweep off — it's alive and rooted into the pores of the concrete, which is why a broom and a garden hose barely touch it.
Why it's a real slip hazard — not just an eyesore
The reason to deal with green or black concrete isn't only that it looks neglected. That biofilm is slick, and it gets especially slippery when wet — after a summer rain, around a pool, or under a dripping downspout. Steps, walkways, and pool decks are exactly where people move fast in bare feet, and a shaded, algae-covered slab is one of the more common places a homeowner slips at home. If a walkway feels greasy underfoot or you can see a footprint scuff through the film, that's the growth telling you it's built up enough to be a safety issue, not just a cosmetic one.
Why a surface cleaner beats a hand-held wand
Concrete is one of the few exterior surfaces where higher pressure is the right tool — but only when it's applied evenly. A professional cleans it with a rotary surface cleaner: a spinning bar of jets under a housing that lays down an even pressure field across the whole slab for a uniform, stripe-free finish. An open wand waved by hand leaves the zebra-stripe pattern that often looks worse than the growth did. Just as important, we pre-treat the concrete with a cleaning solution that kills the algae and mold at the root before rinsing — so it's gone, not just knocked back to grow again in a few weeks. See driveway & concrete cleaning and patio & paver cleaning.
Getting rid of it for good — and keeping your plants safe
Because the growth is alive, killing it is what makes the result last. A solution-first clean removes the biofilm at the root rather than just rinsing the top layer, so a properly cleaned slab stays clear far longer than one that was only blasted. The organisms need shade and moisture to come back, so the prevention is about reducing both: keep gutters and downspouts flowing so water isn't dumping onto the concrete, trim back overhanging branches to let sun and air reach the surface, and clear leaf debris out of shaded corners. And because the cleaning solution is applied right next to lawns and garden beds, plant safety matters — a careful cleaner pre-wets and rinses surrounding landscaping so nothing gets burned. More on that in plant & pet safety.
Common questions
Why is my concrete green or black?
It's living organic growth, not dirt. The green is algae and the black-to-dark-green is that algae plus mildew and mold that have had longer to establish. Concrete is porous and holds moisture, so the growth takes hold fastest on the shaded, damp areas — the north side, under trees, and along downspouts and lawn edges.
Is green or black concrete slippery — is it dangerous?
Yes. The algae and mold biofilm gets very slippery when wet, which makes shaded steps, walkways, and pool decks a real slip hazard after rain. It's one of the more common places people slip at home, so it's worth clearing off high-traffic and barefoot areas rather than just living with the look.
Will a garden hose or scrubbing remove it?
Not really. Because the growth is rooted into the pores of the concrete and is alive, a hose and a stiff broom only take off the loose surface layer and it comes right back. It needs a cleaning solution to kill it at the root, then an even rinse or surface-cleaner pass to lift it out of the pores.
How long until green concrete comes back after cleaning?
If the growth was killed at the root rather than just rinsed off, a cleaned slab usually stays clear for one to three years with normal conditions. Concrete that stays shaded and damp — under heavy tree canopy or a leaking downspout — comes back faster, so improving drainage and sunlight keeps it clean longer.