The smell comes from decomposition and ammonia released by microorganisms breaking down wood chips
Fresh mulch, especially dark-colored bark mulch, often smells strongly like manure or ammonia because of what happens inside the pile during storage and transport. Wood chips, bark, and other organic material are colonized by bacteria and fungi that begin breaking down the material when ready. This decomposition process releases ammonia and other nitrogen compounds as a byproduct—the same compounds that give manure its distinctive odor.
The smell is strongest in newly delivered mulch because the decomposition is most active in the first few weeks after the pile is created. The material heats up as microorganisms work, and warm piles release more volatile compounds into the air. Once you spread the mulch in your garden and it cools, the smell fades within days to a few weeks as the pile dries out and decomposition slows.
This is a normal part of the mulch-making process and does not mean the product is contaminated or unsafe to use around plants and vegetables. The smell is unpleasant but harmless to you, your family, and your garden.
Key Takeaways
- The manure-like smell comes from ammonia and other compounds released by bacteria and fungi decomposing the wood chips in the pile.
- Fresh mulch smells strongest because decomposition is most active in the first few weeks after delivery, and the pile is still warm and moist.
- The smell fades within days to a few weeks once you spread the mulch in your garden and it dries out.
- A strong ammonia smell is normal and does not indicate the mulch is unsafe or unsuitable for vegetable gardens and edible plants.
- Mulch that smells like rotten eggs or sewage, rather than manure, may indicate anaerobic decomposition and should not be used until it dries and the smell clears.
Why the decomposition process creates an ammonia smell
Mulch piles are a feast for microorganisms. Bacteria and fungi in the wood chips, bark, and any soil clinging to the material begin breaking down cellulose and lignin—the structural compounds that make wood rigid. As they consume these materials, they release nitrogen compounds, primarily ammonia, into the air.
The ammonia smell is strongest when the pile is warm and moist, which is exactly the condition inside a freshly delivered mulch pile. Decomposition generates heat, and the pile retains moisture from rain or watering during storage. Warm, moist conditions speed up microbial activity and cause more ammonia to volatilize—to turn into a gas and escape into the air.
This is why mulch stored in a warehouse or yard for weeks before delivery smells worse than mulch that was freshly chipped. The longer the pile sits, the more decomposition has already occurred, and the more ammonia has built up in the material.
How long the smell lasts after you spread the mulch
Once you spread mulch in your garden, the smell begins to fade almost when ready. The material is no longer in a dense, warm pile—it is spread thin, exposed to air, and cooling down. Ammonia volatilizes faster in open air, and the pile dries out as the sun and wind work on it.
Most gardeners report that the smell is barely noticeable within three to seven days, and completely gone within two to four weeks. The exact timeline depends on how thick you spread the mulch, how much sun and wind your garden gets, and how much rain falls. A thin layer in a sunny, breezy spot will dry and stop smelling much faster than a thick layer in shade.
If the smell bothers you, you can speed up the process by spreading the mulch thinner than you normally would, or by letting it sit in a pile in your yard for a week or two before spreading it. The smell will fade during that waiting period as the pile dries and ammonia escapes.
When a mulch smell is a warning sign
A strong ammonia or manure smell is normal and harmless. However, a smell like rotten eggs or sewage is a different problem. This smell indicates anaerobic decomposition—decomposition that happens without oxygen—which produces hydrogen sulfide and other sulfur compounds instead of ammonia.
Anaerobic decomposition usually happens when mulch is stored in a very dense, wet pile with poor air circulation, or when it contains a lot of fresh grass clippings or other high-nitrogen material that packs down and excludes oxygen. The mulch is not dangerous to handle or use, but it should dry out and air out before you spread it in your garden. Spread it in a thin layer in a sunny spot and let it sit for a week or two. Once it dries, the smell will clear and the mulch will be safe to use.
If the mulch smells like rotten eggs even after it has dried out in your garden for several weeks, do not use it. This is rare, but it can indicate contamination or a problem with the source material.
Using fresh mulch around vegetables and edible plants
The ammonia smell does not mean the mulch is unsafe around vegetables, herbs, or fruit plants. The ammonia is a gas—it escapes into the air and does not stay in the mulch or leach into the soil in harmful amounts. Once the smell is gone, the mulch is indistinguishable from aged mulch in terms of safety and plant health.
If you are concerned about the smell near a vegetable bed, you can wait a week or two before spreading the mulch, or you can spread it and straightforward accept the temporary odor. Both approaches are fine. Some gardeners prefer to use aged mulch (mulch that has been sitting for several months) around edible plants just for peace of mind, but this is a preference, not a requirement.
Mulch itself does not add nitrogen or other nutrients to the soil in the short term—it breaks down slowly over months and years. The ammonia smell is a sign that decomposition is happening, but it does not mean the mulch is "hot" or will burn plants. You can use fresh mulch around any plant in your garden.
How to reduce the smell when you receive mulch
If you are ordering mulch and want to minimize the smell, ask the supplier how long the mulch has been sitting in their yard. Older mulch that has already begun to dry out will smell less than freshly made mulch. Some suppliers can tell you whether the mulch was recently chipped or has been aging for several weeks.
When the mulch arrives, you can also spread it when ready rather than letting it sit in a pile in your yard. The sooner it is spread thin and exposed to air, the sooner it will dry and stop smelling. If you cannot spread it right away, break up the pile with a shovel or rake every day or two to increase air circulation. This will help it dry faster and reduce the ammonia buildup.
Some gardeners add a thin layer of aged mulch or compost on top of fresh mulch to mask the smell, though this is more about comfort than necessity. The smell will fade on its own within a few weeks regardless.
Frequently Asked Questions
Is the ammonia smell bad for my health or my family's health?
No. The ammonia concentration in the air around fresh mulch is not high enough to cause harm, even if you are working in the garden for hours. You may find the smell unpleasant, but it is not toxic. People who work with mulch professionally are not at risk from the smell.
Can I use fresh mulch around my vegetable garden right away?
Yes. The ammonia smell does not indicate the mulch is unsafe or will harm your plants or vegetables. You can spread it when ready. The smell will fade within a few weeks as the mulch dries out.
What if my mulch smells like rotten eggs instead of manure?
That smell indicates anaerobic decomposition, which happens when the mulch was stored in a very dense, wet pile. Spread it in a thin layer in a sunny spot and let it dry for a week or two. Once it dries, the smell will clear. If the smell persists after drying, do not use that mulch.
Does the ammonia smell mean the mulch is adding nitrogen to my soil?
No. The ammonia is escaping into the air as a gas. The mulch will eventually break down and add organic matter to your soil, but this happens slowly over months and years, not during the initial smell phase.
Can I speed up the smell fading by watering the mulch?
No—watering will slow it down. The smell fades faster when the mulch dries out. If you want to speed up the process, spread the mulch thin and let the sun and wind dry it out.