Help - should limestone steps turn green?!

Hi: we had the bottom step in the photo installed last summer. Since then it (and all the new limestone) has turned green. It’s some kind of plant matter, obviously. But why? The old limestone (to which we’ve done nothing) remains nice and clean and lime-colored. Only the new stuff put in. Our contractor is coming to clean it with muriatic acid, but won’t say why his limestone is turning green. (He doesn’t speak english very well.) Can anyone help explain this – and better yet – offer a long-term solution for keeping the limestone NOT green? Many thanks and beers for the right answer! Thank you! C:

chuck

in Driveways, Patios, Sidewalks and Steps 11 years and 8 months ago

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windsorknot | 11 years and 8 months ago

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Skip all harsh chemicals and use the one chemical that you already have pumped into your house. Good old fashioned H2O pumped through an electric pressure washer that you can pick up at Home Depot for about 99 bucks. I have the same issue in the back of my house where the steps don’t get much light and this works like a charm…

brooklynny | 11 years and 8 months ago

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Am I the only one who likes the look of the moss?

snowman2 | 11 years and 8 months ago

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I doubt it is defective. Maybe it is just not acclimated to your local conditions. My stoop gets uniform sun distribution and grows moss on the right side of each step but not the left. I shovel snow from the left side of my stoop and pile it on the right side. I scatter de-icer on the shoveled side. The moss does not grow there so I’m guessing it has something to do with the ph value being affected by de-icer. Moss wants acidic conditions. Bleach is fine. I skip the bleach and just hose it off with a regular garden hose. Yours looks dense enough to need bleach.

chuck | 11 years and 8 months ago

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Cool. Thank you, Bob, Arkady and HDL. Calling off the acid and using bleach. C:

BobMarvin | 11 years and 8 months ago

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Perhaps the new step gets less sunlight than the others. If not I have no other explanation. In any case the bleach solution will take it off efortlessly.

chuck | 11 years and 8 months ago

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I will try the bleach and water – thank you! But still – why the new limestone step and not the old ones? Could it be a poor quality of limestone? Should it be sealed, or sanded?

Arkady | 11 years and 8 months ago

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I’ve just done mine w/ water/bleach & it was fine. The wet spring has caused a lot of this kind of thing & it may be there’s more lime in the recently applied mix & that encourages moss, algae, etc.

hdlbklyn | 11 years and 8 months ago

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Any type of acid on limestone will cause harm. Don’t do it. Stick to bleach as Bob suggested.

BobMarvin | 11 years and 8 months ago

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There’s no need for anything as harsh as muriatic acid, which can damage the limestone. A 1:4 bleach:water solution, applied with a scrub brush, will remove the mold in seconds. It’s an easy DIY job.A