Is there a way to kill the mold on soil surface in pots?

Question

I’ve had mold growing on the surface of my pots for a long while. I kept removing the soil from the surface but that didn’t help. So I went and bought expensive fancy soil!! I took out the plants from the old soil and tried to clear them ip as much as I could and replanted them in brand new pots with the new soil. But bingo! There’s mold growing again.

These are citrus plants that I’ve grown from seeds and I use citrus feed when I water them. I live in a reasonably cold and very damp climate and I don’t water them too often (once a month or so?) they’re indoors behind south facing windows. Is there something I could spray onto the soil to kill the mold?

I searched thoroughly before posting this and did read the relevant posts which mostly suggest preventing mold. However I’m wondering if there’s any anti fungal spray I could use on soil, ideally home made.

Oh, by the way, could it be due to the plastic planters? I notice the other plants taken out from the same old planters and planted in clay pots don’t have any fungus. Maybe the plastic isn’t breathable? But for now I’d like to kill the mold if it’s an option as I don’t have the time to buy other pots and replant again.

Answer 1

This is usually a saphrophytic fungus - its there to consume dead material in the potting soil, but its continued presence indicates a watering issue, as well as air flow problems. If you are not watering too often so that the soil is constantly moist, assuming there's a drainage hole or three in the pot, if you allow water to collect in any outer tray or pot, that will keep the soil too moist.

Water only when the surface of the soil in the pot feels dryish to the touch - water thoroughly, and empty out any outer pot or tray after 30 minutes - check again 30 minutes later to make sure it's empty. Airflow will help too - a closed environment with no moving air also encourages this type of growth. Hopefully, you washed and sterilised the pot before repotting (if you reused the same pot). More information here http://homeguides.sfgate.com/dustylooking-mold-soil-houseplants-41109.html. If your pot has no drainage holes, you need to repot into something that does have drainage holes. In regard to its being a plastic pot, it doesn't exactly encourage this type of problem, but it does mean you need to take extra care with your watering routine.

Answer 2

You didn't mention what type of expensive soil you used but citrus needs a citrus potting mix which is very freely draining.

You've got fungal growth because your mix is remaining wet. Only water when it's dry, determined either using a moisture probe or your finger.

If you want to kill the fungus you can spray with hydrogen peroxide, or, even isopropylalcohol. Or, if you don't have that, burn it off with a magnifying lens taking care not to burn the roots.

https://garden.org/learn/articles/view/4077/