Mold on the soil: white fuzz, crusts, green film and mushrooms in a pot

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People say "mold on the soil" about several unrelated things that appear on top of potting mix. A fungus makes a soft white or gray layer. Algae make a green one. A hard pale crust is not alive at all: it is salt left behind by water and fertilizer. Mushrooms and slime molds turn up now and then as well. (British guides spell it mould.) Only the salt crust injures a plant directly; the living growths are mostly messengers, reporting a surface that rarely gets a chance to dry.

Which one is it?

• Fuzzy or cottony, white to gray: a fungus growing on the mix.
• Dry, crusty, white or tan: salts. Look for the same deposit on the rim, on a clay pot's outer wall and around the drainage holes.
• A green coat or film: algae, sometimes mixed in with fungus.
• A stalk with a cap, often bright yellow: a mushroom.
• A blob that appears overnight, yellow, orange, white or black: a slime mold.

White or gray fuzz

This is a saprophytic fungus: it feeds on dead organic material in the mix, not on the plant. It spreads when the top stays moist and the air around the pot is humid and still, and by itself it does the plant no harm.

Trouble starts if it is left to thicken. A heavy mat can dry out on top into a crust that sheds water, so less of each watering soaks into the mix below. The mat also feeds shore flies and fungus gnats, whose larvae go on to eat roots and leave plants wilted and stunted. The damp that lets it grow is bad for roots too: waterlogged mix holds little oxygen and encourages root rot.

What to do:

• Break the mat up with a fork, or lift it off with the top layer of mix, throw that away and top up with fresh potting mix.
• Wait until the surface has dried before watering again.
• Leave space between pots so air can move and humidity stays lower.

A pale, hard crust

This one is minerals, not a living thing. Hard water carries dissolved iron, sodium and calcium carbonate; soluble fertilizers and the mix itself add more. As water evaporates, those salts are drawn up to the surface and stay there when the water leaves. A saucer of standing water makes it worse, because the pot soaks the runoff back up along with the salts it had just washed out.

Unlike the other growths, salts do hurt plants. As they concentrate they compete with the roots for water, damage root tissue and can dry and scorch leaves. They can also shift the pH of the mix and keep the plant from taking up nutrients. The usual signs are wilting, yellowing, and brown tips and edges on the leaves.

What to do:

• Scrape off the crust and replace it with fresh potting mix.
• Flush the pot from the top with plain water, running through at least three times the pot's volume and letting it all drain out. Maryland's extension service suggests doing this every four to six months.
• Empty the saucer once the pot has drained. Where salts are building up, avoid watering from below and avoid pots with no drainage holes.
• Feed no more than the label directs, and use rainwater, or other water that carries few minerals, if you can.
• Repot every so often into new mix. A pot crusted on its outer wall is best swapped for a clean one; soak and scrub the old one before using it again.

A green film

Algae are simple water plants, single-celled or many-celled, that coat damp potting mix with green. Like the fungus they seldom harm a plant, and the concerns are the same ones: a thick layer that hardens and keeps water out, and a food supply for gnats and shore flies. Deal with it the same way, by letting the surface dry between waterings, improving air flow, and breaking up or removing the layer.

Mushrooms

The mushroom most often found in a houseplant pot is a small, lemon-yellow one, Leucocoprinus birnbaumii, known as the yellow houseplant mushroom or flower pot parasol. It reaches one to three inches in height, alone or in clumps, with a scaly cap that starts oval and opens into a bell. What you see is only the fruiting part. The body of the fungus is a network of fine threads in the mix, which may have come in the bag of potting mix or from a spore carried on the air.

It lives on dead matter, leaves the plant alone and may even return some nutrients to the mix. It is poisonous if eaten, though, causing stomach and gut upsets, so pick the caps off as they appear where children or pets could reach them. Fungicides generally fail against mushrooms. They pop up in warm, humid spells and fade as fast; if they keep returning, the mix is being kept too wet. Their spores can also travel to neighboring pots.

Slime molds

The slime mold advice from Oregon State's extension service is about bark and other organic mulch outdoors, not pots. It describes a primitive, amoeba-like organism with a group all to itself. It shows up suddenly as a patch of yellow, orange, shiny black or white, creeps over the surface eating decaying matter, fungi and bacteria, then dries, browns, releases spores and vanishes by itself. Plants come to no harm, and no pesticide is called for.

Which plants get it

None of the sources ties these growths to particular plants, so this page has no plant list. Any plant whose potting mix stays wet can show fuzz, algae or mushrooms, and any potted plant can collect a salt crust. If gnats are flying around the same pot, read fungus gnats, since a wet, moldy surface feeds their larvae too. To check how wet the mix really is and what else it might be doing to the roots, start the diagnosis.

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