Why is my concrete driveway turning black and green after only a few months?
A straight answer first, then what you can do yourself and when to call a handyman.
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The black and green is living growth: algae, mould and mildew that settle on concrete and feed on moisture, pollen and dirt. Concrete is porous, so it holds water and grit, and in warm, humid weather that growth can show up within months.
The nuance is where it appears. If the stains sit in shade, along the edges, or under a sprinkler or downspout, the cause is moisture that never dries out. If the whole driveway is evenly dark, it is usually a general film of dirt and growth. The fix is the same either way: wash it off at the right pressure, then deal with whatever keeps it wet.
Why it happens
Concrete looks solid but is full of tiny pores. Rain, dew and sprinkler spray soak in and sit there. Airborne pollen, leaf debris, soil and dust collect in the surface texture and give algae and mildew something to eat.
Heat, humidity and summer rain speed this up, which is why growth can appear within a few months. Shade makes it worse because the surface stays damp for longer. Black patches are typically mould or mildew, and green patches are typically algae. Both spread outward from the first damp, dirty spot.
In colder regions the pattern is different. Winter grit and road salt build up on the surface, and a spring wash clears it. The driveway can look dull or stained there too, but the cause is residue rather than growth.
What you can check yourself
Note where it grows
Walk the driveway and see whether the dark areas sit in shade, near a downspout, or where a sprinkler hits. That tells you what is keeping the concrete wet.
Test one small spot
Wet a patch and scrub it with a stiff nylon brush. If it lightens and feels slick under the brush, it is growth rather than a permanent stain.
Trim and redirect
Cut back overhanging branches and shrubs, and aim sprinklers away from the concrete. More sun and less water means slower regrowth.
Sweep off debris
Clearing leaves, soil and pollen regularly removes the food supply. Take care on damp, green areas, because algae is slippery underfoot.
When to call a pro
Call for help if the growth covers a large area, keeps returning after you clean it, or sits beside windows, stucco or siding. Pressure at close range can drive water into window seals and cracks, so the pressure and angle matter. Pavers need extra care because the joints can wash out.
One Call Handyman Repairs can line up a vetted, independent local handyman. The estimate is free, the handyman looks at the job and gives the price before any work starts, and you approve it first. Licensing requirements for this kind of work vary by state, so check with your state's licensing agency or local building department if you want to confirm what applies. Call (888) 471-8212 or send a request through the website form.
What the technician will do
- Pick the right pressureThe technician sets the pressure for the surface. Concrete, pavers, pool decks and fences each tolerate different levels.
- Use a surface cleaner on flat areasA surface cleaner covers driveways and patios evenly and avoids the stripes and etching that a bare wand can leave.
- Protect plants and openingsPlants are covered or rinsed, and windows, vents and other openings are protected so water is not forced where it should not go.
- Check paver joints afterwardsIf the driveway or walkway is pavers, the technician looks over the joints and re-sands them where washing has pulled sand out.
Related questions
- Will the black and green come back after washing?
- It can, especially in damp or shaded spots, because the surface is porous and the conditions that grew it are still there. Trimming back plants, redirecting water and sweeping debris slow it down. How often a wash is needed depends on your shade, humidity and rainfall.
- Can I rent a pressure washer and do it myself?
- On a flat concrete driveway away from the house, many homeowners can. Too much pressure can etch the concrete or wash out paver joints, and close-range spray can force water into seals and siding. A handyman technician can take on the trickier edges and surfaces.
- Is the green stuff damaging the concrete?
- It mainly makes the surface look dirty and slippery when wet. Keeping concrete clean and well drained is good upkeep, and a technician can point out any cracks or crumbling while they are there.
- Does a handyman pressure wash roofs too?
- No. A handyman does not pressure wash roofs. Washing is for driveways, pavers, pool decks, patios, walkways and fences.
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