The smell comes from decomposing wood and active microbes breaking it down
Fresh mulch often smells like manure, ammonia, or something sour because the wood chips are still actively decomposing. When mulch is first made—especially if it includes bark, leaves, or green wood—microbes and fungi begin breaking down the organic material right away. This process generates compounds that smell like manure or ammonia, even though no actual manure is in the pile.
The strongest smell usually appears in the first few weeks after mulch is made or delivered. During this time, the pile is warm, moist, and full of bacteria and fungi working at peak speed. As decomposition continues over weeks and months, the smell fades because the most volatile (easily airborne) compounds are used up or released into the air.
Key Takeaways
- Mulch smells like manure because decomposing wood releases ammonia and other nitrogen compounds as microbes break it down.
- The smell is strongest in fresh mulch and fades as decomposition progresses and volatile compounds disperse.
- Damp mulch smells worse than dry mulch because moisture speeds up microbial activity and traps odors in the pile.
- Spreading mulch in thin layers and letting it sit before use reduces the smell and allows some decomposition to happen before it touches your soil.
- A strong ammonia or rotten smell that persists for months may indicate the mulch was made from diseased wood or stored in anaerobic (airless) conditions.
Nitrogen compounds released during decomposition create the ammonia smell
The ammonia-like or manure-like odor comes from nitrogen compounds that form when microbes break down the wood. Wood contains nitrogen, and as bacteria and fungi consume the carbon in the wood for energy, they release excess nitrogen as ammonia gas and other volatile compounds. This is the same process that makes compost piles smell strong—it is decomposition at work.
Fresh mulch made from bark, sawdust, or chipped branches often has a higher nitrogen content than aged mulch, especially if the wood came from the outer layers of trees or included leaves. The fresher and "greener" the mulch, the more nitrogen is available for microbes to process, and the stronger the smell tends to be.
Moisture and temperature speed up the smell
Damp mulch smells much worse than dry mulch because moisture activates microbial decomposition. Bacteria and fungi need water to grow and break down organic material, so a wet pile decomposes faster and releases odors more quickly. A freshly delivered pile sitting in rain or in a humid climate will smell stronger than the same mulch in a dry location.
Temperature also matters. Warm mulch decomposes faster than cool mulch, which is why spring and summer deliveries often smell worse than fall or winter ones. In a warm, wet environment, the smell can be intense for the first two to four weeks, then gradually fade as the most volatile compounds are released.
Anaerobic conditions create the worst smells
If mulch is stored in a tightly packed pile with no air flow, it can develop an anaerobic (airless) environment. In these conditions, different microbes take over—ones that produce hydrogen sulfide and other foul-smelling gases. This creates a rotten-egg or very strong manure smell that is worse than normal decomposition.
Mulch delivered in large, compressed loads is more likely to have spent time in anaerobic conditions during transport or storage. If your mulch smells intensely bad—not just like manure, but like rot or sewage—spread it out in a thin layer in the sun and let it sit for a week or two. Air exposure will help it off-gas and improve the smell. If the smell persists for more than a month after spreading, the mulch may have come from diseased wood or contaminated material.
The smell fades as mulch ages and breaks down
Once you spread mulch on your garden beds, the smell decreases fairly quickly. Spreading it thin increases air exposure, which lets volatile compounds escape faster. Within two to four weeks of spreading fresh mulch, most of the ammonia-like smell will be gone, even though decomposition is still happening underneath.
Mulch that has been sitting in a pile for several months before delivery will smell much less strong than fresh mulch. If you are sensitive to the smell, you can ask your supplier whether the mulch has been aged or if it is freshly made. Some suppliers offer "aged" or "composted" mulch that has already gone through the worst of the decomposition phase.
How to reduce the smell when using fresh mulch
If the smell bothers you, let the mulch sit uncovered for one to two weeks before spreading it. This allows some of the volatile compounds to escape and gives the initial decomposition phase time to slow down. Spread it in a thin layer rather than piling it thick—thinner layers dry out faster and smell less.
Water the mulch lightly after spreading it, then let it dry slightly. This may seem counterintuitive, but a brief watering followed by drying helps settle the mulch and can actually reduce smell by allowing some compounds to wash away and others to off-gas. Avoid soaking it, which will make the smell worse by reactivating microbial activity.
If you are laying mulch around plants you plan to water frequently, the smell will fade faster because the mulch will dry and rewet in cycles, which speeds up the release of volatile compounds. In dry climates or during dry seasons, the smell may linger longer straightforward because the mulch stays drier and decomposition is slower.
Frequently Asked Questions
Is mulch that smells like manure safe to use around vegetables?
Yes. The smell comes from decomposing wood and nitrogen compounds, not from actual manure or pathogens. Once spread and exposed to air, the smell fades within weeks. Use it around vegetable beds the same way you would use any mulch—keep it a few inches away from plant stems to prevent rot.
Why does my mulch smell worse after I water it?
Watering reactivates microbial decomposition and traps odors in the wet mulch, so the smell temporarily gets stronger. This is normal. The smell will fade again as the mulch dries. If you are spreading fresh mulch, let it sit dry for a few days before watering your plants.
Can I use mulch that smells like rotten eggs or sewage?
A rotten-egg smell suggests anaerobic decomposition or contaminated material. Spread it in a thin layer in the sun for one to two weeks to let it off-gas. If the smell does not improve or smells chemical, contact your supplier—the mulch may not be suitable for garden use.
Does the smell mean the mulch is not working?
No. The smell is a sign that decomposition is happening, which is exactly what you want. As mulch breaks down, it adds organic matter to your soil and improves its structure. The smell fades as decomposition progresses, but the mulch continues to work even after the odor is gone.
How long does fresh mulch smell bad?
Most of the smell fades within two to four weeks of spreading it, especially if you spread it thin and let it dry slightly. In cool or very wet conditions, it may take longer. Aged mulch from a supplier smells much less strong from the start.