Why Does My Compost Have Mold?

And what do the different types of mold in compost mean?

 Close-up of an open home composting bin full with organic garbage from the kitchen and garden.

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Having some mold in your compost is a natural and healthy thing. In fact, mold in your compost is proof that the system is working as it's supposed to.

In addition to the bacteria that break your food down, mold (a type of fungus) also does this critical work. Fungi are especially important because they break down the tougher materials, which can then be targeted by the bacteria. Since mold is a type of fungus, it provides visual evidence that the microorganisms in your compost are doing their job.

There are a few different kinds of mold to be found in compost—they can vary in color, size, and shape. White, green, pink, and red molds are all common ones you might find, and you might also see some that are powdery, ashy, or slimy looking. Read on to learn about these different types of fungi, what they are doing in your compost, and what red flags to look out for.

Yellow Mold

Fuligo septica, slime mold, scrambled egg slime fungus on tree

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Fuligo septica is a bright yellow, kind of fluffy or spongy-looking mold, also known as dog vomit slime mold or scrambled egg slime mold. It only looks bright yellow (like the image) when it's blooming, and otherwise will be a gooey, mostly transparent mold.

Slime molds like this one work to break down your compost and are normal and harmless.

Slime molds can also be other colors, including white, gray, or purplish-brown, and these are all fine to have on your compost.

Green Mold

Green mold isn't going to wreck your compost, but it is a sign that it's too moist, so you can use it as a good indicator that you need to add more dry material, water your compost pile less, or cover it for a few days if it's been raining a lot.

Bird's Nest Fungus

Group of birds nest fungi (Nidulariaceae) growing on mulch

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As its name implies, this mold looks like little tiny bird nests (but they are only about 1/4 inch in diameter). They are great at breaking down organic matter and especially like the woody parts of your compost, so leave them be to do their job.

White Mold

If you see a white, powdery substance on your compost that looks like it could be mold, it is more likely to be Actinomycetes. These are actually a type of bacteria that generally appear when your compost is getting hot and can build up over time. Beneath their top layer, they grow spider-web-like forms that extend through the compost.

You definitely want Actinomycetes around; they specifically work to break down tough cellulose, like branches and bark. This organism is also responsible for the earthy smell of healthy soils.

Signs That Your Mold Is Causing Problems

Most molds (or bacteria that look like mold) aren't a problem, but in some cases, mold might cause issues or there can be too much mold. Compost is all about balance, and if your compost is unbalanced, you will need to adjust it.

If you notice a bad smell, a lot of bugs, lots of green mold, and mushy compost, that means that your compost isn't getting enough air.

That could be because it is compacted and might need to be aerated, or because it's too moist. In both cases, add some dry brown materials (leaves, cardboard, etc.) to your compost pile and give it a good mix to break up the sludgy chunks. Don't water your compost pile, and cover it if it's going to rain.

Use Caution With Molds

While molds are a natural part of the composting process, you want to avoid breathing them in.

The greatest exposure to molds will be when you turn your compost, and those people with allergies (especially mold allergies) or respiratory conditions should be especially careful, as mold exposure can cause a very serious reaction. Avoid turning compost on windy days, and consider wearing a mask that is rated to keep mold spores out.

Mold can also make pets sick—so keep your animals away from the compost, whether that's in a bin or a pile. According to the American Society for the Prevention of Cruelty to Animals (ASPCA), the sickness can set in very quickly and be life-threatening, so you should contact your veterinarian right away if you suspect your pet ate some moldy compost (or other moldy food).

Can You Put Moldy Food in Compost? 

It's perfectly fine to put moldy food into most composting bins or piles. In fact, it can even be beneficial, as it introduces additional fungi that will travel to other ingredients in your compost and help break them down even faster or more efficiently.

There are exceptions to this rule, however. The first is if you are using a bokashi composter, which you should never add moldy food to—because you are actually fermenting the material, not truly composting it. Check out this step-by-step article on bokashi composting for more on that subject.

You also shouldn't put moldy meat, fish, or dairy into your compost because they can attract pests (but generally, most compost bins don't provide the best conditions for these ingredients anyway, as they can attract pests even when they don't have mold on them).

Finally, some molds might sicken worms used in vermicomposting. Use caution.