Prep first
Before you start
- Wet the concrete with a hose before anything else.
- Wear gloves, safety glasses and clothes you don't mind splashing.
- Scrub with a deck brush, then rinse thoroughly with the hose.
Dark streaks and green patches on concrete are algae, and they take hold wherever the slab stays damp and shaded. The bleach method only suits surfaces that can take it, so if your slab is unsealed or has an unknown coating, test a small patch before you commit. Fresh growth should lighten quickly, but a stain that has sat for months may only fade.
When removing algae from concrete, scrub the black streaks while the slab is still damp so the cleaner can work in. If a stain has sat for months, give the cleaning solution from the steps a full pass with the deck brush before you judge it. If it keeps coming back once the slab is clean, that's the point to call a pro.
Before you start
What concrete, brick & driveways can take
Safe to use3
- Water
- Hot water
- Soaking
Use with care5
- Chlorine bleach
- Oxygen bleach
- Solvents (alcohol, spot removers)
- Acids (vinegar, lemon)
- Scrubbing and abrasives
7 items
What you'll need
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- Plastic bucketHolds the solution, mixed one gallon at a time. (opens Amazon)
- Household disinfecting bleachMakes the standard 1/3 cup per gallon mix. (opens Amazon)
- Garden hose with sprayerWets the concrete and rinses it off afterward.
- Deck brushScrubs the growth off the surface. (opens Amazon)
- Broom or leaf blowerClears loose dirt and leaves before you start.
- GlovesProtect you from splashes. (opens Amazon)
- Safety glasses (opens Amazon)
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6 steps
Step by step
Step 1 of 6
Clear the concrete
Move furnishings, rugs, grills and toys off the concrete and out of the area. Sweep or blow away loose dirt and leaves with a broom or leaf blower so the solution reaches the growth.
Step 2 of 6
Water the plants and wet the concrete
Water nearby plants and landscaping first, then pre-wet the concrete with a garden hose and sprayer. The surface should be wet all over before any solution goes on.
Step 3 of 6
Mix the bleach solution
Put on gloves, safety glasses and clothes you don't mind splashing. Fill a plastic bucket with 1 gallon of water, then add 1/3 cup household disinfecting bleach.
Careful. Never mix chlorine bleach with ammonia, vinegar or other acids, or other household cleaners. Toxic gas can form.
Step 4 of 6
Scrub the solution on
Dip the deck brush in the solution and scrub the concrete from one end to the other. Work in sections so the whole area gets covered before the first part starts to dry.
Step 5 of 66 minutes
Let it sit, then rinse
Leave the solution on the concrete for 6 minutes. Then rinse the whole area thoroughly with the garden hose and sprayer.
Step 6 of 6
Re-water plants and let it dry
Water the plants and landscaping that were exposed to the solution, so they get a clean rinse. Let the concrete air dry completely before you put anything back on it.
Tip. Walk the rinsed area once and look for missed spots before it dries.
If you'd rather buy than mix
Best products for algae on concrete, brick & driveways
Pick a plain disinfecting bleach or an outdoor bleach, and check the label for the dilution, since strengths differ between brands.
Amazon Basics Microfiber Cleaning Cloths
How we picked these: each product is made for this type of stain, its maker doesn't warn against using it on concrete, brick & driveways, and its directions fit the steps above.
Finished the steps? How does it look?
Nice work. Let it dry completely and check it again in good light. Here is what done looks like:
How to tell it's really gone →A faint mark usually needs one more round, not a stronger product.
Heavy buildup usually takes more than one round.
Heavy or long-standing buildup →When to call a professional →How to tell it's working
Each scrub and rinse should lighten the growth, and the tone of the slab should even out as the green fades. Look at it once it's completely dry, since wet concrete always reads darker than it really is. If the streak looks the same after two full rounds, it has likely set in, so move on to the dried-stain section.
What changes for yours
Adjust for your concrete, brick & driveways
Black algae on concrete
Black streaks on concrete usually lift with the same bleach method. Give the solution the full 6 minutes, and scrub a little harder along the streak lines with the deck brush.
If the dark lines are still visible after rinsing, repeat the full method on those spots only. Dark growth that has been on the concrete a long time may need the stronger mix covered below.
Green algae on a vertical wall
Walls and other upright surfaces can't be scrubbed the same way as a flat patio. Use a sponge or mop on vertical sections instead of the deck brush, working from the bottom up.
Keep the solution from running down onto plants at the base of the wall, and rinse the wall fully so no solution runs back down over the same spots.
Moss on concrete
Moss is a plant, and it holds water in its clumps, so it needs more scrubbing than the flat algae film. Work the deck brush into the clumps until they loosen, then rinse them away.
