Why Does My Compost Have Mold? Is It Dangerous

Why Does My Compost Have Mold

If you are new to composting or are keen to get started, you might be wondering if seeing mold on your compost is normal. Being wary of mold seems to be built into humans, we want to avoid moldy food and beverages.  It can quite easily turn your stomach even just seeing it on food items. But is it different for a compost heap?

Why does compost have mold on it? Most things that are capable of decomposing are also capable of having mold on them.  Which is exactly what you are seeing on your compost heap. As organic materials break down in your compost heap you might start to see mold forming.  This is especially true in cold compost heaps rather than hot. A hot compost heap is less compatible with mold growth due to the extreme temperatures present.

If you want to learn more about keeping a healthy compost heap and what the signs of a compost heap in need of some TLS are, then read on.

What Does Compost Mold Look Like?

Mold can come in a variety of forms, textures, and colors, including black, white, orange, green, or purple.

Every day and all through the day you’re exposed to mold and mold spores, and for the most part, it’s harmless in small doses.  Typically you’ll find White or Green mold present in compost heaps.

While Mold is harmless for the most part, it’s not something we want to encourage in our compost and it can be a sign that things are not quite right with your compost heap.

Keep a lookout for sludgy looking mold or mold that smells badly.  This is a sign of anaerobic bacteria and is an unwanted side effect of a misbehaving compost heap.

Can Mold in Compost be Dangerous

For the most part, any mold present in your compost heap is going to be harmless.  As long as you wash your hands after handling it and don’t consume it the risk to you, your family, or any pets is going to be almost non-existent.

The only thing I would potentially lookout for is mold that is growing on spoiled meats, animal waste, or on dairy products.  We want to avoid adding these items to our compost heap in the first place, but if you have, the bacteria and mold present can be harmful and you should avoid handling these items.

If you suffer from hay fever or other allergies, you might find that mold spores can create a mild allergic reaction, so wear a mask if this is something you’re prone to.

To ensure you’re creating excellent compost we will take a look at the ingredients and methods needed to create the perfect compost.

Perfect Compost Every Time

Pefect Compost
Handling home made garden compost from recycled food, vegetable and green waste.

Creating compost is much like following a recipe for a cake.  Bring all the ingredients together, mix well, and then let them bake (decompose).

The essential ingredients include oxygen, moisture, and compostable compounds to break down.


Every biological process requires water to function, and composting is no different.

Moisture allows for the microorganisms, microbes, insects, and bugs to live, breed and eat.  A completely dry compost heap will be devoid of life and will remain inert for a very long time.

Keeping your compost heap moist will allow for bacteria and all the other creep-crawlies to thrive, helping speed up the breakdown of the organic materials.

However, too much water can be equally as bad.  An excess of water can cause anaerobic bacteria to flourish.  Anaerobic can break down organic compounds, but they can also create a smell and leave a sludgy mess in your compost heap.

One of the easiest ways of telling if your compost is too wet or too dry is with what’s called the squeeze test.  As the name implies, you need to squeeze your compost and see how much water is present.  A small handful of compost, when squeezed by hand, should produce a drop or two of water.

If your compost is too wet it can be like ringing out a sponge, with water pouring out. If the compost is too dry, then you’ll struggle to get any water out of it at all and it won’t clump together.


Aerobic bacteria (the good kind) require oxygen to flourish.

You don’t need to do anything fancy in order to make sure there is enough oxygen present, simply aerating your compost heap will be sufficient in most circumstances.

Aeration is accomplished by simply turning with a garden fork.  Some compost heaps can be turned by using a handle or can pivot around central access.

Moving the compost mix around allows for air to permeate the compost heap, providing valuable oxygen to the bacteria that need it.

A more hands-off method of improving airflow is by including more brown materials such as hay, dried leaves, or even cardboard.  These items have more cellulose and provide air pockets that allow air to flow evenly through the heap.

Heavy green items such as grass clippings or kitchen waste tend to clump together and prevent the flow of air.  So it’s important to mix these items into the compost to avoid the cutting of oxygen to the rest of the heap.

Compostable materials

We can’t create a compost heap without having something to compost.

The best compost heaps have a healthy mix of green and brown materials.

