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Four weeks ago, I bought a bag of soil with the label “For In-Door Plants”.

However, I noticed the formation of mold very quickly after I used it:

image of a potted plant with slight mold on the soil

Is it the soil? The watering? Or something else I am doing wrong?

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The conditions for growing mold are warmth, soil consistently wet, little air movement and undisturbed soil, and the presence of spores to feed on the organic matter present. It is natural to find some spores in potting compost, not a big deal. The plant in the pot (probably Chlorophytum) has fleshy roots and can be allowed to dry down between waterings. So one suggestion is to hold off on the water somewhat, reduce the temperature a bit, expose to gentle breezes and, as you are doing already, keep the surface loose.

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This is totally common to have happen with new soil. It must not be sterilized very well or they added organic matter and fungal spores later. Not a problem. Use your fingers to 'fruff' up the top of the soil, might have to do that a couple of times but this fungal growth will stop. Very common when organic matter is added to the soil.

How did you plant your plants in this new soil? Is that pot full of soil or did you put rocks or gravel below the soil and above the drainage hole? If so, you need to redo by turning over, get rocks or gravel out of the bottom completely and re-potting adding more soil. You should only have 1" from the rim to the top of the soil. Do not water too much. Allow to dry a bit and as this is new soil for this plant make sure it gets some water around the roots until it is well established then you can start soaking the soil, allowing it to drain, wait until the top 1" is dry before watering again. Could you take a picture of the ingredients and send?

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  • Isn't it generally (i.e. always) advisable to cover the bottom of a pot with gravel, expanded clay or something similar for drainage?
    – dakab
    Jan 27, 2017 at 8:31
  • No. Absolutely a MYTH. Trust me, using rock or gravel at the bottom of your soil only exacerbates soggy soil. You've made a perched water table with small pore spaces above large. All those pores in the soil have to become saturated before the water begins to flow into the large pore spaces. Big no no. All soil, only potting soil in a pot. Nothing between the bottom of the pot and the sterilized potting soil. Get rid of those rocks or whatever below the soil otherwise the drainage is truly compromised. Lift the bottom of the pot off the surface using tiles to improve drainage.
    – stormy
    Jan 27, 2017 at 8:44
  • @dakab that's a common myth.
    – Stephie ♦
    Jan 27, 2017 at 8:44
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    Interesting. I thought it was used to simulate soil horizons on a miniature level, with water-permeable layers—below the soil—ending in a drainage divide. It’s also extensively advised, so I guess people need something more palpable than “trust” or “that’s a myth”.
    – dakab
    Jan 27, 2017 at 9:53
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    It’s noteworthy how my relatively neutral, explanatory, thus constructive comment made you feel your answer being “dissed”, which it was not. But if you want to feel that way: Consider it a reminder that “trust” doesn’t count if you lay it on that thick, implying that producing academic sources would be no big deal for you. There’s indeed no room for more than that in comments, especially when it starts to get “grin”, generalize “humans” or become implicitly offensive.
    – dakab
    Jan 28, 2017 at 8:28

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