Flagstones are one of the most popular options for garden and yard paving, and for good reason. Being both very durable and beautiful with a wide range of choices in material, shape, and colour, flagstones provide an elegant and natural-looking option that fits a variety of home and garden designs.

    Now, it is quite natural for flagstones—like any other outdoor paving material—to get dirty over time, especially considering the fact that they are exposed to the elements 24/7. But at times, you may notice your flagstone pavers accumulating these black stains that aren’t very easy to remove. 

    In this article, we will show you exactly what they are and what you can do to clean them. 

    What’s Causing It?

    Simply put, black stains on flagstones are caused by certain types of mold and mildew. These fungi find their way onto the stones either from their spores landing on the stones or from simply extending onto the stones from patches of mold or mildew right beside them—a pretty common occurrence for garden pavers.

    Once they make it onto the flagstones, the mold and mildew take root into the pores of the stones, from which they will grow and expand into larger patches that you will see as black stains.

    Source: wikimedia.org

    How to Clean Black Flagstone Stains

    Thankfully, cleaning these black stains out is quite easy and only needs a few common household tools and cleaners. Below is a list of the variety of methods you can use to clean out your flagstones. Each method should work quite well for most mold and mildew stains, but if one method hasn’t done the trick, you can try any of the others until the stains are completely gone. 

    Cleaning Flagstone Stains with Vinegar

    Vinegar is one of the most basic cleaning products that you may use to remove black flagstone stains from your floors. Because of its acidic nature, it is very effective in removing organic stuff, which is exactly what mold and mildew are.

    When using vinegar as your cleaning chemical, it’s important to use white vinegar specifically, as opposed to other variants like balsamic vinegar or apple cider vinegar. Other types of vinegar have some colorations that will end up staining your flagstone even if you do remove the black mold stains.

    1. Pour white vinegar over the flagstones. 
    2. Let the vinegar sit for about a minute. Allowing the vinegar to sit too long can corrode the stones.
    3. Clean the flagstones by rinsing them with water. A pressure washer on a low power setting is recommended.
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    If you want to play things a bit more safely, you can dilute the vinegar with water, then let the mixture sit for about 15 minutes or longer depending on how diluted the vinegar is. 

    Cleaning Flagstone Stains with Dish Soap

    The use of normal home dish soap is a suitable option if you don’t care for the scent of vinegar (which is likely the case). The following are the steps to take in order to use it for your flagstone cleaning.

    1. Prepare a mixture of water and dish soap, making sure that there is enough soap to produce suds.
    2. Pour a small amount of the mixture over the flagstones, trying to avoid getting the soap onto nearby vegetation (if there is any).
    3. Using a rough brush or a cleaning pad, scrape away the black spots from the surface of the water. The dish soap should aid in the killing of the mold or mildew’s roots, allowing it to be removed more easily.
    4. Clean the flagstones by rinsing them with water. 

    Cleaning Flagstone Stains with Bleach

    Depending on how stubborn the black stains are and whether or not the other solutions have been successful, bleach may be the only option left. Both ammonia and oxygen bleach can be used here, and we’ve outlined the procedures to use them below 

    1. Dilute the bleach with water in a 1-to-1 mixture. You can adjust the concentration to your preference, but it is generally advisable to avoid making the mixture too concentrated to avoid damaging the flagstones. 
    2. Pour a small amount of the bleach mixture over the flagstones. 
    3. Allow the bleach to sit for about 5 minutes, adjusting the time depending on how diluted the bleach is. You can also use a brush to remove the stains more directly. 
    4. Clean the flagstones by rinsing them with water. 

    Preventing Flagstone Stains

    Now, it is always a good idea to clean off these unsightly black stains from your flagstone pavers. However, we would more strongly recommend taking measures to prevent them from taking root in the first place.

    Despite being constantly exposed to nature, flagstones can be kept quite clean and free of mold and mildew if you keep up with their general maintenance. Below, we’ve created a list of some of the things you can do to keep your flagstones in tip-top shape. 

    1. Spot-clean new stains on your flagstones as soon as you find them—removing the mold or mildew as they are still taking root makes them easier to remove than if you allow them to grow into larger patches. Any of the methods we’ve described above will do the trick. 
    2. Remove debris from your flagstones. Dirt and dust always accumulates on your flagstones over time, and sometimes this dirt could carry mold and mildew spores that produce the stains we have been trying to remove. Dry sweeping your flagstones regularly is an effective way to prevent this from happening. 
    3. Keep your flagstones dry. One of the most common and significant causes of mold and mildew growth is the presence of water and excessive moisture. Clearing away standing water (as in the case of rain puddles or runoff from potted plants) will remove this critical catalyst, thereby reducing the possibility of mold growth. 
    4. Apply a stone sealer to your flagstones. There are special products that you can purchase at gardening supply stones that can seal the tiny holes in the structure of the flagstones. This solution helps minimize the ability of mold and mildew to take root in the small crevices of the flagstones, which should help preven these fungi from growing. 
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    avatar for Matt Greenfield

    Matt Greenfield

    Matt Greenfield is an experienced writer specializing in home improvement topics. He has a passion for educating and empowering homeowners to make informed decisions about their properties. Matt's writing focuses on a range of topics, including windows, flooring, HVAC, and construction materials. With a background in construction and home renovation, Matt is well-versed in the latest trends and techniques in the industry. His articles offer practical advice and expert insights that help readers tackle their home improvement projects with confidence. Whether you're a DIY enthusiast or a seasoned professional, Matt's writing is sure to provide valuable guidance and inspiration.

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