What Makes Mulch Smell?

Why Does Mulch Smell Bad? Causes & Solutions 

A big part of the fun of gardening in Texas is spending time in nature, which can be a delight for all five senses. But if you smell something funky, vinegary, rotten-egg-like or just plain bad while spreading mulch in your garden, trust your nose. You might have sniffed out a case of "sour mulch."

Usually seen -- or in this case, smelled --  in mulch that wasn't properly aerated or maintained during production or storage, bad-smelling mulch isn't just trouble for your nose. Those pungent odors are often caused by the chemical byproducts of anaerobic fermentation, a condition that can happen when mulch piles are too large, too moist, too compacted or not turned often enough to promote airflow through the pile.

Often highly acidic or even partially fermented into wood alcohol, bad-smelling mulch can do more than just stink up your garden. Depending on how long anaerobic conditions were allowed to progress, sour mulch can cause plant wilting, discoloration, and other forms of damage, sometimes in less than an hour after application.

At Texas Native, we know mulch -- including how to properly process and store it to avoid anaerobic conditions and a sour, bad-smelling result. Since 1987, we've been recycling natural land-clearing debris like brush, tree trimmings, and wood chips into products to help your garden and landscaping grow and look their best.

Open a bag of Texas Native mulch, and you'll immediately see (and smell) the difference between our products and the competition.

Read on for everything you need to know about why mulch smells bad, sour mulch, and the complications that can arise if you use bad-smelling mulch in your garden or landscaping. Or do yourself a favor: start strong with Texas Native brand mulch, processed with skill and care right here in Texas.

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Why Do Some Types of Mulch Smell Bad?

Everything in nature has some smell to it. But if you turn a shovel full of fresh mulch and are hit with a scent that reminds you of vinegar, ammonia, rotten eggs, or alcohol, you may be dealing with "sour mulch."

Sour mulch smells bad due to a process called anaerobic fermentation that usually occurs when mulch is stored improperly after processing. The condition happens when mulch piles are too large or not turned over often enough. As a result, mulch piles can become compacted, especially when moist, leading to a lack of oxygen reaching the deepest parts of the pile.

Under these conditions, tiny organisms called anaerobic microbes can easily thrive. If anaerobic microbes are allowed to take over, they'll start to break down the carbohydrates in the wood, producing chemical byproducts with unpleasant odors.

Compounds produced by anaerobic microbes that can make mulch smell bad include:

• Acetic Acid: Smells like strong vinegar or pickles. White vinegar you purchase in the grocery store is just watered-down acetic acid.

• Ammonia Vapor: A sharp, chemical smell that can potentially irritate the eyes.

• Hydrogen Sulfide: Smells like rotten eggs, sewage or sulfur.

• Propionic Acid: A deep, rancid odor, something like smelly socks or body odor.

• Butyric Acid: Smells like spoiled butter or pungent cheese.

• Wood Alcohol: A strong, faintly sweet, alcohol-like smell.

What Should Good Mulch Smell Like?

Tolerances for what smells bad can vary widely from person to person. That said, good, properly processed mulch shouldn't smell overwhelmingly terrible or unpleasant.

Healthy mulch should smell like freshly-cut wood or have an earthy smell, somewhat like a handful of rich garden soil. This indicates the mulch was stored and cured in properly-sized piles and was turned regularly, avoiding the anaerobic conditions at the middle of the pile that can result in sour mulch.

READ TEXAS NATIVE'S COMPLETE GUIDE TO MULCH 

Other Ways to Detect Sour Mulch (Besides the Smell)

Excess Heat

Sour mulch sometimes feels noticeably warm or hot to the touch, even if it's in a shady spot. This excess heat production can indicate ongoing activity by anaerobic microbes as they continue to break down carbohydrates in the mulch.

Acidity and Potential of Hydrogen (pH)

Another way to determine if you have sour mulch is a pH test. The pH scale ranges from 0 to 14, with a pH below seven considered acidic. The lower the pH number, the stronger the acidity.

