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The vast majority of mold and fungi that appears in plant soil is harmless to your houseplants, but it can still be unsightly and an indication of other potential issues.
SIGNS YOUR PLANT HAS SOIL MOLD
The mold can be found both on top of the soil or coming out of the drainage holes at the bottom. It may wrap around the outside of terra cotta pots.
The mold may be yellow, white, brown or grey in color and fuzzy, slimy or looking like cluster of eggs.
Small mushrooms might grow out of the pot.
The soil may also have a slight musty odor.
Most molds that appear in plant soil are not harmful to your plants - they are a type of saprophytic fungus, meaning that they feed on the dead organic material in the soil. They won’t feed on your plants or cause any direct damage to them.
Soil that is higher in organic content will be more likely to promote mold and fungi growth so you're unlikely to see it in, for example, cactus pots if you're using the right soil mix. In some cases, organic fertilizers can lead to excess mold and fungi growth because they live on materials that are not totally decomposed present in the fertilizer.
Seeing mold or an egg-like fungus on your soil surface can be a shock. While it's not eating your plant, you should check for underlying causes such as poor drainage or overwatering.
WHAT TO DO NOW
Usually, it’s enough to just scrape the mold from the soil surface, then allow the soil to dry out - if possible:
Manually remove all visible mold
Cut back on watering for a while - let the soil dry up between waterings and water a bit less each time. This should of course be within the accepted range of what your plant tolerates
Always check the soil before watering - in the Info tab on your plant’s profile, you can find how dry the soil should be before giving more water
If it keeps coming back, consider amending your potting soil with more perlite or pumice. This will promote good aeration and leave less dampness in the soil
As well as causing mold to form, excessively wet soil can lead to greater problems such as root rot, which can be very difficult for your plant to recover from.
HOW CAN I PREVENT THIS FROM HAPPENING AGAIN? As molds develop in damp conditions, try to make sure you’re not overwatering your plant. If the plant type allows it, then you can try letting the soil dry out between waterings (check in the plant’s Info tab to see if this is okay for each particular plant).
HOW CAN I MAKE THE SOIL ODOR GO AWAY? If your soil has an odor, placing the plant in an area with better airflow and allowing it to dry out will help it to go away faster. Be careful to not expose sensitive plants to drafts. Once the soil has dried out, the odor should be more or less gone, or at least not as strong.