Why Does My Compost Have Mold?

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White Mold in Compost

When I was new to composting, I was concerned about the normality of mold in my compost. I did quite a bit of research and asked other gardeners in forums about this. They gave me accurate answers and as time goes on, I have realized that mold is part of the composting process.

So why does my compost have mold? Mold is often seen on dead matter like compost and it signifies full decomposition. Gardeners often wonder if mold is dangerous, but the simple answer is that mold is good in compost as long as it is mixed properly.

In this post, I want to share information with you that will help you to keep your compost in good condition.

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What Does Mold Look Like?

Mold is a cotton-like texture. The color of mold (typically green, black, white or pink) is typically determined by the type of material on which it feeds. It may also depend on the climate and region.

  • White mold is common and usually found on particles of wood in compost. The white mold in a hot compost is a very good sign that compost is decomposing correctly. (see the picture above)
  • Green mold is the most common color in compost. It usually grows in compost that has a large amount of food waste.
  • The pink color of mold is usually caused by cleaning substances in your compost. Those cleaning products have the capability to kill the organisms that help in the decomposition process. It is important to eliminate this type of mold by avoiding using water that may contain soap into your compost pile.

Can Mold in my Compost be Harmful to Me?

In general, the mold in your compost pile probably isn’t going to harm you just by handling your compost during the turning process.  As long as you aren’t consuming the mold, you should be fine!

There are exceptions to this as some people could be sensitive to mold spores so you could always wear a mask when turning your compost to prevent inhaling those spores.

If you are building a compost pile that has meat and dairy products in it, your pile could have some harmful bacteria and mold in it.  These products could attract certain animals and pests that may get sick if they try to consume these items in your compost.

In the next sections, we’ll look at ways to ensure you are building a successful compost pile.

Basic Requirements of Compost

Compost must meet some basic requirements for a great decomposition. These requirements include sufficient moisture, air and organic material. Without these three natural resources, organisms cannot live.

Sufficient moisture

Moisture is the most vital factor in the process of composting. The amount of moisture in compost has a great impact on the success of compost. The organisms that are decomposers need a sufficient amount of moisture to survive. If there is insufficient moisture, bacteria will slow down. On the other hand, if the moisture is excessive, the water will force air out. This will cause your compost to have an unpleasant smell.

Various methods are used to measure the moisture content of a compost pile. One of the simplest ways is called the squeeze test. In this test, if your compost is too dry, then it will not form a ball when squeezing it in your hand. If the compost is too wet, water will continuously drip out when squeezing it. The appropriate compost moisture will form small droplets or glisten if squeezing it in your hands.


Just like you or me, organisms that act as decomposers need air to breathe. Proper aeration is a key factor in composting. Air movement in the pile can be improved by regularly turning it. Think about when you are making fire. It doesn’t burn without air. The same principle applies to the composting process.  You can read more about turning compost piles in our article When Should I Turn My Compost Pile.

Another way to encourage air pockets is to add corn stalks, leaves, straw, or other coarse materials that are high in carbon. If your compost is all the same consistency, like thick layers of grass or full leaves, the material will mat together and become slimy because air cannot move through sufficiently.  This is why different sizes of material should be added so allow for those air pockets that let the organisms breathe and thrive.

Organic materials

Even though they are not the only part, organic materials are essentially the heart of compost. Maintaining a healthy and fertile compost demands appropriate organic materials, which is 50/50 of browns and greens. Greens help the microorganisms to grow, while browns help to provide the necessary air in a compost pile.

Simple Ways to Fix Struggling Compost

  • If you are using a container that has limited openings, air exchange will also be limited. It is important to make sure that compost is in an open area. Again, for compost to have even more access to air, you should turn it over regularly using a pitch fork or shovel.
  • If your compost pile has an unpleasant smell, it typically signifies that it is too wet. To solve this, you should add more browns, such as dried leaves, that will help absorb water.
  • When mixing organic materials, the more you break up and shred your materials into smaller pieces, the faster your compost will break down and be successful.  This provides more surface area to each piece of material and allows more access to all the tiny little organisms that want to break it down.

Things That Do Not Belong in a Compost Pile

Some families may be in the habit of throwing any and all leftovers into a compost pile. However, not every type of waste belongs in a compost pile. Some waste may cause damage to compost. Below is a list of things to avoid and the reason of avoidance.

Coal Ashes

When seeing mold in compost, it may be tempting to use ashes to cover it. However, this is not a good idea because coal ashes contain substances that are harmful to plants, such as iron and sulfur.

Meat and Dairy Products

Besides creating an unpleasant smell, waste of dairy or meat products is the main attractor of insects, animals and pests, which may disturb the process of decomposition. It is normally recommended to not include these types of products in your compost pile.

Chemical Pesticides

Compost should not be disturbed by anything that was treated with chemicals. Many individuals may put lawn trimmings that were sprayed, but this may kill the organisms that are beneficial to composting – not a good idea.

Pet Waste

Pet waste from dogs or cats could contain viruses and pathogens that can harm compost, so it is important not to use this for compost.

Breakable Items

It will be difficult for materials such as glass or metal to break into your compost. Instead, they will be causing more disturbance to the process.

Benefits of Composted Soil

Finished Compost
Here is some finished compost from my compost bin! Amazing stuff!

Nothing beats rich soil that is produced from compost. Composting greatly improves soil value. It attracts bacteria and fungi that are beneficial to a plant’s growing process. It also increases the soil’s ability to hold nutrients and water; therefore, the growth of plants is promoted. Composting is environmentally friendly as it does not disturb your garden or cause harm to the innocent insects. It also acts as the great fighter of plant pests and diseases. Adding compost in soil helps to maintain the acid and alkaline levels. Finally, composting is an easy and economical thing to do in your garden.

Compost is the best food for soil. When compost is correctly made, it will smell like black soil and the decomposed material will not be recognizable. Above all, that rich soil of decomposed organic materials will absolutely give plants healthy growth that will lead to great yields.

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But the seed on good soil stands for those with a noble and good heart, who hear the word, retain it, and by persevering produce a crop.

Luke 8:15

Related Questions

Is it okay to put moldy food in compost? Absolutely! As stated above, mold signifies decomposition, which is the exact goal of a compost pile. By adding moldy food, the decomposition has already begun!

What happens if compost is too wet? If compost is too wet, it will not break down to create the beautiful soil it is able to produce. Try adding dry ingredients (paper shreds, cardboard, dry leaves) to the compost if it seems too moist.

How do I keep my compost from smelling? Turn it to get more air into the compost. Foul smelling compost may also be a sign that the compost is too wet. See previous question for other tips.

Can you put bread in a compost pile? It is generally recommended to avoid putting all bread type products in a compost pile because it can attract animals and pests.


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Corey Leichty

Hi, I’m Corey and I love using gardening as a way to provide food for my family, learn life lessons alongside my wife, Andrea, and teach life lessons to my two sons. Do you have gardening questions? Not finding what you are looking for? Please feel free to Ask a Question (Click Here!) and I will get back with you as soon as I can!

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