Understanding Compost Mold (friend or foe?)

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Are you looking to up-level your gardening game and positively impact the environment? Then starting your very own compost pile might be the answer you’ve been looking for! Composting is a simple practice that transforms organic waste into nutrient-rich soil, which promotes healthy plant growth and reduces landfill waste and greenhouse gas emissions. Although this simple method is a win-win for both your garden and the planet, what about compost mold and its advantages and disadvantages?

Mold found in compost signals decomposition, but too much can cause strong odors and color changes like pink, white, yellow, or green. It’s best to balance your compost with carbon-rich materials, lime/wood ash, and add the appropriate moisture to encourage microorganism activity. Manage food waste within your pile by chopping the scraps finely, layering, and turning your compost appropriately.

brown bag full of kitchen scraps on white wooden floor

So, are you ready to embark on an exciting journey to become a composting expert? In this article, I will review the essential information you need to know about composting and how to handle pesky mold that may arise and try to throw a wrench in your composting plans. 

Fortunately, not all mold growing in your compost is detrimental. And you can keep your backyard compost pile healthy and happy with the proper knowledge and tools. Even in the face of runaway mold, you can join the millions of backyard gardeners who service their green oasis with a reliable compost pile and become confident in creating it correctly.  

Humble Highlights

  • Save time by achieving these 4 common steps to keep your compost pile’s moisture levels on target so you can let Mother Nature work hard on your behalf. 
  •  Discover the four most common types of mold that may appear in your compost so you can understand how well your pile is performing with a simple glance. 
  •  Save a TON of energy by avoiding these primary compost hassles so you can quickly troubleshoot your pile – even if you’re a complete composting rookie!  

Composting Purpose And Benefits 

Composting is a natural process miming how nature recycles organic materials, like leaves, dead plants, and debris, to create healthy soil. Composting is also a simple and effective way to transform organic waste materials, such as food scraps and garden waste, into a valuable resource that can help improve soil health and promote optimal plant growth. By creating nutrient-rich soil, composting also helps reduce the need for chemical fertilizers, which can negatively affect the environment and human health.

mold and compost
Creating compost at home helps recycle waste while providing nutrients to your garden that your plants can use.

Composting is a sustainable and environmentally-friendly practice that helps reduce landfill waste and its accompanying harmful greenhouse gas emissions, such as methane gas. Unfortunately, when organic waste materials are sent to landfills, they do not decompose properly and can release this methane gas into the atmosphere. 

Methane, as we all know, is a potent greenhouse gas contributing to climate change. Fortunately, composting can reduce greenhouse gas emissions by averting organic waste from landfills and turning it into nutrient-rich soil you can reuse to promote plant growth and reduce the need for synthetic fertilizers.

There’s more than just one way to create your compost successfully. This video details six various kinds that you can start doing today. So choose one that suits your gardening goals, and let’s get after it.

Creating compost goes a long way in helping your garden plants stay happy and healthy. Remember, choose a method that works for you, master it, and your garden will be better for it.

In addition to reducing waste and greenhouse gas emissions, composting also improves overall soil health. Nutrient-rich compost provides plants with the necessary nutrients to grow strong and hardy, leading to better yields, improved plant quality, and fewer plant diseases – saving you loads of time. 1

Composting also helps garden soil retain moisture, reducing water usage. Furthermore, it encourages biodiversity by creating a habitat for earthworms and other beneficial organisms that call your garden home. Overall, composting is an easy, effective, and practical way to reduce waste, conserve natural resources, and support a sustainable way of living while growing your own food.

The Basics: Achieving The Perfect Moisture Level For Successful Composting

There are four common strategies for maintaining the perfect moisture level in your composting pile, including:

  • Keep Your Pile Damp, Not Waterlogged
  • Carbon And Nitrogen Are Key Nutrients For Composting
  • Proper Air Flow Techniques For Your Compost Pile
  • What To Include And Avoid In Your Compost Pile
compost molding process
Just as there are simple methods you adhere to in the garden, there are sound composting strategies that will yield consistent and quick results.

