Can Mulch Go Bad? | How to Fix Moldy Mulch

Mulch can go bad if it is damp and left in bags. Mulch can smell and turn rancid but is easy to fix. Empty the bag of mulch out into an open area or on a tarp. Use a rake to spread the mulch out and the rain and sun will help the mulch to dry out and balance any acidity, mold or fungus. After 1-2 weeks the mulch can be added to your garden.

Factors that can make mulch go bad include excessive moisture, lack of oxygen, and high temperatures. Signs of bad mulch include foul odors, mold growth, and clumping. There are steps you can take to prevent mulch from going bad. This article will explore more about dealing with bad mulch.

How to Tell if Mulch is Bad

Mulch that has gone bad will have a strong acidic or vinegar smell and may have large amounts of white mold or slime molds throughout. This is particularly common in bark mulch which has been kept damp without air movement. Here are some more ways to tell if mulch has gone bad.

1. Unpleasant Odor

One of the most obvious signs that your mulch has gone bad is a strong, unpleasant odor. If it smells sour, like vinegar, or has an ammonia or sulfur-like scent, it’s time to replace it. Good mulch should have an earthy or woody smell.

2. Mold Growth

Another sign of bad mulch is the presence of mold. If you notice white mold growing on the surface of your mulch, it’s time to replace it. Mold can cause health problems and can also damage your plants.

3. Pest Infestation

Bad mulch can also attract pests like termites, ants, and cockroaches. If you notice an increase in pest activity in your garden, it could be a sign that your mulch has gone bad. Be sure to inspect your mulch regularly and replace it if you notice any signs of pest infestation.

In summary, if you notice an unpleasant odor, mold growth, or pest infestation in your mulch, it’s time to replace it. Keeping an eye on your mulch and replacing it when necessary will help keep your garden healthy and thriving.

How to Fix Bad Mulch

Leaving mulch in bags without air holes can allow it to go bad but it is easy to fix. Here are the simple steps to fixing bad smelling or moldy mulch.

Step 1: Wear gloves and a mask to and empty the bag of mulch onto a tarp.

Step 2: Use a rake to spread the mulch out to 2-3 inches thick to allow air movement and water to drain through.

Step 3: Move the mulch around each day with your rake to allow more air to reach the mulch pieces and dry out.

Step 4: Wait for 1-2 weeks and when the mulch stops smelling, it can be used on your garden.

Factors That Can Make Mulch Go Bad

Mulch is a great way to keep your garden healthy and looking great, but it can go bad if certain conditions are not met. Here are some factors that can make mulch go bad:

Moisture Levels

Mulch needs to be kept at the right moisture level to prevent it from going bad. If it gets too wet, it can start to mold or rot. On the other hand, if it’s too dry, it can become a fire hazard. It’s important to keep an eye on the moisture level of your mulch and add water or remove excess water as needed. Leaving mulch in bags can trap water and cause it to grow mulch.

If you have any leftover mulch after mulching your garden, it is best to add it to your compost to break down. You can also keep mulch in large piles in the corner of your garden. It will slowly start to break down and will be great for your garden.


Temperature is another important factor to consider when it comes to mulch. If the temperature gets too hot, it can cause the mulch to dry out completely. On the other hand, if it gets too cold, it can freeze and become compacted, which can prevent air and water from reaching the soil.

Type of Mulch

The type of mulch you use can also affect how long it lasts and whether or not it goes bad. Organic mulch, such as wood chips or straw, tend to decompose faster than inorganic mulches, such as rocks or rubber.

This means that you may need to replace organic mulches more frequently to prevent them from going bad. They also will help to improve your soil over time unlike inorganic mulches.

Why Mulch Grows Mold in Garden Beds

Mulch can grow mold if it stays damp for long periods of time.

Bark mulch that is laid too thick can grow mold or fungus when laid on garden beds. Mulch that is in shade can also go moldy while on top of garden beds. To prevent and treat mold, make sure the depth is no more than 3 inches. Move the mulch around with a rake to allow the water to drain through and to add extra air.

Sugar cane or straw mulch can also form mold if it is laid too thick. The best way to prevent this is to mix through a small amount of bark chips as this will help to aerate the straw and prevent mold.

Sugar cane mulch can go moldy if it is laid too thick or kept too wet.

How to Use Old Mulch

Old mulch can be left on top of garden beds to break down. Top up with new mulch and worms and soil bacteria will continue to break down the old mulch.

Old mulch is half way to breaking down to a rich humus which is a nutrient rich addition for your soil. Mulch must be broken down completely before being mixed through the soil but you may not need to do this yourself at all.

