How To Get Rid Of Bad Mulch Smell

How To Get Rid Of Bad Mulch Smell

A garden filled with beautiful greens and colorful flowers is something we all dream of. The lovely tunes of hummingbirds, visually enchanting butterflies, and the fresh air that greets you every morning – these simple joys are just some of the garden perks to look forward to!

But, when the garden suddenly smells foul, then clearly something is off. Nobody wants to spend time in a stench-filled garden! 

The most common cause of this stink is the anaerobic decomposition of mulch. However, it does not have to smell this way. If you have been struggling with this offensive garden odor, we might have the solution for you. Read on to learn how to get rid of a bad mulch smell, as well as some of the best options for organic mulch.

What natural mulch smells like

Mulch normally smells like grass, freshly-cut wood, soil, bark, or leaves, depending on what organic material it is made of. If all is well, the earthy smell of mulch is mild and does not overpower the other scents coming from your flowering plants.

Issues arise, however, if the mulch starts to develop a rotting smell that emanates throughout the garden. Many times this has something to do with poor aeration, a damp environment, or excessive heat. Mulch stored in piles or compacted bags encourages anaerobic decomposition, which causes unwanted odors. Over time, bacterial byproducts can start to accumulate in the material, causing it to become toxic to the soil, plants, and beneficial organisms. 

Why does my mulch smell so bad?

Mulch is not supposed to smell bad. A normal, healthy mulch usually takes the smell of whatever material it is made from. For example, mulch made of wood should smell like freshly-cut wood, while one made of grass clippings should smell like grass.

If your garden starts giving off sour or manure-like odors after applying mulch, this is a red flag! Mulch that emits unpleasant odors is usually not healthy for your plants. In some cases, a bad-smelling mulch can cause soil toxicity and even irreversible damage to your crops and ornamental plants.  

If you are bothered by such undesirable odors in your garden, they might be caused by one or a combination of the following:

1. Contamination

If your mulch smells like manure, there is a good chance that the bark or grass you have used has been contaminated by other organic matter or waste. For example, during the collection process, the mulch might have been contaminated with animal urine, feces, or other waste matter.

2. Excess heat and moisture

Warm weather can alter the natural odor of your mulch. On top of the excess heat, a damp or humid environment can further promote bacterial growth. Since mulch is comprised of biomaterials, the presence of bacteria can hasten the fermentation process, releasing an unpleasant or offensive odor. 

3. Anaerobic conditions

Storing mulch in airtight containers promotes anaerobic conditions that cause the release of a sulfuric odor. This is why commercially produced organic mulches often smell like rotten eggs or ammonia, since they are likely stored in heaps. 

The anaerobic degradation due to the absence of airflow can also make the mulch more acidic, dropping the pH by up to 2.0. When combined with bacterial byproducts, namely methane and acetic acid, the mulch can become toxic to your plants. Hence, if your mulch starts smelling sour, it is probably best to restore it to its usable form before applying it to your garden.

4. Mulch decomposition

Mulch made of organic materials will naturally decompose, thanks to the bacterial and fungal pathogens present in the soil. Actinomyces is the most common anaerobic bacterium responsible for the degradation, causing the release of unpleasant odor.

Decomposition is an essential process to make the macronutrients available for your plants. You might be temporarily bothered by its stench, but do not worry – the strong odor should go away naturally after a few days. Expose your mulch to sunlight and air to promote aeration and prevent the buildup of toxins. 

5. Dyed mulch or black rubber mulch 

If your mulch makes your entire garden smell like chemicals, then you are probably using colored mulch or black rubber mulch.

The dye used in colored mulch, particularly red oxide, releases a rust-like odor when exposed to moisture or hot temperatures. On the other hand, black rubber mulch is made of inorganic materials from recycled tires. Similar to dyed mulch, black rubber mulch can also fill your garden with a stench as the material heats up under intense UV conditions.