Let’s take a closer peek at these composting basics so you can build your pile efficiently and quickly. 

Keep Your Pile Damp, Not Waterlogged

Did you know water is one of the essential ingredients for successful composting? Unfortunately, many beginning composters don’t, and those that do, either under or overwater their compost, creating potential issues that can slow your composting efforts. Luckily, you’ll know better. 2

Enough water in your compost pile can help the microorganisms break down all the organic waste you add. But here’s the thing, you need to get the moisture level just right for your pile to decompose properly. So, if there’s too little water, these microorganisms will have difficulty doing their job. But, on the other hand, they won’t receive enough oxygen if there’s too much water, making finding the right balance all the difference in your compost pile.

To best achieve this moisture balance in your compost, it’s essential to maintain moisture levels around 40-60%. But how can you tell if your pile has enough water? Again, using your senses is vital! 

A well-moistened compost pile should appear damp, not saturated. When squeezing in your hand, compost (also called humus) should have a texture similar to a wrung-out sponge, not crumble dryly between your fingers. Mature compost should also not emit unpleasant odors but rather smell damp, earthly, and fresh. These are important signals your compost is ready to be applied in your garden.

Carbon And Nitrogen Are Key Nutrients For Composting

Organic materials are essential for composting as they contain the necessary amounts of carbon and nitrogen that microorganisms need to break down waste into rich, nutrient-dense compost. Carbon and nitrogen can come from various sources, including food scraps, yard waste, and paper products, to name a few.

Food scraps you’d otherwise toss in the trash are an excellent source of organic material for composting. These can include everything from fruit and vegetable peels to eggshells and used coffee grounds. Yard waste, such as leaves and grass clippings, can also be included in your compost. Even paper products like newspaper and cardboard can be incorporated if they are shredded or torn into small pieces.

compost mold dangers
Create a dedicated compost space in your yard, but keep this pile away from your garden. Compost piles work well in the sun or shade, but if you choose a sunny area, the sun’s heat will quicken the decomposition process.

However, it is essential to note that not all organic materials are created equal when composting. Some materials, like meat scraps and dairy products, should be avoided as they can attract pests and create unpleasant odors. 

Additional materials you should avoid adding to your pile include:

  • Diseased or insect-infested plants
  • Weeds or flowers 
  • Whole eggs (eggshells are acceptable)
  • Animal bones
  • Grease and/or oil
  • Pet waste
  • Large chunks of wood
Humble Tip:
Keeping your compost as organic as possible is crucial. Therefore, I suggest adding grass clippings and yard waste to your compost pile only if you do not currently apply pesticides or synthetic chemicals, which are so common today. 

These chemicals will remain in your pile yet will likely not be fully processed when you spread your compost back into your garden. So, to keep your garden as organic as possible, skip these additions and opt for alternative carbon and nitrogen sources that have not been chemically sprayed.    

Likewise, for the cardboard mentioned above, avoid any dyed and colored boxes, as you’ll want to keep these chemicals out of your pile as well. 

Proper Air Flow Techniques For Your Compost Pile

Proper airflow is crucial for making compost as it allows oxygen to reach the microorganisms inside the pile responsible for breaking down the organic materials. Without adequate airflow, these microorganisms can become sluggish or even die off, slowing down or halting the composting process altogether. 3

One way to ensure proper airflow is by turning the compost pile regularly. This process involves mixing the contents with a pitchfork or shovel while introducing more oxygen into the pile’s center. Turning the pile also helps evenly distribute moisture and nutrients, promoting uniform decomposition and a well-mixed final product.

Humble Tip:
A good compost practice that helps improve airflow is layering the nitrogen (greens) and carbon (browns) like you would if making homemade lasagna. When layering your compost, by alternating greens and browns, you’ll create natural channels for air to circulate through the pile, improving overall decomposition rates. This method is an excellent process to follow when first starting. 

It’s also important to remember that the two basic composting methods most gardeners choose are hot and cold composting. Hot composting creates a more aerobic environment, encouraging faster decomposition speed with more turning to promote airflow, while cold composting takes longer with less. 

