Cleaning a Concrete Patio in Charleston, SC: Green Growth, Black Spots and White Bloom

Short answer: a Charleston concrete patio cleans up with a diluted sodium hypochlorite mix and a surface cleaner at moderate pressure - but only if what you are looking at is organic. Green film and black spots are living growth, and they wash off. A chalky white haze is not growth at all: it is mineral salt pushed up out of the slab from underneath, and bleach will not touch it. Work out which of the three you have before you buy anything.

What is actually on the slab?

Look at the patio after a dry day rather than a wet one. Almost everything you will see on Lowcountry concrete falls into one of three groups, and they do not respond to the same treatment.

• Green film - algae. It grows wherever the slab stays damp, which under a mature live oak is most of the year. It is the easiest of the three to remove and the fastest to come back.
• Black or dark grey spots that resist scrubbing - usually lichen. Lichen anchors into the pores of the concrete rather than sitting on top of it, so it needs chemical dwell time, not force. Trying to blast it off is how patios end up permanently striped.
• A powdery white bloom - efflorescence. Moisture moves up through the slab, dissolves salts inside the concrete, evaporates at the surface and leaves the mineral behind. Charleston's high water table, marsh-edge lots and low-lying streets make this common on ground-level patios, and it often shows up after a long wet stretch or a king tide.

How do you clean the green and black growth?

The order matters more than the equipment does.

1. Clear and sweep first. Move furniture, planters and grills off the slab entirely. Anything left in place leaves a clean outline you will be looking at all summer.
2. Soak the plants around the patio with plain water before you start, and rinse them again when you finish. Wet foliage takes far less damage from overspray than dry foliage does.
3. Apply the diluted mix with a surfactant and let it dwell. Dwell time is what kills the growth - pressure only removes what is already dead. On a hot afternoon, keep the surface wet so the mix cannot dry on the concrete.
4. Pass over it with a surface cleaner - the round rotary attachment - rather than a wand. It holds a consistent height and overlaps evenly, and that is what prevents striping.
5. Rinse the whole slab, working toward a drain or a lawn edge rather than into a bed you just soaked.

How much pressure is safe on concrete?

Less than most people assume. Concrete is durable, but the cement paste at the surface is thin, and a narrow or rotary tip held close will cut into it and expose the aggregate underneath. That damage is permanent - you cannot sand a patio back. A homeowner machine in the 1800 to 2500 psi range with a wide fan tip, kept moving and kept well back, is plenty for algae once the chemical has done its work. Older downtown slabs, decorative broom finishes and anything stamped or colored deserve more caution than a modern pour beside a garage. Oil drips and rust bleed are their own problem and need a different product - we cover those in our guide to removing oil and rust stains from concrete.

What removes the white bloom?

Efflorescence is a different job entirely. Start dry: a stiff brush and a shop vacuum lift a surprising amount of it, and doing that before you wet anything down stops you dissolving the salt straight back into the slab. What is left usually needs a masonry efflorescence remover - a mild acid, not a bleach - applied to pre-wetted concrete, kept off metal and plants, and rinsed thoroughly afterward.

The honest part is that cleaning efflorescence only treats the symptom. If moisture keeps moving up through the slab, the bloom comes back. Look at where water actually goes - a downspout discharging beside the patio, a bed graded toward it, a slab sitting low in a yard that holds water after a storm - and deal with that first.

How often should a Charleston patio be cleaned?

Once a year suits most properties, timed for after the spring pollen drop finishes so you are not cleaning just ahead of the heaviest fall of the year. Patios in deep oak shade, on the north side of the house, or backing onto marsh often want a second pass in late summer, because those slabs genuinely never dry out. A rinse and a sweep between cleanings does more good than one hard annual scrub. If you would rather not handle the chemical side yourself, that is what our Charleston deck and patio cleaning service is for.

Frequently asked questions

Can I just use household bleach straight from the bottle? Undiluted it is wasteful and hard on the plants around the slab, and without a surfactant it runs off the concrete instead of clinging long enough to work. Dilution and dwell time do the job, not concentration.

Will the black spots come back next year? Lichen re-establishes slowly, over years. Algae can return in a single wet season if the patio is shaded. So if green film is back within a few months, the issue is drying time and airflow, not a failed cleaning.

Should I seal the patio afterward? A sealer on clean, fully dry concrete makes the next cleaning easier and slows staining. The common mistake is sealing over trapped moisture or unresolved efflorescence - it clouds, and the salt keeps pushing out behind it. Let the slab dry for several genuinely dry days first. You can see the full range of surfaces we handle on our Charleston pressure washing page.