Hardware stores and home centers sell products designed to kill moss and algae, but you can save money by using inexpensive chlorine bleach or a non-creamy hand dishwashing detergent, such as Dawn.
Using bleach might sound scary because it can damage plants if it’s splashed on leaves; however, with reasonable care, you can keep your plants looking fine. Put on old clothes that you don’t mind getting spotted with white from the bleach, and slip on rubber boots and long rubber gloves. Goggles are a good idea, too. Make sure small children and pets are out of the area, and connect a hose that has a spray nozzle on the end; you’ll need it to rinse the path at the end, but it’s also good to have ready to go in case bleach spills or splashes where you don’t want it and you need to quickly rinse it off.
In a bucket, dilute one gallon of bleach with three gallons of water, or use one quart of bleach to three quarts of water if you have a small area to clean. Using a long-handled scrub brush, spread the solution evenly across the bricks. You might want to pour some of the solution directly on the bricks and then spread it with the brush. Or, depending on how open the joints are between the bricks, it might work better to dip the brush in the solution and then quickly move the dripping brush to the bricks. Brushing on the solution makes it easier to control where it lands than if you apply it with a garden sprayer, as some people recommend.
Wait a while — maybe 20 minutes — but not so long that the solution dries on the bricks. Then give the bricks a good scrub and hose them off, using just the force of the water from the spray nozzle. The rinse water dilutes the bleach, which by this point has largely broken down and turned into salt water because of its reaction with the algae or moss. I have been cleaning a brick walkway this way for 20 years and have never seen plant damage. One of my brothers does the same thing on our mother’s brick walkway, also with no obvious plant damage.
Another brother, though, has switched to the dishwashing detergent method, which a maintenance person at a golf course recommended to him. I plan to try it the next time my bricks get slippery. The process is the same, but use one cup of soap to one gallon of water. One obvious benefit: You don’t need to be so careful about protecting your skin and eyes or keeping children and pets away. There are no dangerous fumes to worry about, either, unlike with chlorine bleach, which can cause serious lung damage when inhaled (although using it outdoors minimizes this risk).
Some people recommend using salt or vinegar, or a combination of them. Using salt as a de-icer has resulted in damage to nearby plants, which argues against using it to clean a path. Vinegar can etch masonry materials, but if it’s not left on very long and is then diluted with plenty of water, it might not cause any noticeable damage.
However, it’s really important not to mix the different treatment options without being sure of what they do in combination. Never use chlorine bleach with any cleaner that contains ammonia; the combination produces dangerous chlorine gas, which irritates eyes, skin and the respiratory tract and, in large doses, can even cause death. Mixing bleach with acids, even vinegar, also releases chlorine gas. It’s fine to combine hand dishwashing soap and bleach or dish soap and vinegar, though.
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