Why Does Concrete Turn Green in Memphis?
If one side of your driveway is green and the other is not, nothing is wrong with the concrete. It is doing exactly what Memphis humidity makes it do.
What the green actually is
It is algae, and often a mix of algae, moss and lichen. Concrete is porous, so it holds moisture in its surface. Add shade, add humidity, add organic debris from trees, and you have created a growing medium. The green is a living colony, not a stain.
Why Memphis is so good at growing it
Long humid summers, mild wet winters, heavy tree cover and afternoon storms that leave surfaces damp for hours. North-facing surfaces and anything under a canopy stay wet longest, which is why the shaded strip of a driveway greens up first and worst while the sunny half looks fine.
Why it matters beyond looks
Algae holds moisture against the concrete. In freeze and thaw cycles that trapped moisture expands in the pores and accelerates surface spalling. On walkways and pool decks it is also genuinely slippery when wet, which is the reason we get called about pool surrounds more than any other single surface.
Why scrubbing and bleach from the hardware store disappoint
Household bleach at hardware-store dilution kills some surface growth but does not reach what is rooted in the pores, and it runs off before it has time to work. You get a visibly better surface for a few weeks, then it returns looking as though nothing happened.
The combination that works is the right solution at the right dwell time, then mechanical removal with a surface cleaner that holds even pressure across the slab.
How long a proper clean lasts
In our conditions, a well-cleaned driveway typically looks good for one to two years. Shaded areas under trees are at the shorter end. Full-sun areas often go longer.
You extend it by improving airflow and light where you reasonably can: trimming back overhanging branches, keeping leaves off the slab in autumn, and making sure downspouts are not discharging across the concrete.
Questions
Will the green come back after pressure washing?
Eventually, yes, because the conditions that grew it are still there. A proper clean that kills the growth rather than just blasting it off typically holds for one to two years in the Mid-South.
Does green algae damage concrete?
Over time it contributes. It holds moisture in the surface pores, which accelerates freeze-thaw damage and surface breakdown.
Can I just use bleach on my driveway?
It will lighten the surface briefly. Without the right dilution, dwell time and mechanical removal, the growth rooted in the pores survives and returns quickly.