A Midlands guide to identifying what is growing on Columbia siding, roofs and concrete - mold, mildew, algae or lichen - why the difference changes the treatment, and what removes each one without damaging the surface.
Almost every homeowner in the Midlands calls the dark stuff on their siding "mildew." Some of it is. A good deal of it is something else entirely, and the distinction is not academic - the four organisms that commonly grow on a Columbia house exterior respond to different treatments, return on different timelines, and carry very different consequences if you ignore them. This guide explains how to identify what you are actually looking at from the ground, and what each one requires.
Mildew is a specific kind of surface-level fungal growth. It stays flat against the material it grows on, it is usually white, grey, or a light powdery yellow when it starts, and it lives on the surface film of dirt and organic residue rather than in the material itself. Mold is a broader category of fungus that grows in three dimensions - it has depth and texture, it comes in darker colors, and it can push root-like structures called hyphae into anything porous. In practical Columbia terms: mildew is a stain you can often disturb with a fingernail, and mold is a colony that has settled in.
That is the textbook line. On the outside of a real house in Richland or Lexington County, the picture is muddier, and two other organisms get mislabeled as mildew far more often than actual mildew does.
Those dark vertical streaks running down a Columbia asphalt shingle roof, heaviest on the north slope, are a cyanobacteria commonly identified as Gloeocapsa magma. It is an algae, not a fungus at all. It feeds partly on the limestone filler used in modern shingles, it holds moisture against the roof surface, and it spreads by airborne spores - which is why one streaked roof on a street in Irmo tends to become several streaked roofs over a few seasons.
This matters because the treatment is genuinely different. Roof algae must be treated with a cleaning solution and rinsed at very low pressure. Putting real pressure on an asphalt shingle strips the granules that give the shingle its UV protection, and you will not see the damage as damage - you will see a clean roof that ages five years in one. Anything involving a roof surface belongs in a soft wash roof cleaning process, not a pressure washing one.
The second most common misidentification is the green cast that appears on the shaded elevation of a house, on the lower courses of vinyl siding, and along the north edge of a driveway or walkway. That is green algae, and it is the single most common growth on Columbia exteriors. It shows up where three conditions overlap: shade, retained moisture, and a surface with enough texture to hold a film.
Green algae is the easiest of the four to remove and the fastest to come back. It has no root structure, so a proper cleaning solution and a low-pressure rinse takes it off completely. But because it is airborne and opportunistic, it returns to exactly the same elevations as soon as the conditions return. If your north wall greens up every year, the growth is not the problem - the shade and moisture pattern is.
Lichen is the crusty, raised, grey-green or pale-orange patch that looks almost like it was glued to the shingle, fence board, or brick. It is a symbiotic organism - an algae and a fungus living together - and unlike the other three it anchors physically into the surface it grows on. On a roof it works into the shingle mat. On a wood fence it holds into the grain.
Lichen is the reason the "just blast it off" instinct causes so much damage in the Midlands. It genuinely will not come off with a normal wash, so people escalate pressure until something gives - and what gives is the shingle, the wood fiber, or the mortar joint, not the lichen. Established lichen requires a dwell-time chemical treatment, often more than one application, and patience. Frequently the correct answer is that it will release over the following weeks rather than during the visit.
The Midlands sits in a humid subtropical climate with long, hot summers, and the overnight dew point stays high for months at a stretch. That combination - warm surfaces that never fully dry overnight - is close to ideal for every organism described above. Columbia's position near the confluence of the Broad and Saluda rivers, and the heavy tree canopy across older neighborhoods in Shandon, Forest Acres, and along the lake in Irmo and Lexington, adds the other half of the equation: shade and constant organic litter.
Pine straw and oak litter are not neutral. As they break down against siding, gutters, and concrete, they leave an organic film, and that film is the food source. This is why the growth is almost never evenly distributed across a house - it concentrates under the eave line, behind downspouts, along the bottom two or three courses of siding where splash-back lands, and anywhere a shrub is planted tight against the wall.
