A slippery green driveway in the Ohio Valley is usually live algae growing in the pores of the concrete, not dirt sitting on top. Pressure washing lifts the visible film, but because it does not kill the organism, the green often returns on shaded, damp surfaces.
Why it grows here
Ohio Valley humidity, shade, and a driveway that stays damp after rain create the conditions algae needs. North-facing and tree-covered driveways grow it fastest, and it gets dangerously slick when wet.
What pressure washing does and does not do
A surface cleaner gives an even, stripe-free clean that removes the visible growth and the slick film. But pressure alone does not kill algae at the root, so on heavily shaded driveways it can regrow the next humid season. For lasting results, a soft-wash treatment of the algae followed by sealing is more effective than pressure alone.
Why sealing matters
Sealing fills the pores of the concrete so algae has less to grab onto and water does not sit in the surface. It is the step that turns a one-time clean into a lasting result, especially in freeze-thaw country.
Five-minute self-check
- Walk the driveway after rain and note where it feels slick.
- Look for dark, green-tinted patches, especially in shade.
- Check where water sits and drains slowly.
- Note how much sun the driveway gets during the day.
- See whether it has been more than a year since the last clean or seal.
Related reading
See our driveway cleaning and concrete sealing pages, and our post on orange rust stains on concrete.