Pressure washing erases the green and black staining you can see, but in South Florida's humidity the algae and mold spores that caused it often survive in the surface, which is why it comes back faster than most homeowners expect.
Most homeowners in South Florida have had the same experience. The driveway or pool deck gets pressure washed, it looks brand new for a season, and then the green and black staining starts creeping back in the exact same spots, sometimes within a few months. It feels like the wash did not work. In most cases it did exactly what it was supposed to do. It just did not do the part that actually stops the growth from coming back. Pressure washing is a cleaning step. What grows on concrete, pavers, and pool decks in this climate is a biological problem, and biological problems need a biological answer, not just more water pressure. Understanding what is actually growing, and why it grows back so fast here compared to a drier climate, changes how you think about maintaining outdoor surfaces. What Is Actually Growing on the Surface The dark staining on roofs, driveways, and pool decks is usually a mix of algae, lichen, and mold, with one organism doing most of the damage on hard surfaces: a type of blue-green algae that thrives in warm, humid, shaded conditions. South Florida gives it almost everything it wants year round. There is rarely a true dry season cold enough or dry enough to knock growth back the way a northern winter does. These organisms do not just sit on top of concrete and pavers. They send fine filaments and spores into the pores of the material itself. On broom finished concrete, exposed aggregate, and the sand joints between pavers, that surface is porous enough to give algae and mold spores somewhere to hide well below the layer that a pressure washer tip actually reaches. Why Pressure Washing Alone Does Not Finish the Job A pressure washer is extremely good at blasting away the visible layer of growth and the dirt holding it in place. That is real cleaning, and it is the right first step. The problem is what it does not do. Water pressure, even at a few thousand PSI, does not reliably kill the organism. It knocks the colony apart and rinses most of it away, but spores that were driven into pores, cracks, and paver joints are often still alive. Give them a few weeks of Florida humidity and afternoon rain, and they start colonizing the surface again from the inside out. This is also why straight high pressure water, used without any treatment, can sometimes make regrowth faster rather than slower. Blasting a porous surface opens it up further, roughing the texture and giving the next generation of spores more surface area to grip. On pavers, aggressive pressure can also wash out the sand in the joints, which is where algae loves to establish itself once it is loose and full of organic debris. Why Some Areas Come Back Faster Than Others If you have ever noticed that one side of the driveway stays clean for a year while the other turns green in eight weeks, that is not random. A few things decide how fast growth returns to a given spot: Shade. North facing areas, and anything under a tree canopy or roof overhang, stay damp longer after rain and get far less UV to slow the algae down. Irrigation overspray. Sprinkler heads that clip the edge of a driveway or pool deck keep that strip wet on a schedule, which is close to ideal growing conditions. Drainage. Low spots that hold water, or areas where a downspout dumps runoff, give organisms a reliable water source even between rain events. Organic debris. Leaf litter, pollen, and grass clippings