Common Questions Answered
It's one of the most common questions we hear from homeowners across St. Peters, O'Fallon and St. Charles County — "why does my driveway keep turning green?" You clean it, it looks great, and then by the following summer it's green again. Here's exactly what's happening and what you can do about it.
That green coating on your driveway is algae — specifically a type of green algae that thrives on concrete surfaces in humid, shaded environments. Missouri's summers are essentially perfect growing conditions: warm temperatures, high humidity, frequent rain and plenty of tree canopy creating shaded, damp areas on your driveway.
Algae spores are airborne and land on every outdoor surface constantly. On a dry, sunny driveway they die off quickly. But on a shaded, damp concrete surface — especially one that hasn't been sealed — they take hold and spread rapidly.
Missouri sits in a humid continental climate zone. Summer relative humidity regularly exceeds 70%, and the greater St. Louis area gets around 40 inches of rainfall per year. Add in the mature tree canopy that shades driveways in established neighborhoods across St. Peters, O'Fallon, Chesterfield and Ballwin, and you have ideal conditions for rapid algae growth.
The north and east facing sides of your driveway are typically the worst — they get less direct sun and stay damp longer after rain. You'll also notice more growth near downspouts, under overhanging trees, and along the edges near your lawn where moisture is higher.
Important: Algae on concrete isn't just cosmetic. It creates a slippery surface that's a genuine safety hazard — especially when wet. Green driveways and walkways are significantly more slippery than clean concrete.
The main reason your driveway turns green again after cleaning is that most cleaning methods don't actually kill the algae — they just remove the visible growth. The root system stays in the pores of the concrete and regrowth begins almost immediately.
The second reason is unsealed concrete. Concrete is porous, and those pores absorb moisture and provide an ideal habitat for algae to establish itself. Sealed concrete is much more resistant to algae growth because the surface is less hospitable to moisture retention.
The most effective approach is a two-step process: professional pressure washing that removes all current growth, followed by a concrete sealer that closes the pores and makes future algae growth much harder to establish.
A properly cleaned and sealed driveway can stay algae-free for 2–3 years versus the 6–12 months you might get from cleaning alone. The sealer also protects against oil stains, road salt damage and Missouri's freeze-thaw cycles — making it one of the best investments you can make in your concrete.
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