Good mulch should have a relatively neutral pH of around 7. Sour or bad-smelling mulch, on the other hand, often has a pH in the range of 1.8 to 2.5.

That's very acidic. By contrast, white vinegar used in the kitchen has a pH of around 2.5, while straight lemon juice has a pH between 2 and 3. Imagine dumping lemon juice or vinegar all over your plants, and it's easy to see why using bad-smelling mulch is a potential disaster for your garden. pH testing kits can be found at most lawn and garden stores.

Does Texas Native Mulch Smell Bad?

At Texas Native, we've spent decades perfecting our mulch-processing protocols and techniques, from pile size to moisture levels to how we turn the mulch to prevent oxygen-deprived conditions.

The result? Products with none of the unpleasant odors and microbial byproducts that can spoil your good time in the garden or harm your plants. Open a bag of Texas Native brand mulch, and all you should detect is the sweet, earthy smell of gardening success.

Don't know how much mulch you'll need for your project? Try Texas Native's free online mulch calculator! 

Is Sour Mulch Dangerous to People, Livestock, or Pets?

While the vapors produced by sour mulch can stink, they're not considered a serious health risk for most people, livestock, or pets.

That said, some compounds produced by sour mulch -- including ammonia, propionic acid, and hydrogen sulfide -- can cause minor eye or respiratory irritation in some people, especially if they're sensitive to strong smells or have medical conditions that involve breathing issues.

Can Sour Mulch Harm My Plants?

Due to issues like increased acidity, pungent vapors, compounds that can harm plant tissues, and more, spreading sour mulch in your garden or landscaping can definitely harm your plants. That's especially true for sensitive or new plantings.

Low-growing plants in close contact with sour mulch are most susceptible to injury, and it can happen fast!  Depending on the extent of anaerobic microbe action in the mulch, symptoms can sometimes appear in as little as half an hour. However, most issues will become noticeable within 24 hours of application.

The plant damage caused by bad-smelling mulch can reveal itself in many ways, with issues often widespread in areas where it was applied. These issues may include:

• Visible and rapid wilting.

• Yellowing of leaf margins or entire leaves.

• Grass near sour mulch turning pale yellow.

• Leaf scorching, with the edges of leaves looking "burned" or blackened.

• Bleached white, brown, or light tan leaves.

• Losing leaves (also called defoliation or "leaf drop").

• Stunted or slowed growth or lack of growth.

• Plant death, especially in tender or bedding plants close to the ground.

For these reasons, play it safe: If your mulch smells bad -- like feet, rotten eggs, ammonia, vinegar, alcohol or any of the other odors we've discussed -- don't apply it! You could save your harvest or flowerbeds.

Texas Native has the shortcut to great gardens and landscaping! Shop our full line of mulch and compost products here.

How to Fix and Prevent Bad Smelling Mulch

The best way to avoid the issues related to bad-smelling sour mulch is to purchase mulch from Texas Native! Through consistency and sound science, we work hard to prevent the issue so you don't have to deal with it.

We've spent decades perfecting our techniques, resulting in a complete line of premium mulch products that are safe, all-natural, properly cured, and great for your garden, landscaping, and plants.

Our carefully-controlled process is designed to avoid the conditions that allow anaerobic microbes to create smelly byproducts and plant-destroying acids within the mulch pile. Open a bag of Texas Native brand mulch, and all you should be able to detect is a rich, clean, natural odor, not unlike that of fresh wood or turned earth.

If you're storing mulch in bulk on your property for later use, however, here are a few tips that can help keep your mulch healthy and stink-free:

Aerate and Turn

Oxygen is crucial to creating healthy, sweet-smelling mulch. Turning the mulch pile on a set schedule allows beneficial aerobic microbes to thrive, eliminating the bad-smelling compounds produced under anaerobic conditions. Thoroughly turning the pile over once or twice a month with a shovel or tractor scoop is usually ideal, though you'll want to turn the mulch more often in damp or humid conditions.

