Article extracted from https://dengarden.com/gardening/Moldy-Potting-Soil
White, Fuzzy Growth in Potted Plants
All is fine and dandy until you wake up one morning to discover that your favorite potted plant is growing fuzzy, white mold. Worried, you run to Google, type in "moldy potting soil," and end up here. That's a good thing too, because I've got the answers!
That stuff is probably a harmless saprophytic fungus. Below, you'll learn whether or not to be worried, how to remove the mold naturally and safely, and a few proactive steps you can take to ensure that it doesn't return. The process is quick and painless, for both you and your plants!
Is Moldy Soil Bad for Plants?
The quick answer is no, that white stuff growing in your potted plants will probably not harm them. Although you don't always see them, molds and fungi are present in every organic gardening mix. In fact, many organic gardeners believe that "living soil" is the ideal environment for growth. So it's a sign of life, although it might not be one you want to look at, since it's not exactly pretty.
On the other hand, a saprophytic fungus might also be a sign that your plant is not getting what it needs in terms of sunlight, air circulation, and moisture. The mold might also be competing for nutrition with your plant, so it is also a sign that you need to pay attention to.
How to Remove Mold From Soil
If there's mold growing under your houseplants, or in any container plant for that matter, here's what to do:
The first step is physical removal. Wearing a breathing mask, scrape off and discard the affected bits of soil.
Lightly dust the soil with ground cinnamon. Cinnamaldehyde, the stuff that gives regular cinnamon its flavor and scent, acts as the perfect natural fungicide and prevents mold growth. Try to get an even distribution and remember that it only takes a thin layer.
Do not water until the top two inches of soil are dry. For smaller containers (a gallon or smaller), wait until the top quarter inch has dried before returning to a water regimen. Use your finger to gauge moisture levels.
What Else Can You Do?
Never let pots sit in saucers full of water for more than 5 minutes. Drain off excess moisture.
Place plants in sunlight or strong artificial light to help them dry.
If you see any mold, take the plant outside for a day to expose it to natural light and air. When you bring it back in, choose a new home for the plant in a spot that is slightly more sunny and breezy.
You might also consider transplanting to a larger pot full of fresh dirt. Make sure you choose a pot with plenty of drainage holes.
Is It Safe to Use a Bag of Old, Moldy Potting Soil?
Sometimes, you may not use all the potting soil at once; later when you go to use some more, you discover that fuzzy white stuff has bloomed inside the bag. It might also be that you buy a new bag of potting soil, bring it home, and discover the same thing. The question is, can you still use that soil?
If you are transplanting, planting, or just replenishing dirt levels, the answer is yes. Before you use it, simply mix up the contents of the bag and work the white stuff back in with the dark. You might also add a little fresh compost. Cut the bag open and leave it outside, exposed to the sun and air, for a day or two, turning occasionally. You could also mix in some fresh compost.
On the other hand, if you're planning to sow seeds, you should NOT use that soil. Since the mold will compete with the seedlings for nutrients, it's best to give the new plants a fighting chance in some fresh dirt.
Is the White Stuff Growing in Your Potted Plants Dangerous to Humans?
If we see mold, most of us react by covering our mouths and running the other direction. But, although most mold-removal companies will try to persuade you otherwise, the mold in your potted plants is not particularly dangerous to humans.
On the other hand, if you are extremely sensitive to mold allergies, you might not want to take any risks. Some people have claimed that the spores trigger allergic reactions.
Outdoor container plants are less likely to grow mold, but it does happen. The plant likely needs less water and more sun and wind. | Source
How to Prevent Mold From Growing in Soil
Mold can never be totally eliminated. The truth is that mold spores are a regular part of soil and are normally harmless. The real threats to your plant are heat, humidity, and low ventilation. Under these conditions, mold spores grow into their adult fungi form and release even more spores. Indoor planters and container gardens are common hosts, as they retain more moisture. To prevent mold growth, follow the simple steps below:
Don't Overwater. Overwatering is the main cause of mold growth in container plants. Soil that is constantly moist is much more likely to harbor happy spores. Water only after a quarter of the pot's total soil volume has dried out. For example, if your plant's soil is 8" deep, don't water it until the top 2" have dried out. For most indoor plants, watering once a week should be sufficient.
Reduce Humidity & Increase Ventilation. Together, these two conditions create the stale environment in which mold thrives. By already not overwatering, you're also reducing the humidity, so to reduce moisture even further, place your plants in sunnier, well-ventilated areas or use a small fan to constantly push new air around the soil.
Is It Mold or Perlite?
Don't mistake the two: Perlite is a white volcanic glass that is used to improve drainage. It's not harmful to plants.
Overall, mold in potting soil is very easy to treat. My one word of advice is to avoid the use of vinegar. Mold can survive the acidity, but your plant's roots may not! Cinnamon is a much safer and effective option.
Comments, questions, and suggestions are always appreciated!