When White Mold Grows on Potting Soil

White mold at the base of a grape ivy (Cissus alata, formerly C. rhombifolia). Photo: matusskaaaaa, depositphotos

By Larry Hodgson

When you garden indoors long enough, you eventually run into this situation. White mold appears on the surface of the soil of a houseplant or perhaps some seedlings. What is it? And is it harmful? And how do you get rid of it?

What It Is

The mold is a type of filamentous fungus. (I can’t be more precise than that, as there are dozens if not hundreds of species that all look pretty much alike.) Such fungi are ever-present in our environment: their spores float around in the air both indoors and out and germinate when they find a substrate that suits them. And spores may be present in the potting soil you buy. Or might have been present in the greenhouse where the plant was purchased. Or carried on a potting tool. Or your fingers …

What can I say? Fungus spores are everywhere!

This kind of fungus is saprophytic: it lives off decomposing particles of organic matter (wood, peat, leaf mold, etc.) found in the potting mix. It isn’t pathogenic or directly harmful your plants … nor is it harmful to people or pets. In fact, it actually helps your plants, as the organic matter it digests contains minerals that are then freed up. Your plants can therefore absorb and use them for their growth.

In other words, white mold is part of the composting process, but composting in a pot rather than in a bin!

The slow decomposition of potting soil due to various soil organisms (most invisible to the eye) is just part of the circle of life and usually goes on unnoticed. The appearance of white mold is one of the rare examples where you can see Mother Nature at work.

How to Get Rid of White Mold

So, white mold on potting soil is normal and even beneficial to a certain degree, but you still probably don’t want it. First, for esthetic reasons, perhaps. However, some types form a network of mycelium so dense that it impedes air and water circulation to the plant’s roots and you certainly don’t want that. In either case, it’s best to remove it.

Simply take a fork and scrape off the upper surface of potting soil (it can go into compost pile or even directly into the garden) to a depth of about 1 inch (2 cm), then replace with fresh soil. That’s usually enough.

If ever it comes back (that’s fairly rare, but some species are more persistent than others), you may need to repot the plant, removing the old potting mix and replanting it in fresh soil in a clean pot.

Prevention

Decomposition goes on in all potting soils, but usually sight unseen. The presence of mold is usually related to excess humidity, poor drainage or poor air circulation: things you could correct. The current resurgence on the market of pots without drainage holes is one reason the presence of white mold is more common than in years past.

Here are some suggestions for preventing mold on potting spoil:

1. Let the soil dry out between waterings

Just a bit for seedlings, cuttings and most houseplants; more deeply for succulents and other arid climate plants.

2. Increase air circulation

You could have a fan operating in the room, for example, or crack open a window.

3. Avoid dense, heavy soils

Soils from the garden, notably. Light, well-aerated potting soils, widely available commercially, are better choices.

4. Remove fallen leaves or other debris

They can be a source of fungi or help nourish fungi.

5. Replant plants growing in pots without drainage holes into more appropriate containers

What can I say? Growing plants in pots without drainage holes is simply a bad gardening practice and can be harmful to plants in so many ways!

6. Give your plants more light

That way, they’ll use water more rapidly and the potting mix will dry out more quickly, something mold fungi don’t like.

7. Remove mulch

Mulch is usually good for plants, but in situations where poor air circulation, high humidity or poor drainage prevail, it can stimulate fungal growth. Or use chopped sphagnum moss as mulch: it has natural antifungal properties.

8. Apply a fungicide

I mention this as a last resort, as I feel it really shouldn’t be necessary to “pull out the big guns” for such a minor problem. However, yes, commercial fungicides will help prevent mold or slow its growth, as will home-made products like powdered cinnamon sprinkled over the soil or chamomile tea watered into it.

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White mold on houseplant soil: it’s only a minor problem and it’s usually one that’s very easy to fix.