Is Mold Bad for Your Compost? Yes, Except One

Have you ever noticed mold growing in your compost? Let me guess, you were alarmed the first time you saw it and immediately thought something was wrong. But, I am here today telling you to relax. Molds are actually helpful in compost and I am going to prove that today.

White, yellow, and green molds growing in the compost are generally not harmful. Rather, they help break down organic matter and hasten the decomposition process. Pink molds, however, are concerning as it slows down decomposition and so the formation of compost.

What are the things that you should not throw in your compost? Pet feces? Meat scraps? Horse manure? Discover which of these materials are for your compost, and which of them are not by reading on!

Can Mold Make Your Compost Go Bad?

Molds are natural components of compost since they aid in breaking down plant polymers in kitchen wastes, thereby speeding up the decomposition process. Therefore, molds are not bad for composting.

Let us now break the myth.

Molds are a subtype of fungi that are known decomposers.

Let me introduce a new term to you—saprophytes. To put it simply, this refers to organisms that need decaying materials, like your banana peel, to live. Saprophytes get their energy to thrive from organic materials such as dead plants—or even animals!

Molds are Saprophytes
Molds are Saprophytes

The amazing thing about them is that they can spread and reproduce very quickly such that their hairy structures can attack organic matter and consume them.

So instead of harming the compost, molds act as your micro workers!

Since mold accelerates decomposition, it is also safe to put moldy food in your compost. It is just like hiring more workers in your compost pit!

Compare compost to other soil agents in our article on soil conditioner vs compost.

4 Compost Mold Colors and Their Causes (Only 1 is Bad!)

Compost molds can be seen as white or gray hair-like structures and colonies along the surface of the compost. However, pink, green, and yellow-colored molds can also develop due to biological and environmental factors.

I summarized the reasons for the emergence of differently colored molds you may find in your compost in the table below.

Mold ColorCauseIs it bad for your compost?Recommended Action
WhiteDecomposition processNoNone
YellowThe transition phase of fungi in the compostNoNone
GreenToo moistNoLower the moisture by adding
more dry organic matter;
water your compost less
PinkChemicals in the compost (e.g. cleaning materials)Yes, it can kill helpful microorganisms and slow down decompositionMake sure you are watering
your compost with pure water (without soap or any disinfectant)
Mold Colors: Causes, Dangers, and Recommended Actions

The Only Mold Color That May Cause Problems in Your Compost

As you can notice from the table above, the only critical mold color to look out for in composting is pink.

Now, you may be asking, “how can I avoid problematic mold development in my compost?” Don’t worry, I’ve got you covered!

To prevent the development of pink mold, you need to make sure that no cleaning solutions can be mixed into the water you are using for your compost. Commonly, this happens when your water source for composting is in your dishwashing area as well.

9 Things That Should Not Be Thrown in a Compost Pile

Plant parts affected with disease and treated with pesticides, weeds with flowers, scraps of meat, whole eggs, any type of bones, oil, pet wastes, and big chunks of wood are not ideal materials for a compost pile.

To avoid problems such as growing molds and bad odors, take note of the following materials that you should never add to your compost pile!

1. Disease-Affected Plant Parts

Plant pathogens can be transferred to the soil through diseased plant parts. These can infect the plants to which the processed compost is applied.

In order to kill these microorganisms, the temperature must be above 150°F—which is not achieved in compost piles.

2. Plant Parts Treated With Pesticides

As mentioned earlier, chemicals can cause pink molds. Therefore, it is important that we make sure no trace of chemicals is inside our compost pits!

Pesticides are included in the chemicals that can slow down the decomposition process since microorganisms cannot break them down.

3. Weeds With Flowers

Weeds with flowers must not be placed in compost piles because these flowers contain the seeds. Just like plant diseases, weeds can only be destroyed by extreme temperatures (>150°F).

Therefore, if they are not destroyed, they will grow along the plant where you’ll use the compost and compete with nutrients!

