Why Is My Driveway Turning Green or Black?

Quick Answer

Green discoloration on concrete is usually algae feeding on moisture and organic debris, while black or dark gray patches are typically mold or mildew growing in shaded, damp areas of the slab. Both thrive in low-sunlight spots like under porches, near downspouts, or beneath dense tree cover, and both can be removed with the right cleaning solution and pressure washing.

Green Stains: Algae Growth

Green discoloration on driveways and walkways is almost always algae. It needs moisture, organic matter like leaves or grass clippings, and limited direct sunlight to thrive, which is why it tends to show up on the shaded side of a driveway or in low spots where water pools after rain. Algae itself isn't usually harmful to the concrete, but it does make surfaces slippery and unattractive.

Black or Dark Gray Stains: Mold and Mildew

Dark, almost charcoal-colored patches are typically mold or mildew, which favor the same damp, shaded conditions as algae but tend to dig deeper into the concrete's surface. Mildew can also create a musty smell, especially after rain, and in some cases can make the surface noticeably more slippery than algae alone.

Why It Keeps Coming Back

If the underlying moisture and shade problem isn't addressed, algae and mildew will return even after a thorough cleaning. Poor drainage, clogged gutters dumping water onto the driveway, and overhanging tree branches are the most common repeat offenders here in Cuyahoga, Lorain, and Medina County, where humid summers and shaded yards are the norm.

How Professional Cleaning Helps

A surface-level pressure wash can knock visible algae and mildew loose, but a cleaning solution formulated to kill the organism is what actually prevents it from regrowing within a few weeks. We pair hot water pressure washing with the right cleaning agents to fully clear stains and slow regrowth, then can recommend a sealer to make future cleanups easier.

Frequently Asked Questions

Is green algae on my driveway dangerous?

It isn't toxic, but it does make surfaces slippery, especially when wet, which can be a real fall hazard on steps and walkways.

Will bleach get rid of mildew on concrete?

Bleach can lighten mildew temporarily, but it doesn't penetrate concrete pores the way a dedicated cleaner does, so regrowth tends to happen faster.

How can I stop algae from coming back?

Improving drainage, trimming back shading trees, and applying a concrete sealer after cleaning are the most effective long-term solutions.

Get Your Driveway Clean and Safe Again

Combat Power Washing is a veteran-owned, family-operated concrete cleaning company serving Cuyahoga, Lorain, and Medina County. We'll clear away algae, mold, and mildew stains and help you understand what's causing them to keep coming back. Request your free, same-day quote today.