What the black actually is
Algae, mold and mildew — a biofilm growing on and slightly into the surface of the concrete. It needs three things: moisture, shade, and organic material to feed on. Concrete under a tree provides all three at once.
Shade means the surface never dries fully. Concrete is porous enough to hold that moisture. And the leaves, needles, sap and pollen dropping from above provide a continuous food supply. It's close to ideal growing conditions.
Why the north side is always worse
Sun is what kills this stuff naturally. Surfaces that get direct sun for part of the day dry out and get UV exposure, both of which suppress growth. North-facing sections and areas under permanent canopy get neither.
That's why a walkway can be visibly clean on one side and black on the other with no other difference between them.
Why rinsing doesn't work
A hose removes loose surface debris. The biofilm has structure in the concrete pores, and what you leave behind regrows quickly — often faster than before, because you've removed the competing material and left moisture behind.
The same is true of pressure alone. You can blast a walkway clean and have it visibly returning within a couple of months if nothing killed the growth at the root.
What actually removes it
A cleaning solution that kills the organism, given time to dwell and do that, followed by surface cleaning to remove the dead material. That sequence matters — clean first without pre-treating and you're just removing the top layer of a living colony.
Done properly, a shaded walkway stays clean for a year or more rather than a couple of months.
The safety part people underrate
Algae on wet concrete is genuinely slippery. A shaded front walkway or pool surround with organic growth on it is a real slip hazard in wet weather, and it's a common reason people call us about a walkway rather than a driveway. If you have anyone elderly using that path regularly, it's worth handling sooner.
Can you prevent it coming back
Not entirely, unless you remove the shade — and nobody wants to cut down a mature oak to keep a walkway clean. Trimming branches to let more light through helps. Improving drainage so water doesn't sit helps. Sealing concrete reduces how much moisture the surface holds.
Realistically, shaded flatwork is on an annual cleaning cycle, and that's normal rather than a sign anything went wrong.