Algae on concrete

Asked January 20, 2019, 10:01 AM EST

I have persistent yellow and green algae on concrete walkways, deck, and siding. I have tried many cleaners including bleach, simple green, RD20, 409, copper salts, Method All Purpose, OxiClean, hydrogen peroxide, Dawn dish soap, and maybe something else I’m forgetting. Some work for about a week and then the algae returns. See attached picture. Any suggestions?

Baltimore County Maryland

1 Response

If you have primarily had this problem during this past year, the good news is that last year was abnormal, with double the rainfall Maryland normally gets. So, there have been many water-related problems, included a lot of mold, mildew and algae. In that case, you may not have as much of an issue in the future.

That said, the places with the algae probably are shady and/or damp, or even have sitting water. Address these conditions if you can.

Besides moisture, algae also need the nutrients on the surface where it is growing. Keep the areas as clean as you can. Power washing once or twice a year may be required. Dust and particles are always being blown around outside and they settle on any surface, so a one time treatment will never be a permanent solution if the moisture and nutrient conditions favor algae.