If your exposed-aggregate driveway or patio keeps turning green or black, you’re not alone, especially in Wilmington-area humidity. That color usually comes from algae or mildew feeding on moisture and organic grime that gets trapped in the pebbles and pores, which is why it can come back quickly after rain or shade.
What makes exposed aggregate tricky is that the same texture that looks great also holds water longer and gives growth more places to hang on, so “just crank up the PSI” can leave you with a patchy result or even a roughened surface. In the sections below, you’ll see why certain spots regreen first and how to clean in a way that kills the growth without damaging the finish.
Why Exposed-Aggregate Shows Green/Black Faster
If you understand why the same few areas darken first, you can stop treating the whole driveway like a losing whack-a-mole job and start getting longer stretches of clean between washes.
Exposed-aggregate concrete turning green or black sooner is usually because it gives algae and mildew more places to hang on and stay fed. The rough pebbles and tiny pits trap dust and pollen. That micro-texture holds water longer after rain or in Wilmington-style humidity. That extended wet period gives algae and mildew more time to take hold. That’s why the staining can return quickly after a wet stretch.
For example, the shaded side of a driveway near shrubs often regreens while the sunny, breezy section stays cleaner. Stop blaming “bad concrete.” This Old House would tell you to fix the dampest spot first.
Find The Moisture Source In 5 Minutes
Walk the slab right after a rain or morning dew and look for the last places to dry. Those are your regrowth zones. If you don’t change why they stay wet, you’ll keep cleaning the same green algae on concrete on a loop.
In five minutes, check four common “feeders”: sprinklers hitting the surface (overspray stripes), downspouts dumping next to the concrete (a darker fan-shaped stain), and shade or blocked airflow from shrubs/fences (the consistently slick section). If you can’t point to one of those patterns, you’re not done diagnosing yet.
If you have any nearby planting beds, pre-wetting and rinsing thoroughly can help reduce the risk of cleaning runoff stressing your landscaping. Read more in our article: Protect Landscaping Driveway
Clean It Safely: Soft-Wash Logic, Not Max PSI
A homeowner rents a big pressure washer, tries pressure washing exposed aggregate, and ends up with a lighter, rougher strip that still speckles dark a week later. The fix is usually the opposite of what the machine makes you want to do.
On exposed aggregate, “hit it with the pressure washer” usually backfires. High PSI can etch paste and pop stones, like sanding a textured countertop, and the growth survives. The lower-risk approach is a soft-wash method. You kill the growth, let it dwell, then rinse under about 1,000 PSI.
For example, if your first pass with a rental washer made the paste between pebbles look lighter but left dark specks on the stones, you didn’t need more pressure, you needed better chemistry and patience. DIY if you can do a small test spot and protect plants; hire a pro if you’ve got lots of landscaping or you’ve already started roughening the finish.
Plant and pet protection is often the deciding factor between a simple DIY wash and calling a pro when you’re using any kind of treatment. Read more in our article: Cleaning Chemicals Landscaping Pets
Will It Come Back After I Clean It?
Only if those same areas keep staying damp longer than the rest of the slab. If you fix the moisture pattern (overspray or drainage), you’ll usually go from “weeks” to “months” before you notice regrowth.
Is The Black Stuff “Black Mold” And Is It A Health Hazard?
On outdoor concrete, the dark staining you’re seeing is usually algae or mildew, not the indoor “black mold” problem people worry about in walls. Treat it as a slip-risk and maintenance issue, and focus on killing the growth and keeping the surface from staying damp.
Is Bleach Safe On Exposed-Aggregate Concrete?
Chlorine-based cleaners can work because they kill the organism, but the risk is what they do to plants and runoff if you apply them carelessly. If you can’t thoroughly pre-wet and rinse landscaping and control where the rinse water goes, you’re better off hiring a pro.
Chlorine-based cleaners can burn leaves fast if you don’t pre-soak plants and manage where the rinse water flows. Read more in our article: Cleaning Chemicals Harm Plants Pets
Should I Seal Exposed-Aggregate To Stop The Green Film?
Sealer can slow regreening by reducing absorption. Sealer is not a magic fix if the slab still holds moisture. Only seal exposed aggregate concrete after the concrete is genuinely dry for several days and you’ve addressed the “stays wet” cause.
How Often Do I Need To Clean In Wilmington-Area Humidity?
Plan on at least a seasonal rinse/maintenance clean, and more often for shaded or north-facing sections that stay slick after rain. If you’re cleaning the same spot every few weeks, stop. Even the Home Depot or Lowe’s weekend-project mindset cannot out-pressure bad drainage.
Roof not getting any younger? Contact us at Contact us or call 910-241-1152 to find out where you stand.
