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Why Does Mulch Smell? The Best & Easiest Way to Fix It!

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Have you ever decided to boost yourselves up by going out to smell the illusive earthly scent of your garden? And instead, you sniff in a foul rotten aroma through your nostrils that cause your olfactory bulbs to suffer.

Well… that’s the mulch GONE BAD!

But why does mulch smell so bad?

The mulch itself doesn’t have a disturbing odor. It’s the toxic gases and chemicals that emit a foul smell. These gases and chemicals are formed in mulch due to anaerobic decomposition or fermentation. They include methane, hydrogen sulfide, carbon dioxide, methanol, and acetic acid. The mixture of these would smell something like a mixture of ammonia, rotten eggs, and vinegar.

Why does mulch smell
“Why does mulch smell?” – Background Image via Jon Moore.

Why Does Mulch Smell?

The mulch smells when manufactured and stored in the combination of high moisture levels and the absence of oxygen.

The anaerobic decomposition causes the organic raw material to decay and form toxic gases like methane, hydrogen sulfide, carbon dioxide, and chemicals such as acetic acid and methanol.

These products build up in the mulch and make it smelly and acidic because the mulch can’t find any air to breathe in the airtight packages — abiding in the anaerobic conditions.

Therefore, the answer to “Why does mulch smell?” would be:

The mulch smells because of the toxic buildup of different gases — aka biogas — and chemicals resulting from anaerobic decomposition and fermentation of the organic raw material present in it.

How Should Mulch Smell Like?

In general, mulch doesn’t have a particularly bad smell. It is not strong or unpleasant.

Fresh and healthy mulch smells like the mulch itself. 😅

You got it, right? I mean that mulch should have a woody smell or a smell like fresh compost.

This type of healthy mulch is very beneficial for your plants. Mulch provides your plants with a protective layer in harsh summers and freezing winters.

It also helps to conserve moisture and allows better air circulation. Moreover, it is a direct source of nutrients for the plants.

Many healthy bacteria and fungi are naturally present in the mulch. They decompose the mulch over time by a process called composting.

The composting process amends the soil by converting complex organic raw material into simpler substances that are easily absorbed by the plants.

So, if your mulch smells fine, it is good to go for mulching.

The Smell of Bad Mulch?

If your mulch has gone bad due to anaerobic decomposition, it would have a very unpleasant smell. The smell really depends upon the chemical composition of the mulch and the amount of compost present in it.

The mulch itself doesn’t contain manure. But mostly, the rotten or decayed mulch would have a similar smell that actually comes from hydrogen sulfide.

Why does mulch smell? Well… it’s because of the nasty chemicals.

So, if your mulch smells something like a mixture of ammonia, rotten eggs, and vinegar, it indicates that it has undergone anaerobic decomposition.

Is Smelly Mulch Bad for Your Plants?

You can tell by sniffing your mulch if it is good for your garden or not.

The smell of your mulch indicates its level of toxicity. It might also give you an idea of the mulch’s pH value.

Both of these magnitudes are directly related to your plant’s health. If plants are grown in sour or acidic mulch, they are prone to severe damage.

Toxic mulch can result in yellowing, scorching, and wilting of the foliage. The harm caused by acidic mulch is usually irreversible. The damaged plants sometimes reach a point where they ultimately die.

Therefore, you should properly examine the mulch — particularly its smell — before applying it to your plants or trees.

If it has an upsetting and foul odor, it may contain all the harmful elements explained before. And it would have the pH lying anywhere in between 2 and 6, which is injurious to your plants.

How to Fix Smelly Mulch?

Now that you know how to diagnose your mulch with just a little snuffle, it’s time to fix your foul-smelling mulch.

So, the easiest method to fix the smelling mulch is by spreading it on the ground — preferably under sunlight — and letting it dry. Disperse the mulch in such a way that there’s maximum air circulation through it.

This process is called aerobic breakdown. It is opposite to anaerobic decomposition and occurs in the presence of oxygen.

This is an efficient way to get rid of excess moisture. As excess moisture could also be a cause of the rotten smell of the mulch.

If the smell is due to natural decomposition and is not very unpleasant, it will go away on its own in a couple of days.

But if you follow the process of aeration as explained above to remove the sour smell (due to anaerobic decomposition), the smell would leave the mulch in a day or two at maximum.

This makes the easiest method to fix your foul-smelling mulch.

How to Properly Mulch Your Plants?

Now that you know all about the smell and toxicity of your mulch, it’s time that you learn how to properly apply mulch to your plants.

There are a few things you should keep in mind while mulching your plants.

  • Always use organic mulch for your gardens.
  • You should never apply mulch deeper than 2 to 3 inches.
  • Never apply mulch in contact with the base of trees. Mulch helps to retain moisture. If you build up a large pile of it around the trees, it can cause stem or root rot.
  • And lastly, you should reapply or entirely change the mulch every one to two years.
  • Mulch (particularly the one with high wood content) gets compact with time. Therefore, before reapplying, spend some time turning the mulch around.


To conclude the talk, I must appreciate the fact that you are concerned for the health of your plants, although the problem is just related to the mulch.

It is always better to take proper care of our rooting media like potting mixes — or mulch, as it also provides the plants with the necessary moisture and nutrients.

I hope that I was able to answer your question “why does mulch smell?”

If you found this article helpful, please share it with your friends or family.

Also, feel free to let us know if you have any questions in the comments below.

Regards, Mahad H.