Although this layering technique works fine for both the hot and cold methods, it’s vital to understand that if your browns to greens ratio become unbalanced, you won’t end up with compost. For example, many first-timer composters (especially urban gardeners) create a pile with more greens than browns, making for a slimy and smelly mess. 

Instead, strive for a 3:1 ratio of three parts browns (carbon) to one part greens (nitrogen), especially in the hot composting method mentioned above. Turn your pile regularly, add moisture, and you should start seeing tangible results in no time. 

What To Include And Avoid In Your Compost Pile

Many organic materials can find their way into your compost pile, including fruit and vegetable scraps, coffee grounds, tea bags, eggshells, grass clippings, leaves, cardboard, and small twigs. These materials are high in either carbon or nitrogen and can break down quickly when adequately managed. 

While many organic materials belong in a compost pile, some should be avoided. For example, as mentioned above, meat, dairy, and oily or greasy foods should not be added as they can attract pests and create foul odors. 

Additionally, plastic, metal, and glass materials should never be included in a compost pile as they will not decompose and can harm the environment. Lastly, and not to beat a dead horse, it is best to avoid adding plants treated with herbicides or pesticides, as these chemicals can interfere with the composting process and harm your vegetable plants when you incorporate your compost into your garden soil.

Easy Composting Tips: Using Mold To Your Advantage

Seeing mold in your compost pile should not be a reason to panic, as it’s not necessarily bad. In reality, the appearance of most molds within your compost pile signals your pile decomposing naturally and does not impact the quality of your resulting compost. In addition, mold can provide a host of advantages to those seeking to cultivate a flourishing garden.

how to prevent mold in compost bin
Seeing mold in your compost should not elicit panic. Generally, finding mold within your compost means your pile is breaking down appropriately.

Have you ever caught a whiff of a foul odor emanating from your compost? Mold may be the culprit. As previously mentioned, spotting mold in your compost is a positive sign of decomposition taking place. In addition, it is a natural and essential part of the composting process, as it helps break down organic materials, making it easier for microbial life to do their task. 

However, mixing the mold thoroughly into the compost when turning your pile while monitoring for other signs of potential problems is essential to avoid any significant issues moving forward. This also means you don’t need to worry about throwing in moldy food scraps – it adds beneficial microorganisms to your compost. 4

Although mold in compost is generally not dangerous to humans, it is still necessary to use caution. For example, while mold in compost is usually not dangerous, it can cause respiratory issues for people with allergies or weaker and compromised immune systems. So, it’s best to watch for these and other signs of potential problems so you can address them early on. 

Humble Tip:
It is generally safe to put moldy food in your compost since it is a rich source of organic matter that can help nourish the microorganisms in your pile. So long as the food is still recognizable, mold is just one resulting method of how different microorganisms promote decomposition. 

If you feel safer, remove the visibly affected areas by cutting away the mold before adding the food to your compost bin. However, moldy or spoiled fruits or vegetables from a plant or tree that has been treated with chemicals or pesticides should be thrown out and not added to your compost heap. 

Understanding Different Types Of Mold In Compost

Mold is a common sight in compost and can indicate that composting is taking place effectively. Different types of mold can appear in compost, each with its own distinct colors and characteristics. The four most common types of mold you may see in your compost are:

  • Yellow Mold (Fuligo septica)
  • Slimy Green Mold
  • Spider-Web-Like White Mold
  • Chemical Pink Mold
mold in compost pile
As home gardeners, we should embrace the opportunity to minimize waste and care for the planet by managing compost piles that provide nutrients to our garden.

Let’s take a closer look at each type of mold so you can understand the meaning of each within your compost.

Yellow Mold (Fuligo septica)

Yellow mold, also known as Fuligo septica, is a fascinating mold that grows on the surface of your compost and can have a spongy appearance. This mold blooms in a bright yellow hue, giving your compost a strange yet unique pop of color. This type of mold is caused by fungi that break down the carbon-rich materials within your compost and is a tell-tale sign that indicates your compost is breaking down as it should.

