Retaining walls are often meant to be decorative as well as practical, and when they look good, they can add a significant amount of appeal to your garden or yard. The only problem is that any retaining wall will eventually accumulate dirt, mildew, plant material, and various kinds of gunk that can leave the blocks looking stained and ugly. Luckily, getting the blocks clean again is relatively simple, and with the right tools and some attention to detail, any stained or dirty retaining wall can be returned to its former glory.
Part of what you need to consider before you try to clean a wall or a few blocks is that the method that you should use depends on what the wall is made of. If it’s an old stone wall that has stood for decades or longer, or if it is made of prized antique bricks, it’s a bad idea to go in guns blazing and ruin the wall by using harsh chemicals or the wrong cleaner. Also, if you go about things in the wrong way, you can drive dirt deeper into the blocks and make the entire job harder. Think carefully before you pull out the pressure washer!
The best place to start, no matter what your retaining wall blocks are made of, is with an ordinary garden hose with a sprayer attachment. (You’ll need the kind that has a reservoir that allows you to add a cleaner to the spray.) Start off by gently spraying down the whole wall or the blocks that you need to clean. The entire area needs to be soaked so that when you go to clean out the dirt, there is no wicking action drawing moisture (and dirt) into the blocks or into the wall.
Consider whether or not the wall is in an area where you need to protect the greenery. If you are concerned about runoff from the cleaning process, especially if you are going to use some type of harsh product to remove stains, then you should consider putting down tarps or plastic drop cloths in order to shield the plants from the worst of the runoff.
Then, these are the basic cleaning options:
* If you are cleaning a stone retaining wall, invest in some professional-grade degreaser, and put it directly into the reservoir in your garden hose sprayer. Soak the entire area to be cleaned, and wait ten to fifteen minutes for the degreaser to work. (The wall will very likely look dirtier than before, as a result.) After waiting, take a power sprayer, and spray down the wall, working from top to bottom and from one side to the other. Keep it up until the wall stops foaming. Then, let it dry.
* If you are cleaning a retaining wall made of bricks or concrete blocks, start off by using some ordinary dish soap in your garden sprayer. Using a dish soap solution and a toothbrush, carefully scrub at stained areas. Spray off the soap with a power sprayer, and let the wall dry.
* If you are cleaning concrete blocks and soap does not remove the stains or dirt, fill your garden sprayer attachment with bleach. (Remember to take appropriate safety precautions, by wearing goggles or safety glasses, and appropriate clothing.) Spray down the wall with bleach, as above; then allow to dry.
* If none of these options work, there is hydrochloric acid. It can be effective in removing stains on some kinds of blocks, but it is too harsh for use on all materials. If you must resort to using hydrochloric acid, make sure that you follow the manufacturer’s directions exactly — or, consider bringing in a professional who can safely work with this type of hazardous substance.
Ultimately, most dirt and stains on retaining wall blocks are not that difficult to remove, and the average homeowner should be able to do the job. Just remember that you should never start by using a power washer, and that powerful chemicals and cleaners need to be used with an appropriate level of caution.