The least expensive option is to use a little chlorine bleach (just $2.78 for half a gallon at Home Depot) diluted in water. Bleach solution doesn’t damage concrete, but it isn’t something you want in your eyes or on your skin or clothing. You could wind up with spots of white or odd colors because bleach affects fabric dye colors unevenly. So before you start working on your steps, put on old clothes, long rubber gloves and goggles, and have a bucket, a sponge and a synthetic scrub brush handy.
(On a small job such as cleaning a couple of steps, there’s probably little risk of splashing a lot of bleach solution onto nearby plants. But just to be safe, you can cover the plants closest to the steps with an upside down cardboard box. Don’t use clear plastic tubs or a sheet of clear plastic, especially if it’s a sunny day. Enough heat can build up under the plastic to wilt the leaves or even kill the plants. Or, instead of covering your plants, you can mist the leaves with water before you apply the bleach solution to the concrete. This is the best approach when there is a lawn right next to concrete that needs cleaning. Pre-moistening dilutes any splashes, protecting the plants.)
When everything’s ready, sweep or vacuum the steps to remove loose dirt and debris. Then mix three-fourths to a cup of bleach with one gallon of water, and sponge it onto all the areas where you see the green stains. Keep sponging on more as needed so the concrete stays wet for five to 10 minutes. Scrub with a synthetic brush, then rinse thoroughly. Repeat the process if the concrete is still stained. Rinsing with a hose will dilute whatever cleaning solution is still on the concrete, so it won’t harm plants where the water drains.
If you don’t want to use chlorine bleach, there are other ways to kill mold or algae, but don’t use vinegar, which is often touted in advice pieces online. Vinegar is acidic, and acids degrade concrete. Stick with cleaners that have a neutral pH (seven) or are alkaline (above seven). Acids are below seven, and those that have a pH of three or lower are especially harmful to concrete. Distilled white vinegar has a pH of 2.4 in the 5 percent strength sold in jugs at grocery stores.
One product that’s labeled for use on concrete, as well as numerous other materials, is Wet & Forget Moss, Mold, Mildew & Algae Stain Remover ($24.38 for a half gallon at Home Depot). It’s designed so that you can dilute it and spray it on, then just wait for the growth to die and the stains to disappear; no rinsing necessary. The directions say to use a pump sprayer, but for a small area, such as your steps, you could probably make do with a spray bottle.
Or you can try using a non-chlorine bleach, such as OxiClean Versatile Stain Remover ($13.48 for a 7.22-pound box) or Scotts Outdoor Cleaner Plus OxiClean ($10.98 a gallon). These products are appealing because you don’t have to take any precautions to protect plants or worry about any splashes damaging your clothes (although you still need to protect your eyes and skin). However, if you read the information for the Scotts product, you’ll see that it doesn’t actually kill mold; it just takes out mold stains. Does that distinction matter? Probably not, because killing mold is a rather futile goal. There are so many mold spores floating around in the air that when conditions are right, mold will grow. Whether it’s dead or alive, you just want it off your concrete.
If you already have OxiClean that you use for laundry, see if it works. Mix four scoops of the powder with a gallon of warm water. Apply and scrub as if you were using a chlorine bleach solution, except wait 30 minutes before rinsing. Or if you buy a specialty cleaner, follow the instructions on the package. If you get the surface clean, it worked. If not, use chlorine bleach or a cleaner that says it kills and removes mold.
Whatever method you use, once the concrete is clean and dry, apply sealer to the risers so they get the same protection that you’ve already given to the treads. Although you look at your steps now and notice the drips of your “bad” application, the sealer clearly did work: The drips aren’t moldy, nor are the treads.
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