While this yellow mold may not be the most visually appealing to the standard backyard composter, it’s harmless and just another exciting aspect of the composting process. 5

Slimy Green Mold

Green mold is a type of mold that looks fuzzy or slimy and is, surprise, green in color. This mold proliferates by breaking down the food scraps you’ve added to your compost. However, while this mold is usually not harmful, its growth signals that there may be too much moisture within your pile.  

green mold in compost
Regularly turning your compost encourages your pile to effectively “cook,” as beneficial microorganisms generate heat as they decompose your pile.

To effectively address this issue, adding more dry materials, along with more browns (carbon) like cardboard, leaves, or sawdust, to the compost is crucial. Also, be sure to cover your pile during periods of heavy rainfall to prevent an overabundance of moisture. In addition, consider turning your pile more often to help aerate and dry your compost.

Spider-Web-Like White Mold

This white powdery substance visible on your compost pile is actually not mold at all but rather a particular type of bacteria known as actinomycetes. This particular bacteria forms as heat builds up within your compost and can spread in a spider-web-like pattern throughout the pile. 

Contrary to what some may think, actinomycetes are a good sign of a healthy compost pile and are beneficial for the composting process. So, rather than worry about its appearance, enjoy that your compost is thriving.

Chemical Pink Mold

Finding pink mold resembling cotton candy within your compost is typically caused by bacteria that break down sugars in your heap. While this mold is a sign of decomposition, it can also indicate that the compost is too moist and needs more dry materials to bring it back into balance. 

white fuzzy mold in compost
You don’t need a massive space for your compost. Bins, buckets, bags, and tumblers of all shapes and sizes all work equally.

Pink mold is the only type of mold that can actually destroy your compost. It’s presence may be caused by cleaning agents or chemicals in the decomposing materials, and it can slow down the biodegradation process occurring in your pile. It is important to note that pink mold is a type of bacteria and may sometimes appear more orange than pink.

Signs That Compost Mold Is Causing Problems

Some types of compost mold can cause problems if it becomes too prevalent and starts to overtake your compost pile. When harmful mold takes over your compost, it prevents proper decomposition from occurring. Signs that compost mold may be causing problems include an unpleasant odor, slower than normal decomposition, and a sticky or slimy texture to the compost when handled.

mold in compost bin
Just like you keep an observant eye on your garden, you’ll want to be watchful of your compost to avoid any potential problems mold may cause.

How To Tell If Your Compost Is Bad

It is crucial to be observant and quickly identify tell-tale signs that your compost may not be completely healthy if mold seems invasive within your pile. One primary way to tell if your compost is not performing as well as it should is if it exhibits a strong, unpleasant, and pungent odor. This overpowering smell indicates that your compost is too wet or contains too much nitrogen-rich (greens) material, a common ailment in urban gardening. 6

A slimy or sticky texture is also a common sign that the compost may be too wet, leading to anaerobic (lack of oxygen) conditions that slow the decomposition process.

orange mold in compost
Consider placing several logs and branches on the bottom of your compost to avoid compaction and promote airflow. Then, layer your browns and greens on top accordingly and turn your pile often for faster results.

Another sign that your compost may need to be healthier is if decomposing takes longer than usual. Remember, depending on your technique, composting can take a few short weeks to a year or more. If you notice your compost pile is performing slower than your chosen method, it could be due to anaerobic conditions, meaning your pile lacks sufficient oxygen. As mentioned above, oxygen is necessary for microorganisms to break down organic material. If your pile isn’t getting proper airflow, consider turning your pile more regularly and monitor moisture levels accordingly.

As stated above, the two primary composting methods are hot and cold. Hot composting involves turning and incorporating your compost weekly and, depending upon the size of your pile, can take anywhere from a few weeks to several months to be adequately used in your garden. Cold composting takes anywhere from 18 to 24 months to be fully mature but requires little, if any, turning. 

