How to Identify and Treat Mold, Algae, and Lichens on Stone Surfaces

Frederick M Hueston  StoneForensics.com

Have you ever noticed different types of growths on stone surfaces and wondered what they are? Let’s chat about how to spot the differences between mold, algae, and lichens—three usual suspects that can change the look and even the structure of stone surfaces.

Starting with mold, you might see it in shades of black, green, blue, white, and sometimes brown or orange. It tends to feel fuzzy or slimy and shows up in irregular, splotchy patches that can spread quickly under the right conditions. Mold loves moist, warm spots, especially in shaded areas where water tends to gather, like near water features or in poorly drained spots. Although mold itself doesn’t harm stone, it signals high moisture which can cause other problems down the line. Plus, it can discolor the stone and make it slippery, which is a safety risk.

Now, onto algae. This one is usually green, but can also be blue-green, yellow, or red. Algae form a slimy, slippery film over stones and tend to spread over large areas more uniformly. You’ll find algae thriving in sunny, damp places—think stones near ponds or along coastlines. While algae don’t damage stone directly, they can make it look less appealing and become dangerously slick. Over time, they also build up organic material that can encourage more growths.

Lichens are quite interesting; they come in colors like gray, green, yellow, orange, and black, and their color can change with moisture levels. Their texture might be crusty, scaly, or leafy, and they usually form circular, irregular patches. Lichens are a combo of fungal and algal components, which makes them look more complex than mold or algae. They can thrive in many environments, particularly where the air is clean, since they don’t like pollution. Lichens generally don’t damage stones, but they can trap moisture, which might cause issues in colder climates.

If you’re trying to figure out what’s growing on your stone, look at the color and texture—mold is fuzzy or slimy, algae are slippery and film-like, and lichens are crusty or leafy. Check how they grow too; mold forms splotchy patches, algae in thin layers, and lichens in distinct, often circular patterns. Consider their preferred environments as well.

For treating these growths, you can use a fungicide or a water and bleach mix for mold, ensure good air flow, and keep the area dry. Algae can be washed off with water and a mild detergent, and try to expose the stone to more sunlight. For lichens, a stiff brush and a gentle detergent should do the trick, and try to avoid harsh chemicals to protect the stone.

By understanding these differences, you can better manage these growths on your stone surfaces. Regular cleaning and controlling moisture and sunlight exposure are key to keeping your stone looking great.