The Ins and Outs of Composting Mold

Mold is a common sight in any compost pile. As organic waste breaks down, mold naturally moves in to help decompose the materials. But how much mold is normal, and at what point should you be concerned? This guide covers everything you need to know about managing and preventing mold growth in your compost.

What Causes Mold in Compost?

Mold is a type of fungus that spreads through microscopic spores in the air. It thrives in warm, moist, organic environments – making compost bins the perfect habitat. As beneficial microorganisms start breaking down waste, the temperature rises, creating prime conditions for mold.

Food scraps often already contain mold spores when they go into the compost. And since mold plays a role in decomposition, some growth is inevitable. Excess moisture is the biggest factor in uncontrolled mold growth. Too much water and green materials creates anaerobic conditions, slowing decomposition and allowing mold colonies to expand.

Is Mold Growth Always Bad?

Not necessarily! In most cases, mold is a sign your compost pile is actively decomposing. The presence of fungi is part of a healthy ecosystem of microorganisms that break down organic matter. So some mold is to be expected. Mold becomes a problem when it grows unchecked, forming thick layers that overwhelm the compost.

The most dangerous molds produce toxic substances called mycotoxins. Mycotoxins can cause health issues for anyone exposed. Fortunately, mycotoxin-producing varieties rarely show up in backyard compost piles. Mold growth is only concerning if it overtakes the entire compost pile.

Identifying Common Compost Molds

There are thousands of mold species, but a few commonly pop up in compost:

Green Mold

Green mold can appear as velvety spots or fuzzy tufts on decaying vegetation. Two common compost varieties are Aspergillus and Penicillium. They produce enzymes that break down cellulose and lignins. Excessive moisture enables these fast-growing molds to spread rapidly.

Black Mold

Black mold refers to many dark-colored fungal species. Two associated with compost are Cladosporium and Alternaria. They grow on plant matter, causing discoloration and decay. Black mold grows well in cool, damp conditions with little air flow. It produces a distinctive musty, rotten smell.

White Mold

White mold can look like fine cobwebs or cottony growths on plant material and food scraps. In compost, Sclerotinia sclerotiorum is a prevalent culprit. It’s aggressive and spreads quickly in overly moist piles. As it decomposes waste, this mold also consumes beneficial microorganisms.

Yellow Mold

Yellow patches of mold are often in the genus Aspergillus. A. fumigatus is one frequently found in compost piles. It’s associated with breaking down tough, fibrous plant tissue. Yellow mold grows optimally in slightly acidic conditions with sustained humidity.

Is it Okay to Add Moldy Food to Compost?

Moldy produce like fruit, vegetable trims, and garden plants can be safely added to backyard compost piles. The existing mold will help kickstart decomposition.

However, moldy baked goods, grain products, and nuts can potentially introduce mycotoxins. These foods shouldn’t go directly into compost bins. Discarding them in the trash is recommended.

Always use caution and wear gloves when handling moldy waste to avoid direct contact. Make sure food scraps with mold are thoroughly buried under other compost materials.

Tips for Preventing Excessive Mold Growth

Too much moisture is the main cause of unchecked mold growth. Here are some tips for keeping your compost pile balanced:

Mix in dry, coarse materials. Counteract wet food scraps by adding dry “browns” like leaves, straw, sawdust, wood chips, etc. These help absorb excess moisture and improve aeration.

Turn the pile. Turning or stirring the compost once a week prevents soggy spots. It mixes drier outer layers into the center and distributes air pockets.

Pile it right. Make sure your compost maintains the right shape and volume. Piles smaller than 3x3x3 feet don’t self-insulate well. And shape with sloped sides promotes drainage.

Protect from rain. Use a tarp or lid to cover the compost from getting waterlogged during wet weather.

Monitor moisture. The compost should feel moist but not soggy. If water drips out when squeezed, it’s too wet. Add more browns and turn it.

Let it breathe. Turning the pile and using coarse materials allows air circulation. Anaerobic conditions promote mold, while oxygenation inhibits it.

Removing Excess Mold Growth

If you spot extensive mold growth, prompt action can get your compost back on track:

Remove moldy sections. Scrape off any mold-covered materials and don’t put them back in the compost. Discard woody stems or piles overrun with white mycelium.

Stir in amendments. Mix in straw, wood chips, or shredded newspaper to create more air pockets and absorb moisture. Urine, lime, or wood ash can raise pH.

Pile it up. Assemble the compost into a mound at least 3x3x3 feet tall. The mass helps generate heat to dry out the contents.

Let it heat up. Leave the pile alone for a week or two to allow beneficial bacteria to repopulate and increase temperature. Monitoring the internal temperature can ensure it reheats.

Repile later. Once the amended compost heats up again above 120°F to kill pathogens, repile and mix the contents to normalize moisture and oxygen.

Is a Small Amount of Mold Okay?

A tiny bit of mold on finished compost is nothing to worry about. But extensive mold suggests anaerobic conditions prevented thorough decomposition. It’s recommended to screen out moldy bits rather than using heavily molded compost on food gardens.

Sprinkling thinly layered compost with a few visible mold flecks across a flower or tree bed is generally safe.

However, when growing edibles like fruits and vegetables, even small amounts of mold are best avoided. Try using the compost for ornamental plants instead of food crops if mold was an issue during decomposition.