Turning compost creates usable compost. But, the more you do it, the faster you’ll reach your result, assuming your technique and carbon-to-nitrogen balance are sufficient. This video explains a process for how you can achieve garden-ready compost in only a few short weeks.

Like gardening, composting is a subtle art that takes practice to understand and master. Have patience and learn by doing. Check your compost for feedback and then adjust accordingly.

Large amounts of unusually colored or textured molds, such as black or fuzzy mold, may indicate something wrong with your compost pile. While some mold is customary and even beneficial for the composting process, excessive amounts of mold can cause alarm, cause issues with the decomposition process, and influence the resulting quality of the finished compost.

Regularly monitoring your compost pile is crucial to ensure it remains healthy and free of mold and other issues that may stunt the decomposition process. You can maintain a healthy and productive compost pile by monitoring the moisture level, turning the pile regularly, and ensuring proper airflow.

Troubleshooting: Fixing Common Composting Problems

Your compost may need help to decompose efficiently if the moisture and nutrient balance between nitrogen and carbon becomes unbalanced. There are several common ways to restore this crucial balance in your pile, including:

  • Avoid Adding Too Much Fresh Material At Once
  • Proper Use Of Plastic Compost Bins
  • Balancing Wet Composting Ingredients With Lime Or Wood Ash
  • Manage Food Waste To Reduce Mold In Compost
is compost mold dangerous
One primary way to avoid many composting challenges, especially for beginners, is to compost the “right” materials while avoiding others.

We’ll take a better look at each of these basic troubleshooting methods here and see if your compost pile may need a quick retool to get back on track. 

Avoid Adding Too Much Fresh Material At Once

Adding too much fresh material (like kitchen waste) at once to your compost pile can create an imbalance in the carbon-to-nitrogen ratio, causing the moisture level within the pile to rise unnecessarily. When moisture becomes rampant, it may lead to foul odors and harmful substances, such as organic acids and alcohols, that can slow down and impair composting. You’ll also tend to notice green slimy mold accumulating in your pile. Therefore, to prevent this scenario, it is essential to balance the moisture level in your compost by mixing in more carbon-rich (brown) materials to absorb this extra moisture.

Carbon-rich materials, such as shredded leaves, yard prunings, sawdust, straw, and cardboard, are essential for maintaining a healthy compost pile. They also help to absorb any excess moisture and provide aeration, which is crucial for the beneficial microorganisms that work so hard on your behalf. Adding more carbon-rich materials can balance the moisture level in your compost pile and prevent clogging, ensuring the composting process works efficiently. 7

Humble Tip:
One excellent product that helps to absorb moisture within your compost pile and adds carbon is pelleted bedding used for horses and other small animals. These tiny pellets are tiny when first purchased, yet begin to expand once in contact with moisture. 

In fact, I think so highly of these mighty pellets because they saved my compost pile. Many moons ago, when I first started composting, I made the same mistake many newbies make: placing excess nitrogen from too many kitchen scraps in my pile and not nearly enough carbon (browns), creating an imbalance.

As a result, I encountered a similar problem many urban gardeners face – slimy and rotting compost. After researching, I stumbled upon these pellets and incorporated them into my pile. A few weeks later, my compost was headed in the right direction and not nearly as moisture-retentive. 

Proper Use Of Plastic Compost Bins

Plastic compost bins require extra care to balance dry with fresh materials. Therefore, it’s advisable to refrain from stamping or forcing materials into one of these bins to see how much more stuff can fit within. In addition, over-compacting compost can artificially stimulate anaerobic conditions, which, as we’ve covered, are not ideal.

Moreover, avoid adding thick layers to the compost bin when layering your browns (carbon) and greens (nitrogen). Remember, fresh grass clippings already have high water content. So, avoid adding thick layers of grass to your compost which will only create a slimy and soggy mat. Instead, sprinkle grass clippings in thin layers and balance them accordingly with drier ingredients.

