Mulch can stink if it has become anaerobic and does not get enough oxygen. Bacteria and fungi will break down the mulch without oxygen and can release strong smells like methane, hydrogen sulfide gas, acetic acid which smells like vinegar and ammonia gas. Mulch can stink if it becomes too wet and becomes too compact causing anaerobic break down releasing bad smells.
How to stop mulch from smelling
This will support a range of bacteria to break down the mulch aerobically which will avoid bad smells.
What mulch should smell like
Mulch will smell different depending on what it is made from. Good quality mulch made from pine trees will have a sweet pine smell, mixed bark mulch will smell like a leaf and timber mix and straw mulch should smell fresh. Sugar cane mulch will have a sweet smell and grass mulch smells like dry grass.
Bad smells that come from mulch
Mulch that is breaking down without oxygen will have a strong smell similar to manure even though it may not contain any at all. This can come from the methane gas released by the bacteria and fungi breaking down the compost without oxygen. Other bad smells including a strong vinegar or ammonia smell comes from the release of acetic acid.
Sour mulch smells
Mulch that smells sour is a sign that it is breaking down without oxygen and can smell like vinegar, sewage or strong ammonia. This happens when large piles of mulch trap the gases released by bacteria breaking down the organic matter without oxygen. These gases can include methane, hydrogen sulfide gas, acetic acid which smells like vinegar and ammonia gas.
Sour mulch can damage plants
When mulch becomes sour gases can be released which can damage plants. If you place sour mulch around your plants they can wilt, leaves will turn yellow and brown, the leaves can drop off and the plant can die. This will happen faster on hot days where the gases release quickly from the mulch.
Young plants and those with new growth will be damaged most by the gases from sour mulch. Plants that grow close to the ground will also be damaged first as they are closer to the mulch where the gases are released.
Do not add sour mulch to garden beds, pots or around plants because they can be damaged. Lay the mulch out, water it, allow the gases to release and after around 3 days it should stop smelling.
How to fix sour mulch smell
Sour mulch that releases these gases can also be acidic so remove some of the mulch material from the pile and turn it over with a fork. This will add extra oxygen to the mulch and allow it to break down aerobically. This process will produce less bad smells and help the mulch to break down into compost over time.
Spread the mulch out in an area of your yard out of the way. Water the mulch and turn the mulch over regularly. This will release the gases and stop the anaerobic breakdown. The mulch should stop smelling after 3 days and it will be ready to add to your garden.
Once the mulch has stopped smelling, it can be added to the top of garden beds 2-3 inches thick to stop weeds, keep soil moisture in and break down to add extra nutrients to the soil.
How to get rid of bad mulch smell
The best way to get rid of the bad smells from mulch is to turn it over and reduce the mulch volume. This will add extra oxygen to the mix and help it to reach a neutral pH and release any bad smelling gases.
Mulch will naturally break down over time and if it has plenty of oxygen it will not release bad smells. Mulch that has broken down completely will look like compost and have a sweet earthy smell. This is how you can tell that it is breaking down with lots of oxygen and will avoid releasing methane, a bad smelling gas.
Mulch that doesn’t stink
Fresh mulch from pine trees, mixed bark mulch, hay, straw, lucerne and sugar cane mulch will not smell bad. These mulch types will have a fresh smell and will make your yard smell great. To prevent mulch smelling, make sure you lay it at a maximum of 3 inches thick. This will allow oxygen, water and a good range of bacteria to break down the mulch mixing in oxygen and avoiding bad smells.
How to tell if mulch is bad & how to solve it
Mulch that has gone bad will have a strong vinegar, sewage, manure or acidic smell. Mulch can be fixed by laying it out flat in an area of your garden out of the way giving it more air and sunlight and the anaerobic reaction that is causing the smells will stop.
Mulch that smells like cow manure
Mulch can smell like cow manure if it is breaking down without oxygen and producing methane. Methane is produced when cows eat and ferment grass in their stomach so mulch can smell like cow manure if it is breaking down without oxygen.
Stopping the anaerobic reaction and the release of methane gas is the best way to fix this problem, so mix your mulch, reduce the volume and it should stop smelling within 1-3 days.
How to deodorize mulch
Deodorize mulch by laying it out in an area of your garden out of the way away from plants. Turn it each day to give it extra oxygen and water the mulch well. This will help to wash away any chemicals that smell bad and will give the mulch and bacteria the space to break down the organic matter aerobically.
Why does mulch stink? – Summary
Mulch can stink if it is broken down without oxygen releasing smelly gases that can smell like cow manure, vinegar or even sewage. Mulch should smell sweet and earthy and like the substance it was made from like tree bark or straw. Stinky mulch can be fixed so don’t throw it away. Just give it lots of air, water and in 3 days you will have better smelling mulch for your garden.
I am an accredited practicing dietitian, experienced gardener and a dedicated cook. I love writing and sharing my experience so you can learn from my successes and mistakes.