Moss usually grows in shaded, damp corners, so expect to repeat the method there. Check those spots again after the concrete has dried completely.
Algae on a shaded patio
A shaded patio stays damp longer, so the growth often comes back faster than it does on sunny concrete. Give the area a thorough rinse after cleaning and let it dry before you use it again.
If the shade is heavy, moving some furniture or planters so air can reach the surface helps slow regrowth. Keep the pre-wetting step in place, because a dry surface doesn't take up the solution evenly.
Heavy or long-standing buildup
Old growth that has built up over months or years is the hardest to shift, so expect one pass to lighten it rather than erase it. Start with the standard mix, then move to the stronger option if the stain is still dark.
Growth that has dried is more stubborn, so wet the area again before the stronger mix goes on. Mix 2 ⅔ cups outdoor bleach with 13 ⅓ cups water to make 1 gallon, load it into a pump sprayer, and apply it evenly. Let it sit for up to 15 minutes, scrub with the deck brush as needed, then rinse.
Once it's rinsed, let the concrete dry completely and check it in daylight. If the stain is still there, repeat the stronger treatment once more, and accept that a faint shadow may stay on very old growth.
Vinegar, baking soda, peroxide and more
Do home remedies work on algae on concrete, brick & driveways?
Most household remedies don't do the job on algae on concrete, so the bleach steps are the only treatment here. Anything beyond the bleach solution adds nothing for this stain.
Skip6
What makes it worse
Mistakes to avoid
Don't skip the plant watering
Skipping the plant watering is the easiest way to end up with damaged leaves. The solution can land on them, and a rinse afterward doesn't undo that.
Don't put furniture back too soon
Wait until the concrete is completely dry before moving furniture, grills or toys back onto it. Items set down on a damp patio keep that spot wet, which is the condition algae likes.
Don't mix bleach with an ammonia cleaner
Bleach and ammonia together release a toxic gas, and that includes residue from an ammonia-based glass cleaner left on the patio. Rinse the slab fully before you use any other product on it. Keep the two in separate buckets and sprayers so they never meet.
Don't let the solution dry on the slab
Bleach that dries on concrete can prevent a complete rinse and leave a residue behind. Work in sections so each one stays wet until it's rinsed. Give the whole area a final pass with the hose before you walk on it.
Don't assume the bleach method fits every surface
The bleach-and-water method is written for sealed, non-porous outdoor surfaces such as stucco, painted wood, tile and patio stone. Raw or unsealed concrete can soak the solution up, so test a small patch and watch for discoloration. If the patch looks off, stop and reconsider the method before you treat the whole slab.
When to call a professional
A restoration contractor should handle historic masonry, and laser cleaning or abrasive blasting need a specialist operator. If an old sealant or water repellent keeps the cleaner from working, don't push harder on your own. An unidentified mark can go to a testing lab, which is safer than trying harsher products.
Questions
Why does algae appear in concrete?
Algae spread through the air as spores, and they settle wherever the surface stays damp. Shaded spots and areas that rarely dry out are the most common places to see the dark growth. Keeping those areas drier and in more sun helps slow it down.
Will the algae come back after I clean it?
It can, because spores keep arriving on the wind and any damp spot can host new growth. Cleaning removes what's there now, and drying the surface quickly makes it harder for it to return.
How long before I can use the patio again?
Wait until the concrete has air dried completely before you put furniture, grills, toys or décor back on it. Rinse everything well first, so no solution is left sitting on the surface.
What if a dark stain is still there after one treatment?
Repeat the steps on the spots that still show, or move to the stronger option described above. Give old growth more than one pass rather than expecting it to vanish at once.
Do I need to protect my plants?
Yes. Water them before you start and again after rinsing, and keep the solution off leaves and the soil around them.
Will a pressure washer clear algae off concrete?
Light pressure can work, but high force is a bad idea on masonry. Use a wide fan tip, keep the nozzle well back, and rely on warm water with a mild detergent rather than more pressure. Historic stone should never be power washed at all.
Why do I need to wet the concrete before cleaning?
Dry concrete soaks up the cleaner along with the dirt it loosens, and that can leave a mark that's harder to lift. Wetting the slab first keeps the solution on the surface where your rinse can carry it away. Keep it damp until the final rinse so nothing dries onto the surface.
Can I use bleach on concrete?
Yes for algae and moss growth, provided the surface is one that can take it. Test a small patch first, and keep the solution off nearby landscaping. Never combine it with ammonia, which creates a dangerous gas.
Algae on other materials
Other stains on concrete, brick & driveways
Start from the bigger picture
How we know this
Every method, amount and warning here comes from university extension services, industry bodies and product makers. We read them, check every step for unsafe combinations and changed amounts, and write it up so you can follow it.