Balancing Wet Composting Ingredients With Lime Or Wood Ash

Sometimes an abundance of wetter ingredients or naturally acidic material, such as citrus fruit, can upset the balance and make the compost heap slow to decompose appropriately. Counteract this by sprinkling handfuls of ground lime or wood ash into the mix, plus add plenty of browns like cardboard, newspaper, or the pelleted bedding mentioned above if the bin is wetter than average. 8

black mold in compost
To make great compost, you’ll want your heap to be damp, not saturated. So regularly monitor your pile for appropriate moisture levels.

On the other hand, if your compost is too dry, the bacteria and fungi responsible for the composting process will not be able to work effectively. So wet the heap again by watering it, ideally with rainwater, or add lots of fresh green material, such as kitchen scraps, to restore your nutrient balance.

Manage Food Waste To Reduce Mold In Compost

Proper food waste management is essential to reduce mold growth in compost. Therefore, chopping or shredding food scraps into small pieces is recommended to help them break down faster and reduce potentially harmful mold growth.

As previously mentioned, it is also crucial to avoid adding too much wet or green material, such as fresh grass clippings, as this can rapidly increase the moisture level in the compost pile and promote excessive mold growth. 

is mouldy compost ok to use
Layering green and brown materials properly can also help maintain a balance of moisture and nutrients in the compost pile.

Finally, turning the compost pile regularly can help aerate it and promote decomposition, reducing the risk of mold growth. This process is fundamental if you opt for the hot composting method and you’re looking for a final result sooner rather than later in your garden. 9

Humble Tip:
There are various additional strategies for fixing struggling compost aside from the proper food waste management methods mentioned above. These supplementary methods include:

1. Adding more carbon (browns) materials, like dried leaves or cardboard, to balance moisture levels.
2. Adjusting the carbon-to-nitrogen ratio by adding more nitrogen-rich material, such as grass clippings, if your compost becomes too dry.
3. Adding a compost activator or accelerator, which contains particular microorganisms that speed up the composting process.
4. Incorporating air by turning the pile or using a compost aerator to reduce mold growth and break down the compost faster.


Composting is a game-changing practice transforming organic waste into nutrient-rich soil, reducing landfill waste and greenhouse gas emissions all while promoting healthy plant growth in the garden. 

While finding mold in your compost may not be a pleasant or expected sight, most mold is typical and indicative that your compost is decomposing as nature intended. 

The four primary colors of mold that may appear in your compost are yellow, green, white, and pink. However, pink mold is the only one of the four that may present a challenge to your composting efforts as it can slow down the biodegradation process. 

Remember, to keep your compost healthy and to prevent mold from becoming too invasive in your pile, it’s best to ensure your nitrogen-to-carbon mixture is balanced. Likewise, you’ll want to incorporate lots of fresh oxygen by turning your pile often while monitoring moisture levels. Finally, strive to maintain moisture levels between 40-60%. You’re looking for a damp pile, not a saturated one.

With responsible composting practices, you can turn your organic waste into nutrient-rich soil for your plants while contributing to a healthier planet. If you haven’t already done so, don’t wait any longer to join the composting revolution. Let’s show Mother Nature some love and make the world a better place, one compost pile at a time.

Have you made a compost pile in the backyard to enrich your vegetable garden? We’d love to know how you did it and your best maintenance practices that keep it that way. So take a minute and drop a line in the comments below.


  1. Wikipedia – Compost
  2. Cornell University – Science & Engineering – Compost Chemistry
  3. Texas A&M University – Compost Pile Valuable
  4. National Library Of Medicine, National Center For Biotechnology Information – Fungal And Bacterial Successions In The Process Of Co-Composting Of Organic Wastes As Revealed By 454 Pyrosequencing
  5. Permaculture Research Institute – Fungi And Mould, The Great Decomposers
  6. Wikipedia – Ruligo Septica
  7. International Association Of Certified Home Inspectors – Compost Pile Hazards
  8. American Society Of Agronomy – Leaf Mold Compost Shows Benefit For Tomato Plants In Degraded Urban Soils
  9. Los Angeles County, Public Works – Frequently Asked Questions (FAQ)

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