You can usually settle the question without getting on a ladder. Look at color first: white or pale grey powdery patches on a shaded surface suggest true mildew; flat green film suggests green algae; dark vertical roof streaks suggest roof algae; raised crusty circular patches suggest lichen. Look at texture second - if it has height and you can see it standing off the surface at an angle, it is lichen or an established mold, not mildew.
Then look at pattern. Growth that follows water is biological. Growth that follows traffic - tire tracks, walkways, entry doors - is usually soil, not an organism. And growth that is a solid orange-red rather than green or black is not alive at all; that is iron staining from Midlands red clay, and it needs an entirely different chemistry. We cover that separately in the Columbia driveway guide.
For most exterior surfaces the practical treatment converges: a cleaning solution appropriate to the surface, adequate dwell time, and a low-pressure rinse. Mildew, green algae, and roof algae all come off that way. So if the question is only "how do I get my siding clean," the identification matters less than the method.
The distinction becomes important in three situations. First, lichen - because it will not respond to a single application and pushing pressure to compensate causes real damage. Second, anything on a roof - because the correct pressure there is close to none. Third, and most importantly, when the growth is on the inside of the wall rather than the outside.
Exterior mold, mildew, algae and lichen are surface and appearance problems. Interior mold is not, and the two are sometimes connected. If you have persistent growth on the outside of a wall that keeps returning far faster than the rest of the house, in one localized area, it is worth asking why that spot stays wet. A failed gutter seam, a downspout discharging against the foundation, a leaking hose bib, or a grading problem that holds water against the wall will all produce that signature.
Cleaning the outside of that wall will make it look correct for a while. It will not address a moisture path into the structure, and no exterior cleaning company should tell you otherwise. If you suspect moisture is getting into the wall assembly, or you are seeing or smelling growth inside, that is an inspection and remediation question for a licensed professional, not a wash.
Every organism here reproduces from airborne spores. Nothing you apply to a Columbia exterior sterilizes the air around it, so return is a matter of when, not whether. What you control is how quickly the conditions become favorable again.
The levers that make a real difference are unglamorous. Cut back shrubs and limbs so the wall gets airflow and some direct sun. Keep gutters clear so overflow is not sheeting down the same elevation every storm. Direct downspouts away from the wall and slab. Pull mulch back from the siding line. Clear pine straw off the roof valleys and out of the gutter troughs, because that debris is both food source and moisture sponge. Houses in the Midlands that go longest between cleanings are almost always the ones where the shaded elevation dries out during the day.
A Columbia house with an open, sunny exposure and clear gutters can often go a good while between full washes. A house tucked under mature canopy in Forest Acres or backing onto woods in Blythewood will show green on the north wall much sooner, sometimes annually. Roofs are on a longer cycle than walls, because the streaking is slower to establish - but once lichen appears on a roof, waiting makes the eventual treatment harder rather than easier.
The honest guidance is that frequency should be driven by what you can see, not by a calendar. We go into the timing question in more detail in our guide on how often to pressure wash a house in Columbia.
Once you have identified biological growth rather than soil, the method follows almost automatically. Living growth is removed chemically and rinsed gently; soil is removed mechanically with pressure. Using pressure on growth drives water into places it should not go, damages the surface, and leaves the root structure of anything established behind - which is why a pressure-washed lichen patch often reappears looking worse.
Vinyl and fiber-cement siding, painted wood, stucco, and every roof surface belong in the low-pressure category. Concrete flatwork, brick paving, and most hardscape can take genuine pressure. If you want the full comparison, see soft washing vs pressure washing in Columbia, and for what a full exterior wash involves on a Midlands home, see our Columbia house washing page.
Walk the outside of your house and note where the growth is, what color it is, and whether it stands off the surface. That single walk answers most of the question. If it is flat and green or grey on siding, it is straightforward. If it is streaking a roof, plan for a soft wash rather than a pressure wash. If it is raised and crusty, expect it to need more than one pass. And if one specific area keeps coming back much faster than everywhere else, look for the water source before you book a cleaning.
If you would rather have someone identify it for you, we work across Columbia, Lexington, Irmo, Cayce, West Columbia and Blythewood, and we will tell you honestly when what you are looking at is a cosmetic issue and when it is a moisture problem that a wash will not fix.
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