That green or black discoloration on your driveway isn't just dirt — it's biological growth that keeps coming back if you don't address it properly.
If your driveway has green patches near the edges or shaded areas, or dark black/gray staining across the surface, you're looking at biological growth — not dirt, not tire marks, and not a concrete staining issue. It's algae and mold, and it has specific causes and specific solutions.
Green Stains: Algae
Green discoloration on concrete is almost always algae — the same single-celled organisms that turn your house siding green. Concrete is porous, which means algae can penetrate slightly below the surface rather than just sitting on top. That's why a basic garden hose doesn't remove it: you're not reaching where the growth actually lives.
Algae on driveways is most common in shaded areas (along the north side of the house, under trees) and in low-lying areas where water pools. Tennessee's humidity keeps concrete damp longer than in drier climates, which extends the growing season.
Black Stains: Mold, Mildew, and Exhaust Residue
Black or dark gray concrete staining can be a few different things. In shaded areas or near water, it's typically mold or black algae species — biological growth that looks similar to the Gloeocapsa magma that causes roof streaking. Near the garage apron and driveway approach, dark staining is often a combination of oil residue, exhaust deposits, and tire rubber — these need a degreaser pre-treatment to break down before washing.
Why Regular Hosing Doesn't Work
Garden hose pressure is too low to dislodge algae or mold that's penetrated into concrete pores. Consumer pressure washers improve on this, but without a rotating surface cleaner (the spinning disc equipment professional crews use), you'll likely get tiger stripe results — uneven areas where the wand passed at slightly different speeds and distances.
Professional pressure washing with commercial equipment removes the growth consistently across the full surface. For algae and mold, adding a cleaning solution that kills the growth means it comes back slower than if you only cleaned it mechanically.
What Makes Concrete Staining Worse in Tennessee
- High humidity year-round keeps concrete surfaces damp longer, extending algae growing seasons.
- Heavy tree canopy provides shade that prevents surfaces from drying out between rains.
- Tennessee's rain patterns — wet springs and wet falls — create two annual growth spikes.
- Concrete with a rough broom-finished surface holds moisture and debris more than smooth-finished concrete.
How to Keep It From Coming Back
After cleaning, applying a concrete sealer slows moisture absorption and makes it harder for algae to penetrate. Trim overhanging branches that keep the surface shaded. Keep gutters clean so water doesn't run across the driveway and pool at the edges. Most Cookeville driveways need cleaning every 1–2 years depending on shade and drainage conditions.
Driveway cleaning in Cookeville starts at $100–$200 for a standard 2-car driveway. Get your price with InstaQuote.