Mulch is a wonderful gardening aid that prevents weeds from germinating and water from evaporating. But inevitably, you always have to buy more bagged mulch than you need. Can bagged mulch go bad? Can you keep mulch from one year to the next? And what do you do with mulch that’s past its prime?
Bagged mulch can go “bad” when it has too much moisture and heat and not enough airflow, but most “bad” mulch can be easily remedied. To extend the life of your bagged mulch, ensure there’s ventilation by poking holes in the bag (short-term) or laying it out on a tarp (long-term).
How to Tell if Your Mulch is Bad
You can tell if your mulch has gone bad if it is:
- Infested with insects. If it’s crawling with insects, you probably will want to toss it away and start fresh.
- Smells like rotten eggs, vinegar or sulfur. When mulch goes anaerobic (decomposing without access to oxygen), it causes a sour smell. Sour mulch will kill plants if applied directly! But you can still save the mulch. To remedy, spread out the mulch into a thin layer and soak with water. Leave it to dry completely. After a few days, the smell should disappear and you can use the mulch on garden beds safely.
- Covered in artillery fungus. Artillery fungus spores look like specks of tar. They’re harmless to plants but can stain your siding, cars, and other surfaces. Replace mulch every 6 to 12 months to keep this fungus from growing, but if you spot it, simply remove it before the spores can land on any nearby surfaces. Clean surfaces as soon as possible.
- Covered in orange, yellow or red slimy molds. Slime molds are temporary and harmless, so just let them be or turn over your mulch to break them up and improve ventilation.
How to Properly Store Bagged Mulch
Ventilation is the key to properly storing bagged mulch, as air slows down the decomposition process and thus extends the mulch’s life.
If you’re storing bagged mulch for the short term (a few weeks or months), you can store your bagged mulch in the bags they came in. Check that the bag has air holes, and if it doesn’t, poke a few in yourself.
If you’re storing it for longer-term use, transfer the mulch to a plastic container with ventilation holes added (remember to label!). For the best results, store it loose on a tarp in a thin layer to ensure the most airflow.
Then put it in a dry area, like an unheated garage or shed.
What To Do With Old Mulch?
What you can do with old mulch depends on the type of mulch you use.
Natural mulches (like bark mulch, straw, or leaves) can be:
- Left in place if it’s in a thin layer. Bark mulch decomposes the slowest, but so long as the fresh mulch layer doesn’t make it over 2 to 3 inches high, then you can leave it in place and refresh it with new mulch. Loosen up the layer of old mulch first to prevent it from compacting.
- Remove it for the compost pile. Top up your compost pile with these carbon sources. Wood mulches use more nitrogen to break down, restricting the nitrogen for other uses, but once completely broken down, the nitrogen is released back into the compost.
Don’t reuse mulches that have come in contact with fungal diseases like root rot, early blight, and wilt disease!
Dyed mulch can contain chromated copper arsenate (CCA), an extremely harmful chemical that you do not want in your garden. Check with your local municipality for guidelines on how to dispose of dyed mulch.
If your municipal recycling facilities are one of the rare beasts that can accept plastic landscape fabric, then you can recycle it. Your best bet is to either avoid using landscape fabric in favour of natural mulches or to continue to reuse it. I reuse small scraps to keep the soil moist when germinating seeds outdoors.
So long as you store leftover mulch with proper ventilation and in a dry place, you can keep bagged mulch for at least a year. Even if mulch goes bad with a sour smell or slime mold, you can easily rehabilitate it.