Any asphalt surface in Central Florida that sits in shade for most of the day will eventually turn green, black or a streaky mix of both. Driveways under oaks in Winter Park and Maitland, the north side of commercial buildings in Orlando, cart paths through landscaped communities, and the strip along a fence line all develop it. Most owners treat it as a cosmetic issue. It is also a maintenance one.
Why it grows here
Algae, mildew and the black biofilm that follows them need moisture, shade and something to feed on. Central Florida supplies all three. Humidity keeps surfaces damp, overnight dew wets them again every morning, tree canopy blocks the sun that would dry them, and the pollen, leaf litter and organic dust that settle on pavement give the growth a food source. Irrigation overspray and reclaimed water, which carries nutrients, make it worse. Our post on irrigation and asphalt covers that side.
What it does to the pavement
- It holds moisture on the surface for longer, which keeps the asphalt wet and slows the drying that pavement relies on.
- It makes the surface slippery, particularly for pedestrians and cyclists, and on a commercial property that is a liability.
- It prevents sealcoat from bonding. Sealer applied over algae sits on the growth rather than the asphalt and peels off with it.
- On sealcoated surfaces it grows on the sealer itself and can lift it in patches over time.
Algae on asphalt is a sign the surface is staying wet. The growth is the symptom; the moisture is the problem.
Cleaning it off
Light growth comes off with a stiff broom and a hose. Heavier growth needs a cleaning solution designed for outdoor mildew, applied, allowed to work and then rinsed, or a careful low-pressure wash. High pressure is the wrong tool on asphalt because it strips the binder and fine aggregate along with the algae. Our post on whether pressure washing asphalt is safe explains how to do it without damaging the surface. Bleach-based cleaners should be used with care around landscaping and rinsed thoroughly, and they can lighten sealcoat if left to sit.
Cleaning is also the first step before any sealcoating. A surface that has been cleaned of growth and allowed to dry fully is one the sealer can actually bond to. Cleaning it the same morning and sealing over a damp surface is one of the mistakes covered in our post on sealcoating mistakes.
Keeping it from coming back
The growth returns as long as the conditions do, so prevention is about changing them where possible. Trimming canopy to let more sun reach the surface helps most. Keeping leaf litter and pollen swept off removes the food source. Adjusting irrigation so the pavement is not wetted daily removes the extra moisture. Improving drainage so water does not sit in low spots removes standing water. And a sealcoat in good condition gives a smoother, less porous surface that the growth has a harder time gripping.
On commercial property
The shaded side of a building, the walkways under a canopy and the drive aisle along a landscaped berm are where algae shows up on commercial lots in Orlando, Altamonte Springs and Lake Mary. On a property with public foot traffic, slippery growth on pavement is a safety issue as much as an appearance one. Regular cleaning of those areas belongs in the maintenance program alongside sweeping, crack filling and sealcoating.
Timing the cleaning matters in the Florida year. The spring pollen drop and the wet summer months are when growth accelerates, so a thorough cleaning in late spring and another as the rains taper off in the fall keeps most shaded surfaces under control. Any sealcoating should be scheduled after one of those cleanings, once the surface has had a few dry days.
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Iron Ridge Pavement LLC cleans, seals and maintains shaded driveways and lots across Orlando, Winter Park, Maitland, Lake Mary and Central Florida. Request an estimate.