Green materials such as grass, leaves, and kitchen scraps provide a meal for the bacteria to grow.

Brown materials provide a structure for air to flow and help prevent to compost heap from getting too wet.

Can Too Dry or Too Wet Compost be Fixed?

While having compost that is too dry or too wet is not ideal, both situations can be rectified with relative ease and planning.

Too Wet

Make sure that your composting container has plenty of airflows and that you frequently mixing and turning your composting mixture, this will help things dry out.

You can also add extra brown materials to absorb the excess moisture.  I find cardboard works well as it’s readily available in our household from old toilet rolls, egg boxes or Amazon deliveries.

Make sure that any clumped together green material is broken up and mixed in well.  Be extra careful with grass clippings as these will compact easily and cause a brown sludge if you’re not careful.

Too Dry

There are only really two ways to fix a compost heap that is too dry, add water or add greens.

It’s normally preferable to add extra green materials if they are available, this will quite quickly increase the water content.

If extra green material is not available, then simply add some water from a hose pipe or watering can.

Don’t Add These Things to A Compost Heap

Compost heaps can break down a wide variety of materials, but there are certain things that you should avoid adding to your compost.  Either they will cause pests, diseases or just will not compost down in a way we’d like. Here are several things you should not add to any compost heap.

Barbecue Ashes

Most barbecue ashes from charcoal briquets should not be added to a garden or compost heap.  While ash from a fire contains lots of beneficial elements for gardens, commercially available charcoal briquets can contain additives that can kill plants and animals. So, unless you know the source of your ashes, its best just to bin them.

Meat and Dairy

Meat and dairy products are likely to rot before they start to compost.  The process of rotting can and will attract pests such as flies, rats, and cockroaches.  Commercial composters can process meats and dairy products as the temperatures in commercial composting plants are much higher than you can attain at home.

Treated Plant Matter

If you’ve used insecticides or weed killer on any of your plants, then these plants should not be added to the compost heap.  These chemicals can stick around for a while and have the potential to kill of the beneficial organisms and bacteria growing in your compost heap.

Pet Waste

Pet poo contains a variety of bacteria, viruses and pathogens that can cause harm to you or your family. Specialized pet poo composters are available, but they work in a different way and are not used to produce compost that you spread around your garden.

Non-Compostable Material

Don’t add plastic, glass, metal, polystyrene, magazines or any other non-organic materials to a compost heap.  They won’t break down and can be a hazard later on when you want to use your compost.

Why Create Compost At Home

Compost is an amazing, fertile, and best of all, free way of enriching your soil at home.  Compost improves the structure of your soil, it attracts beneficial bacteria and fungi and will promote plant growth.

Soil with added compost is better at holding onto nutrients, it is better able to retain moisture without becoming too wet or compacting and it’s completely organic.

If you want to be kind to your garden, your plants, wildlife and reduce your waste footprint, then composting is a fantastic way to do all of these things with spending a lot of money.

Plants with access to soil enriched with compost are better able to fight off pests and diseases, giving you bigger flowers, tastier vegetables and fruit and more vibrant foliage.

There are very few commercially available items that are as beneficial for a garden as compost.  When done properly, it’ll be richly dark with an earthy smell and won’t resemble the original compostable material.

Anyone with a garden can have a compost heap to provide them with a great way to dispose of green waste items with a very little upfront cost.


Can I add moldy fruit and vegetables to a compost heap?

Yes.  Mold signifies that something had already started the composting process.  Adding those items to a compost heap will just help speed up the process.

Is Wet Compost Bad?

Compost that is overly wet will attract the wrong sort of bacteria and will begin to smell and turn sludgy.  If it continues to remain wet then it won’t produce the sort of compost we’d like and you can end up with a pile of sludge.  Allow for plenty of ventilation and add brown items to soak up the water.

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 Can I Stop Compost From Smelling?

Compost should not smell bad.  It should smell of a tropical rainforest, with earthy tones.  If it smells bad then it’s a sign of too little oxygen getting into it.  Add more brown items and make sure to turn it.

Can I Add X Into My Compost Heap?

I advise only adding organic compounds to a compost heap and I would also avoid adding food items.  Unless the food is fruit or vegetables.  Anything else can lead to trouble.