Overtime worms and bacteria will mix the old mulch through your soil. This is the best way to work the organic matter through your soil. This will not disturb plant roots and requires no work from you. Broken down mulch will improve your soil helping to keep water in your soil, add extra nutrients and aerate the soil helping it to drain well.

How to Compost Moldy Mulch

Moldy mulch can be placed into compost bins and mixed through with other ingredients. These other ingredients including soil bacteria and worms will help to break down the mold.

Mix through fresh ingredients including fall leaves, straw, coffee grounds and lawn clippings. Over around 3-6 months the mulch will break down turning into rich compost which can be added back into your soil to increase the organic matter in soil.

Mulch piles will break down over time and will work its way into the soil naturally.

How to Avoid Bad Mulch Damaging Your Soil

Do not mix fresh, moldy or partly broken down mulch into your soil. Mulch can only be mixed through your soil when it has completely broken down into humus. This will happen first to the bottom layers of mulch and gradually reach the top.

Mulch that has not fully broken down that is dug into the soil will temporarily take nitrogen away from your plants. Soil bacteria will take up nitrogen which can starve plants.

Top up old mulch layers on garden beds with new mulch and the lower layers will slowly break down to improve your soil.

Preventing Mulch from Going Bad

To ensure that your mulch lasts as long as possible, there are a few things you can do to prevent it from going bad. 

Proper Storage

Storing your mulch properly is essential to keeping it from going bad. Here are some tips to keep in mind:

  • Store your mulch in a dry, covered area to prevent moisture from getting in. 
  • If you’re storing bagged mulch, make sure the bags are tightly sealed and stacked neatly to prevent air from getting in.
  • Avoid storing mulch near sources of heat or fire, as this can cause it to dry out and become brittle.
  • You can cover your mulch with a tarp if you are expecting heavy rain.

Regular Checkups

Regularly checking your mulch can help you catch any issues before they become serious. Here are some things to look out for:

  • Check for signs of mold or fungus, which can indicate that your mulch is too wet or has been exposed to too much moisture.
  • Look for signs of insect infestation, such as holes or tunnels in the mulch. This can indicate that your mulch is attracting pests.
  • Check the color and texture of your mulch. If it’s become dry and brittle, it may be time to replace it.

By following these tips, you can help ensure that your mulch stays fresh and healthy for as long as possible.

Bags of mulch will keep well if sheltered from the rain and kept dry.


Mulch is such an important addition to any garden, and if it moldy it can still be used. Air will help your mulch to reduce its acidity and any unsightly fungus that may have grown.

Mixing old mulch with new, fresh mulch is another great way to us it in your garden and make sure it doesn’t end up thrown out or wasted. Old much is great mulch and on its way to turning into compost.

Frequently Asked Questions

How long can mulch sit before going bad?

Mulch can last for a long time if it is stored properly. If it is kept dry and away from sunlight, it can last for several months or even years. However, if it is exposed to moisture and sunlight, it can start to break down and become moldy.

What are signs that mulch has gone bad?

The most obvious sign that mulch has gone bad is the presence of mold or fungus. Other signs include a foul odor, discoloration, and the presence of insects or other pests.

What is the average lifespan of mulch?

The lifespan of mulch depends on several factors, including the type of mulch, the climate, and how it is stored. On average, mulch can last for several months or even years if it is stored properly.

Is it safe to use old mulch?

Old mulch can be safe to use as long as it is not moldy or contaminated with pests or disease. However, it may not be as effective as fresh mulch in suppressing weeds and retaining moisture. It is recommended to refresh mulch every year or two for optimal results.

Does bagged mulch go bad?

Bagged mulch will go bad if it is left in the original bag, is damp and does not have air holes. Mulch can last well for 3-6 months in bags if it is dry, there are air holes and it is kept in a protected area. The best approach with mulch is to buy what you need in bags and use it straight away.

If mulch has gone bad, smells or contains lots of mold, allow it to air out and dry in an open area of your garden before placing it on your garden beds.

How long mulch is good for?

Mulch can last for a long time if it is left in an open area, gets plenty of air and can drain freely. Large mulch piles will slowly break down over time. Mulch piles will break down from the bottom up, particularly if they are left on soil.

Soil bacteria will come up to break down the bottom layers of mulch. Worms will join in eating the bacteria and organic matter from the mulch.

Can you leave bags of mulch in the rain?

Leaving bags of mulch in the rain can be fine if it is only for a day or so. If you have bought mulch for a weekend job and it starts raining before you get to lay it out, leave the mulch in the bags. Ideally the bags will be kept out of the rain, but if you have no other option they will be fine.

The key to success is to place the mulch on top of the soil as soon as it stops raining so the mulch doesn’t go moldy in the bag.