If your patio goes green and gets dangerously slick a few weeks after every rain, you don't have a dirt problem — you have a living one. Those black and green patches are algae and mildew, and in shaded, damp central Ohio yards they'll keep coming back until you change the two things they need: a hospitable surface and standing moisture.
Why it keeps coming back
Algae spores are always in the air. They take hold wherever a surface stays damp and shaded — under trees, along the north side of the house, or anywhere water lingers after a storm. A quick blast with a hose or pressure washer knocks back what you can see, but if the growth isn't killed at the root and the surface stays just as damp and porous, it's back within a season. That's the cycle most homeowners are stuck in.
Why Columbus yards grow it so fast
Central Ohio is close to ideal algae habitat. Summers are humid, springs are wet, and the mature tree canopy in older, established suburbs like Worthington, Upper Arlington, and Gahanna keeps patios shaded and damp long after a rain moves through. Add the slow-draining, clay-heavy soil common across the region — which holds moisture at the surface instead of letting it soak away — and a shaded, north-facing patio here can stay damp for days at a stretch. That's exactly the standing-moisture window algae needs to re-establish, and it reloads every season. It's why the same patio that stays clean in a sunny, open yard goes green every single spring on a wooded lot: the conditions repeat, so the fix has to change the surface itself — kill the growth at the root, then seal — rather than just rinse it.
The fix that actually lasts
Breaking the cycle takes two steps, in order:
- Kill and clean, not just rinse. A proper patio power washing uses the right cleaning solution to kill the algae and mildew at the root, then removes it — so it isn't just pushed around and left to regrow.
- Seal the surface. A sealed patio sheds water and dirt instead of soaking it up, which makes it far less hospitable to regrowth. On pavers that means a clean, re-sand, and seal; on flagstone or bluestone it means gentle, stone-safe natural stone care.
Help it along
Anything that reduces standing moisture buys you longer between cleanings — trimming back branches that keep the patio in shade, clearing leaves that trap dampness, and making sure water drains off rather than pooling. You won't make a shaded patio immune, but combined with a proper clean-and-seal you'll go from "green every spring" to "clean for years."
The safety piece
Don't put this one off on a shaded, sloped, or poolside patio — algae is genuinely slippery when wet and a real fall risk on steps and walkways. Getting it cleaned and sealed isn't just cosmetic; it makes the space safe to use again. A free written estimate will lay out exactly what your surface needs.