Staying Safe When Handling Moldy Compost

Mold releases countless tiny spores that can trigger allergic reactions when inhaled. Take precautions when dealing with moldy compost:

  • Wear gloves, long sleeves and pants to avoid skin contact
  • Use a N95 respiratory mask or respirator to avoid breathing in spores
  • Work in short intervals if mold levels are high to limit exposure
  • Stand upwind when turning or spreading moldy compost
  • Wash hands, clothes, and tools thoroughly after handling
  • Keep pets away from the compost until mold and spores dissipate

Significant mold growth requires more intensive mitigation. Consider having a professional assess and remove contaminated compost in severe cases.

Common Causes of Excessive Mold in Compost

Too much moisture

Excess water creates anaerobic conditions where mold thrives. Add more “browns” and turn the pile to dry it out.

Lack of aeration

Densely packed piles prevent airflow. Turning the compost and mixing in coarse material improves oxygen circulation.

Insufficient heat

If the compost doesn’t heat up, pathogens and mold can proliferate. Make sure the pile is large enough to self-insulate.

Too many greens

Too many nitrogen-rich greens make it overly moist. Balance out with browns like dried leaves and wood chips.

Cold weather

Cool temperatures inhibit beneficial bacteria that generate heat. Protect piles from chilling in winter.


Exposed compost can get waterlogged in heavy rain. Use a tarp or shelter to keep it covered.

Unbalanced pH

High acidity creates favorable conditions for fungal growth. Lime or wood ash can raise pH if needed.

Not turned enough

Turning mixes in oxygen while exposing all contents to heat. Unturned piles can stay soggy.

Troubleshooting Excessive Mold Growth

Seeing a mold takeover in your compost? Here are some troubleshooting tips:

Symptom: Soft, smelly compost covered in white mold.

Issue: Lack of oxygen, excess moisture.

Fix: Turn pile, add coarse browns, protect from rain.

Symptom: Moldy spots deep in the pile’s center.

Issue: Insufficient aeration and heat.

Fix: Turn and fluff pile, monitor internal temperature.

Symptom: Pervasive mold despite dry materials.

Issue: High acidity promotes fungi.

Fix: Incorporate wood ash or lime to raise pH.

Symptom: Mold recurring despite preventive measures.

Issue: Stubborn pathogenic spores may remain.

Fix: Discard pile contents and sterilize

When is Compost Too Far Gone?

Seeing a little mold on finished compost is no cause for alarm. But if your entire pile becomes a dense block of green, white, black or other fuzzy mold, it’s best to start over.

Extensive mold growth makes compost potentially unsafe for gardens. Toxic spores can also pose health risks to anyone exposed. Here are some clear signs your compost is too far gone:

  • Mold covers over 50% of the total pile
  • Musty, sour, or nasty sulfur smells
  • Visible mushroom-like mold structures forming
  • Grey, black, orange, or other dark discoloration
  • Squishy, soggy, and dripping wet materials
  • Mycelium webs coating much of the compost
  • Failure to reheat after turning and amending
  • Mold persists after repeated turning and drying

You’ll have to make the judgment call at what point it’s better to ditch the batch and start fresh. If the mold has penetrated deep into the pile and persists through your remediation attempts, it’s best to remove and discard the contents.

Safe Mold Remediation Process

Remediating excessive mold growth in compost requires care to avoid spreading spores. Follow these safe practices:

Use protective gear – Wear gloves, long sleeves, pants, goggles and an N95 mask or respirator. Avoid inhaling airborne spores.

Work in small batches – Remove and handle only one section at a time to minimize exposure.

Seal in bags – Place discarded moldy compost in plastic bags and seal tightly before disposal.

Clean tools – Scrub any shovels, pitchforks, or rakes used to remove moldy compost.

Wash up – Remove protective wear, wash your hands and clothes after finishing up. Shower if extensive mold exposure occurred.

Dry the bin – Allow the now empty compost bin to dry out completely before rebuilding the pile.

Disinfect (optional) – Scrub down the bin with a 10% bleach solution to kill any lingering spores. Rinse thoroughly.

Place bagged waste in the trash, not the curbside compost bin. Handled carefully, even serious mold infestations can be fully remediated.

Can You Use Moldy Compost on Plants?

Finished compost with significant mold growth should not be used for edible gardens or crops. The risk of mycotoxin contamination makes it unsafe for foods you ingest.

Small amounts of mold are generally okay for ornamental plantings like trees, shrubs, and flowers. Diluting mildly molded compost may reduce risks.

However, heavily molded compost is best avoided altogether in gardens. Dispose of any visibly moldy finished compost rather than trying to use it up. The hazards outweigh any benefits.

Containers and raised beds intensify exposure. Soil mixes with molded compost should not be used for vegetables, herbs or other edibles.

When is Moldy Compost Safe for Plants?

The only time using molded compost on plants is advisable is if the pile was remediated correctly:

  • Moldy materials were fully removed
  • Aeration and moisture were corrected
  • Internal temperature reached 140°F for 3 days
  • Compost re-heated after being repiled
  • No visible mold remains after screening
  • Musty smells have dissipated
  • You avoided overwatering afterwards

Take time to troubleshoot the underlying issues before trying to use a moldy batch. And get a clean start if the compost became anaerobic or was abandoned unturned.

With vigilance and proper curing, a mold invasion can sometimes be overcome. But when in doubt, play it safe and start fresh.

In Conclusion

A small amount of mold in compost is no cause for alarm. But left uncontrolled, compost mold can take over and create hazards:

  • Monitor moisture, turning piles as needed
  • Introduce more coarse, dry materials
  • Ensure proper aeration to inhibit mold
  • Discard finished compost if mold is extensive
  • Take safety precautions when handling moldy compost

While mold needs moisture to grow, compost needs moisture to decompose. Finding the right balance is key to preventing uncontrolled fungal growth. With proper management, compost mold can be avoided or overcome.