Mulch is a highly essential organic matter for gardening, and many people love using it because of its ability to retain moisture and prevent weed overgrowth. But despite its widespread use, mulch may sometimes emit odors that may be pretty offensive—discouraging many gardeners from mulching.
The smell of mulch cannot make you sick. The odor is mainly due to the process of natural anaerobic decomposition, which produces several gases. These fumes are non-toxic when inhaled in small amounts.
In the rest of this article, I’ll explain several need-to-know points about mulch and include more details as to why it smells so bad, how dangerous its fumes are, and what mulch smells like when it’s good. I’ll also discuss the best mulch types and how to get the offensive smell out of your home.
Are Mulch Fumes Dangerous?
Mulch decomposition often leaves behind substances like methane gas, hydrogen sulfide, and acetic acid. None of these substances is dangerous when inhaled in small and diluted quantities.
Methane is a natural gas found in landfills, natural gas systems, coal mining, and agricultural areas. Interestingly, it’s the world’s second most abundant greenhouse gas—only behind CO2.
Although it’s harmful to the earth’s atmosphere in the long run, methane gas isn’t toxic to the average human when inhaled in small amounts. However, excess methane can significantly reduce the amount of oxygen in the atmosphere, which may lead to suffocation in tight spaces and crowded areas.
Hydrogen sulfide is another major gas that influences the smell of mulch. If you’re smelling something similar to rotten eggs, this colorless gas is most likely the culprit.
Luckily, Hydrogen sulfide’s smell dissipates as soon as it gets into the air. So, while the rotten egg smell may be unsettling, it will not harm you or any pets you may have.
Acetic acid is responsible for mulch’s peculiar vinegar-like smell. Vinegar comprises about 4 to 6% acetic acid. And although some irritation may occur when it comes in contact with your nose, it doesn’t pose any serious health risk.
Ultimately, you’d have to remain constantly exposed to them for many hours to get sick because of these fumes. Therefore, I’d recommend holding off on buying those protective masks in bulk.
While none of these substances is harmful to humans, they can seriously harm your plants if the chemicals seep into the soil. Your plants may droop, wilt, or even stop growing altogether.
Sour and dangerous mulch, left unchecked, can also lead to the death of a few plants in your garden—or even damage the entire garden in certain situations.
Why Does Mulch Smell Bad?
Mulching is an inexpensive gardening process that gardeners have used for decades. However, the smell is a problem common to many of these gardeners when using mulch. It comprises organic matter such as plant residue, chopped leaves and trees, shredded bark, sawdust—and even paper.
Sometimes, they also include black rubber and dyes. As such, mulch undergoes decomposition under the influence of time, similar to all organic matter.
This decomposition accelerates due to the presence of two significant microorganisms: bacteria and fungi. Actinomyces, a type of bacteria, can be commonly found in the decomposition process. Interestingly, this type of bacteria often thrives under anaerobic conditions.
Mulch typically undergoes anaerobic decomposition because it doesn’t allow enough air to pass through it. After getting some mulch, you would quickly realize that it’s already beginning to smell without being used. That’s why mulch is commonly stored and applied in piles that prevent oxygen passage.
Anaerobic decomposition is necessary to release nutrients into your plants and prevent the growth of weeds. The decomposition process often produces substances like acetic acid, methane gas, and hydrogen sulfide.
Also, because oxygen is absent from this decomposition process, it almost always leaves an unpleasant smell in its wake. These factors can make your mulch smell like rotten eggs, ammonia, or strong vinegar.
Ultimately, it increases the number of nutrients in your soil. I recommend taking soil measurements before and after mulching to see how necessary mulch is for your plant. On many occasions, it means testing for the amount of nitrogen, potassium, and phosphorus in your soil using a soil tester.
More often than not, this smell is tolerated by most gardeners because it’s necessary for plant growth. However, the scent may be too smelly to ignore if you’re a casual gardener.
But are these fumes dangerous to people?
How Should Healthy Mulch Smell?
I always tell my friends that you can’t tell when mulch smells terrible if you don’t know how good mulch smells. And because lots of commercial mulch comes from tree remnants like wood chippings and sawdust, you should expect good mulch to smell that way.
Sometimes, manufacturers add compost when preparing their mulch to make it double as a fertilizer. Typically compost arises from the decomposition of plants, food remnants, and other organic materials.
This decomposition commonly results in the development of worms and fungal creatures. As such, it may smell somewhat pungent, but a mild smell shouldn’t cause alarm.
The stinky smells often dissipate into the air rather quickly. However, if it smells much sourer and more persistent than you’re comfortable with, you may need to change or alter it.
Contrary to popular belief, there’s no manure in most mulch. It only smells like manure when it’s rotten and decomposing.
Black rubber and dyed mulch are equally strong-smelling mulches. These mulches are often aesthetically pleasing, made from rubber tires and organic dyes, but can create consistently off-putting smells.
Dyed mulch is reddish, caused by using red oxide and chromate copper arsenate. Both these chemicals often create terrible smells when they get in contact with too much heat or water. So, take this into account if you’re trying to buy mulch.
The Best Type of Mulch To Use for Your Garden
You can make mulch from organic materials like wood and dead foliage or by including some artificial materials. Some synthetic materials include dyes and black rubber—so natural mulch is the way to go for the best results.
Many synthetic mulches release toxic chemicals upon decomposition, significantly endangering your plants. On the other hand, the gases that natural mulch releases cause relatively little to no harm to you or your garden.
Other good examples include:
- Wood chips
- Bark mulch
- Cypress mulch
- Straw mulch
- Cocoa bean hulls
Should You Remove Old Mulch?
It would be best to replace your mulch yearly. After mulch fully decomposes, it often stops giving nutrients to the soil and loses its ability to retain moisture. When it gets to this point, you need to replace the mulch for your agricultural bedding.
Interestingly, different types of mulches decompose at various rates. And organic mulch decomposes way faster than artificial ones.
Here’s what to do when you notice your mulch is beginning to go old:
- Remove its topmost layer using a rake or any other handy tool.
- Add your new mulch on top.
- Let the fresh mulch settle in over the next few days.
This practice saves you lots of time rather than performing a complete replacement.
The foul smell of mulch isn’t the only thing that can have health concerns for humans. So, you may worry if inhaling mulch dust is unhealthy. Check out my blog post to learn about the health risks of mulch dust: Can Inhaling Mulch Dust Make You Sick?
When taken care of properly, mulch dramatically helps to improve your plant’s health and retain all the moisture your soil needs. For the most part, you can safely ignore the slightly unpleasant smells that come from this organic matter. However, if it smells sour and foul, you may want to let in some air.
Luckily, the smell of mulch isn’t harmful to people and cannot get you sick because the toxic fumes aren’t present in amounts that may be considered dangerous. Remember to replace old mulch to continue to foster your plant’s growth, and your garden should thrive!
Fresh mulch even smokes sometimes. If you wonder why you could check out my other article. I’ll explain how mulch piles build heat and how to prevent them from steaming or smoking: Why Does Fresh Mulch Smoke Sometimes?