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.
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 throughout. This is particularly common in bark mulch which has been kept damp without air movement.
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.
Moldy mulch on garden beds
Bark mulch that is laid too thick can grow mold or fungus. 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.
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.
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 avoid 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.
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.
Leaving 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.
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.
Does mulch go bad? – Summary
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.
For more on treating moldy mulch check out my previous article here.
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.