Start by identifying the spot
Look closely and touch it with a gloved finger or a stick. Mildew shows up as soft, fuzzy or flat gray-black freckles, often in clusters, in shade where the slab stays damp. Lichen is crusty and slightly raised, often round, sometimes with a gray, green or black edge, and it clings hard. Old gum is a flat, hard, perfectly round dark disc, usually near doors, bus stops and sidewalks. Tar is shiny or sticky, smells like petroleum and smears when warm. Tire rubber shows up as streaks or arcs rather than dots.
A simple test helps. Put a few drops of diluted household bleach on one spot. If it lightens within a minute or two, it is organic growth such as mildew or algae. If nothing happens, you are probably looking at gum, tar, rubber or a mineral stain. Rinse the test spot afterward.
Why shaded side yards collect black spots
Many LA homes have a narrow side yard between the house and a block wall or fence. Sun barely reaches it, sprinklers hit it daily, and plant beds keep the edges wet. That steady moisture is all mildew needs, even in a dry climate. The coastal marine layer adds morning dampness closer to the ocean, and the rainy season from roughly November to March gives growth a head start every winter.
Organic spots also come back. If you clean them without changing the conditions, they return within months in the same pattern. Shade itself is not the problem. Constant water is.
How each type is treated
Mildew and algae respond to a soft wash approach: a diluted sodium hypochlorite solution with a surfactant, applied at low pressure, allowed to dwell a few minutes without drying, then rinsed. Pressure alone tends to bleach the top of the spot but leave roots in the pores, so it reappears. Pre-wet nearby plants and rinse them after.
Lichen is tougher because it anchors into the surface. It usually needs a stronger cleaning solution, longer dwell and sometimes a second treatment days later after the lichen dies back. Faint ghost rings can remain. Gum comes off with targeted hot water or steam and a focused tip, one disc at a time. Tar needs a citrus or petroleum-based degreaser to soften it before rinsing, and tire rubber responds to a degreaser and agitation with a brush.
Mistakes that make it worse
Chasing a spot with a zero-degree tip is the most common mistake. It carves a pale crater around the stain that is more visible than the spot was. Mixing cleaning products is the other big one. Never combine bleach with ammonia or acid-based cleaners, because the fumes are dangerous.
People also spot-clean only the darkest marks. On older concrete, that leaves bright polka dots on a dull slab. Treating the spots and then cleaning the whole area evenly gives a much better result.
The last mistake is letting a cleaning solution dry on the surface. In summer heat, a bleach-based mix can dry in minutes, leaving uneven blotches and doing little to the growth. Work in small sections, keep the surface wet during dwell time, and clean early in the day when the concrete is cooler.
Keeping them from coming back
Adjust sprinkler heads so they water plants, not concrete. Trim plants that hang over the walkway and hold moisture against it. Sweep leaves before they sit and rot in the rainy season. In a heavily shaded side yard, a light maintenance clean once a year is often enough to stop mildew before it turns into dark speckling. For green growth on walls next to the walkway, our guide to algae on north-facing walls covers the vertical side of the same problem.
When to bring in help
Scattered mildew on a small patio is a reasonable DIY job with a pump sprayer and a brush. Lichen, dense gum fields, or a full driveway with mixed spot types is where a pro saves time and avoids surface damage. As of October 2026, walkway and sidewalk cleaning runs $0.55 per square foot and driveway cleaning $0.50 per square foot on our instant quote calculator, with a $195 visit minimum. Say Wash It pre-treats the spots by type, then cleans the full surface so it dries evenly.
Key points
Start by identifying the spot
Look closely and touch it with a gloved finger or a stick. Mildew shows up as soft, fuzzy or flat gray-black freckles, often in clusters, in shade where the slab stays damp. Lichen is crusty and slightly raised, often round, sometimes with a gray, green or black edge, and it clings hard. Old gum is a flat, hard, perfectly round dark disc, usually near doors, bus stops and sidewalks. Tar is shiny or sticky, smells like petroleum and smears when warm. Tire rubber shows up as streaks or arcs rather than dots.
How each type is treated
Mildew and algae respond to a soft wash approach: a diluted sodium hypochlorite solution with a surfactant, applied at low pressure, allowed to dwell a few minutes without drying, then rinsed. Pressure alone tends to bleach the top of the spot but leave roots in the pores, so it reappears. Pre-wet nearby plants and rinse them after.