Did you know that there are flowers of weed species that contain thousands of seeds? This is the reason why they multiply so quickly! They can also be dispersed through wind, water, and even when kids play with them.

Do Not Put Weeds in Compost
Do Not Put Weeds in the Compost

4. Scraps of Meat

Scraps of meat, livestock, or fish, can cause bad odors because it is an organic matter that is a rich source of protein, fats, and carbohydrates. Bacteria love those!

To put this in a practical perspective, you wouldn’t leave a piece of meat on your dining table for so long because you know it will produce a bad odor, right? The same thing goes with compost pits!

Aside from this, meat scraps also attract flies and rodents!

5. Whole Eggs

Putting whole eggs in your compost pit will surely produce a very bad smell. This is expected because, in that condition, it will rot.

They can even attract rodents such as rats, which we do not want!

6. Bones

Bones decompose slowly! It takes 6 to 30 years for them to break down, which is a really, really long time. The presence of calcium, which is an inorganic mineral, is the reason why.

7. Oils and Grease

Just like bones, oils and grease are slow to decompose. It may even take 20 to 30 years for oil to be fully degraded in the soil!

Because of this, it is not ideal to put oil in your compost.

8. Pet Wastes

Pet feces may seem a good choice for composting, but it is actually not recommended.

   Explore more on this topic in our article on pet wastes as fertilizer or compost? 

This is due to the fact that your dog or cat’s diet differs from that of farm animals such as horses or cows.

Because most pet foods contain meat and bone, these elements are also present in their waste. In comparison, horses and cows mostly eat grasses, so their waste is mostly made out of this as well.

The only difference between the two is that pet waste may contain inorganic materials, whereas farm animal waste mostly contains organic materials that decompose easily.

9. Chunks of Wood

If you are using chunks of wood in your compost, you might experience problems in turning the compost, which is an important step in composting.

Turning can avoid the development of unpleasant odors and accelerate decomposition. Parts of the compost are inverted or transferred to another container before being placed back in during turning.

Big chunks of wood are also hard to decompose. They must be chopped into smaller pieces, or—better yet—grounded into powder.

5 Hot Composting Mistakes to Avoid

What Can You Add to Your Compost Pile?

The best materials to compost are green and brown organic matter and materials with less odor. Browns give carbon, greens supply nitrogen, and water facilitates organic matter breakdown.

The ratio of browns to greens in your compost pile should be equal. You can choose a compost material from the list below.

  • Vegetables
  • Fruits
  • Eggshells, crushed
  • Coffee grounds
  • Coffee filters
  • Tea sachets
  • Shelled nuts
  • Paper or cardboard shreds
  • Grass trimmings
  • Plant cuttings
  • Straw and hay
  • Plant leaves
  • Sawdust
  • Wood chips
  • Fur and hair
  • Ashes


Is it okay to put moldy food into the compost?

Putting moldy food into the compost can help in speeding up the decomposition process. This is because the molds from the food will continue carrying out the process and are also able to degrade other organic materials in the compost.

What will happen if the compost is too wet?

If the compost is too wet, one can observe a bad odor. This can also be attributed to the lack of proper air circulating inside the compost. Thus, it is recommended to turn the compost and add more dry material to balance the moisture.

How do I keep my compost from smelling?

Bad odor in the compost is a result of too much water. In order to control this, one should place more organic material such as dry soil, more leaves, or wood shredding. Mixing these materials can absorb the excess water and can lessen the compost’s bad odor. Furthermore, remember to not put smelly materials such as meat, eggs, pet feces, and dairy.

Summary of Is Mold Bad for Your Compost?

Molds that grow in compost are not always dangerous. Oftentimes, they are normal growth and can even help fasten the decomposition process.

Only pink molds are a cause of alarm since they indicate that a chemical was mixed into the compost. In this instance, decomposition slows down.

Things that must not be thrown into the compost include diseased plant parts, plants treated with pesticides, weeds with flowers, meat scraps, whole eggs, bones, oils, pet feces, and big chunks of wood.


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