For these reasons, both dyed and rubber mulches can emit offensive odors when the weather is particularly hot or rainy. Aside from the unpleasant odors they emit, these inorganic mulches can also leach the dye into the soil and possibly harm the beneficial microbes, earthworms, insects, and your garden plants. This is why organic mulches are usually the most preferred material to enrich or insulate garden soil.

How to get rid of a bad mulch smell

Thankfully, you do not need to discard your foul-smelling organic mulch, as it is fairly easy to treat. Here are some effective ways to help you fix your mulch and get rid of that stinky smell:

1. Aerate the mulch

As mentioned previously, a lack of proper air circulation often causes the mulch to sour or produce a manure smell. Hence, to get rid of the stench, all you need to do is aerate the pile. You can do this by spreading your mulch thinly – approximately about three inches deep – across a tarpaulin on your driveway or in your garden bed. Let your mulch dry and receive plenty of oxygen for several days until it loses its sulfuric odor.  

2. Mix your mulch with soil

Another quick fix is to give your mulch a good fluff and a mix to improve the aeration. Make sure to mix it well with the underlying soil instead of just layering it on the topsoil. This will also encourage the anaerobic bacteria to continue decomposing the organic matter without causing a toxic buildup in your mulch.

3. Avoid piling the mulch for too long

Letting a pile of mulch sit in one location for long periods can promote anaerobic conditions and rot. If you want to save your mulch for later use, make sure to mix or shovel it well and provide it with enough air. Instead of storing it in large piles, it is best to spread it thinly in a sunny place, or any well-ventilated location. The organic ingredients in your mulch will continue to decompose, but this time with enough oxygen. This should keep your mulch smelling better and prevent the accumulation of harmful byproducts that cause the awful smell.

4. Do not store mulch in airtight bags

Airtight bags promote anaerobic decomposition due to the lack of air circulation. It might sound convenient to store your mulch in an airtight bag for future use, but this can actually affect its quality. 

As mentioned previously, unused mulch is best spread on a roof tarpaulin to keep it well-aerated. But, if you need to store it in a container, it is highly recommended to use an open container or a burlap sack.

Make it a habit to check your stored mulch regularly. Fluff it up at least once a month or so, to encourage the formation of air pockets. You can also poke a few holes in the bagged mulch to keep it dry and airy. With proper ventilation, your mulch should stay fresh and smell better for a couple of months.

How long does it take for mulch to stop smelling?

Mulch gives off an offensive odor due to decomposition. If you have followed all the tips mentioned above, the unwanted smell should be gone within a week. However, other factors such as sunlight, weather conditions, and humidity might also speed or slow down the elimination of the odor. Just keep your mulch in a dry environment and everything should go well.

Lastly, avoid making any changes to your mulch within seven days – trust the process! If you think your mulch still retains the offensive odor after following the steps mentioned in this guide, then the source of the odor might be something else. Check for the presence of other waste materials and compost nearby, and make some adjustments if necessary. You can also add cedar chips to quickly solve the bad odor problems.

Best natural mulches that do not stink

Almost all kinds of organic mulch release a fresh, earthy scent. They do not typically smell bad unless they are improperly stored. The key is to allow the mulch to decompose with the anaerobic bacteria and oxygen to prevent the sour or manure-like odors from taking over.

Additionally, organic mulches are safer and healthier for your vegetable crops and ornamental plants. With the help of soil-borne microbes, they can decompose easily to help enrich your garden soil.

Depending on your preference and the type of garden you have, you can opt for any of these natural mulches:

  • Cedar bark chips
  • Pine bark/straw
  • Natural compost
  • Cypress mulch
  • Grass clippings
  • Pine bark mini nuggets


Anaerobic decomposition is the main cause of stinky mulch. Storing your organic mulch in a moist environment without adequate airflow often traps bacterial byproducts such as methane and acetic acid, leaving behind a sulfuric or manure-like smell. 

But you do not have to throw away your stinky mulch! You can fix the bad smell pretty quickly if you follow the tips mentioned in this guide. Simply give your mulch lots of aeration and the smell should be gone within four to seven days.

Image: / Larisa Stefanuyk