Why Does Mulch Go Bad?

September 1, 2022
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Death begets life. For many ancient Indian religions like Buddhism and Hinduism, this idea is expressed as the concept of “samsara,” the endless cycle of life, death and renewal. Samsara emphasizes the connectedness of all life and its ability to change form. Humble mulch, organic matter spread across garden and landscape soil, provides a perfect demonstration of death transforming into life. 

Yet the gardener and the landscaper must be careful about which life forms they use a mulch to support. As organic matter, often shredded wood, mulch can decay in ways both helpful and detrimental to plant life. This article will explain the benefits of mulch, how it can go bad, how to know if it has, and how to best prevent that end. 

What is Mulch?

Mulch is defined less by what it is than what it does. Most simply, mulch is a layer of material spread over soil. Most often, mulch consists of wood chips, but can also consist of straw, paper clippings, or inorganic substances like plastic, fabric or gravel.  You can get mulch delivery on this very site. 

How Mulch Works

As a layer between soil and air, mulch performs multiple functions. First, it provides shade and cover for soil, helping it retain moisture. Soil moisture level is one of the most important factors in plant growth, especially in hot, sunny climates where soils bake in hot temperatures. By keeping soil cool and damp, mulch can decrease water usage and keep root systems happy and healthy.

Regarding unwanted plants, a good layer of mulch can prevent weeds from sprouting up. By creating a barrier over the soil, mulch can keep weed seeds from reaching the soil. For seeds already under the layer, mulch can limit sunlight and growth. Aside from creating eyesores, weeds compete with other plants for finite nutrients. 

Finally, and especially importantly, mulch nourishes soil as it decomposes. Organic mulches are pieces of dead organisms rich with nitrogen, carbon, and other plant nutrients. The death of these organisms, aided by living decomposers like insects and fungus, restores important compounds that plants absorb through their roots. Mulch comprises an important step of samsara by providing fodder to transform death again into life. 

Why Does Mulch Go Bad?

There are two types of decomposition: aerobic (using oxygen), and anaerobic (not using oxygen). Herein lies the difference between creating food for plants or exposing them to detrimental chemicals. In both types of decomposition, microorganisms like bacteria and fungi digest the mulch in order to grow and reproduce. In this process, they utilize the carbon and nitrogen stored in the wood to build their cells. In aerobic respiration, or decaying mulch exposed to air, microorganisms can efficiently turn wood into energy, emitting carbon dioxide and heat in the process. When these microorganisms die, the molecules they digested from the wood become available in the soil for plants to use.

But microorganisms are highly evolved and incredibly adaptive. Unlike humans, they can exist without oxygen. In these environments, like mulch piles stacked too densely, bacteria and fungus can still digest the mulch to create energy, albeit in a much less efficient way. In this process, known as anaerobic respiration, instead of producing carbon dioxide, microorganisms create either acids or alcohol. This is imperative to producing vinegar, alcohol and other products, but problematic for gardens.

Anaerobic respiration produces “sour mulch” or mulch gone bad when the mulch is stacked too highly and densely, thus cutting off oxygen in the middle of the pile. Mulch can also sour when left in plastic bags with high moisture and no ventilation. To extend the life of mulch in bags for the short term, poke ventilation holes to allow oxygen to enter the bags. For the long term, remove the mulch from the bags completely and spread over a tarp.

The products of anaerobic reactions, especially acetic acid, methanol and ammonia, can be harmful to plants in multiple ways. Firstly, acid lowers the pH (acidity) of the mulch, which in turn can affect the pH balance of the soil. Plants prefer specific ranges and can suffer in imbalanced soils. Secondly, other compounds like ammonia can cause “fertilizer burn” by applying too much concentrated nitrogen to plant soils. Finally, volatile gasses can also damage plants as they escape from the mulch. 

To summarize, air is good for mulch, lack of air is bad. By piling mulch too highly and/or densely and cutting off the supply of oxygen, one can produce sour mulch that in turn can harm gardens and landscaping. 

How Do I Know If My Mulch Is Sour?

Definitely don’t taste it. However, your nose can do a perfectly adequate job at detecting the byproducts of anaerobic respiration that produce sour mulch. These products can smell like vinegar, ammonia or sulfur. If you want to get technical, you can test the pH of your mulch with testing strips–good mulch is close to a neutral pH, so an acidic reading near 2-4 indicates a problem. 

Can My Mulch Go Bad Once I’ve Spread It?

As long as you’ve spread an even layer of good mulch across your garden or landscaping, you shouldn’t have to worry about sour mulch. Instead, your area will benefit from the aerobic respiration for which the mulch provides the fodder. Still, knowing that mulch breaks down in this way means that it has a limited lifespan. Eventually, mulch will be turned back into the soil from which it grew, and more mulch is needed from your local landscape supply company.

We recommend re-mulching twice a year to maintain the benefits of a solid layer of mulch. Turning the mulch with a rake or a hoe can help re-introduce oxygen to the layer and prevent mold growth. Once mulch falls apart in your hand, it’s time to re-mulch. But don’t despair, that decomposed mulch is now plant food. Mulching in the spring and the fall, depending on climate, is generally best practices for keeping a healthy layer atop your garden or landscaping.