Keep the Pile Small

Caused by the sheer weight of a too-tall mulch pile pressing down on itself,  compaction is another issue that can contribute to the conditions that create sour mulch. In addition to regularly turning your bulk mulch, try not to let the mulch pile become taller than four feet high, especially if you're not turning it very often. If in doubt, spread it out!

Keep It Dry

Wet, heavy, compacted mulch is a perfect environment for anaerobic conditions in the middle of a mulch pile. As you turn the mulch, also be sure to allow it to dry. While a little moisture is fine, you don't want the mulch to feel soaked. Store your mulch in a covered area if possible. If it becomes too wet, spread the mulch out over a larger area and allow it to dry.

Neutralize Acidity

Sour mulch is often very acidic, with a pH of 1.8 to 2.5. Spread on a garden or landscaping, it can cause many issues, including wilting, leaf scorching, or even dying plants. If you suspect you have sour mulch in your bulk pile, spread it out in a thin layer (away from your garden), wet it down thoroughly, and let it dry in the sun. Doing this several times can leach away the acid, letting beneficial microbes into the pile. If in doubt, don't risk using it.

How to Prevent Bad-Smelling (Sour) Mulch

Purchase Texas Native Brand Mulch: When you purchase Texas Native brand mulch at a home-improvement store near you, you're not just buying a brand name. You're buying consistency and quality control. You're buying nearly 40 years of processing expertise, preventing issues like sour mulch that can harm your plants and reduce your yields.

Proper Storage: Avoid storing bulk mulch in piles that are too large. Piles should be no more than four feet high. Mulch stored in piles taller than six feet for long periods of time is at high risk of developing anaerobic microbe activity and souring.

Frequent Turning: Mulch piles need to "breathe" to stay healthy. Turn mulch piles once or twice a month with a shovel or tractor scoop. Make sure to reach and stir the deepest part of the pile, especially if the pile is very wet or more than four feet tall.

Ensure Good Drainage: Store bulk mulch in a well-drained area to prevent waterlogging and excess moisture, allowing anaerobic microbes to thrive. If the pile does get soaked, turn it more frequently or spread it out over a larger area to allow it to dry. Putting your bulk mulch pile under some form of cover, like an open shed, hoop enclosure, or even a tarp can also help.

Give It the Smell Test: As we've discussed, mulch that smells bad is more than an assault on your nose. The stink is caused by acids and other byproducts that could harm or kill your plants. Before applying mulch around plants, smell it. If it smells like vinegar, rotten eggs, ammonia, sulfur, alcohol, or stinky cheese, play it safe and don't use it.

Choosing the Right Mulch to Avoid Odors

Mulch producers can say all kinds of things on their packaging, but the nose always tells the truth. If your mulch smells bad when you turn it with a shovel or open the bag, you're likely dealing with sour mulch. As we've discussed, the intensely unpleasant smells of "sour mulch" usually arise from improper storage and anaerobic fermentation.

At Texas Native, we're the mulch experts. Since our founding in 1987, we've turned mulch-making into an art, keeping sour mulch and anaerobic fermentation at bay the old-fashioned way: through hard work and excellent science, so you don't have to worry about it.

Ready to learn more about using mulch, compost, and other Texas Native products in the garden? Read our Native Insights Blog for tips on using mulch, pet safety in the garden, Texas native plants, winter gardening, and so much more.

Contact Texas Native

For over 35 years, Texas Native has been collecting organic materials and transforming them into sustainable landscaping products. We're focused on diverting green waste from landfills and burn piles while helping our local contractors and homeowners conserve water and promote soil health in their landscapes.

Today, the Texas Native brand offers more than 40 different premium mulch, soil, compost, and aggregate products across the state of Texas.

Have questions? Contact us today through our convenient online form, phone (512)259-7430, or send an email to info@austinwoodrecycling.com.

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