Mulch can be very beneficial to your soil and the plants in your beds. At the same time, they are a few things you need to be aware of when using mulch. Using the wrong mulch can also have toxic effects.


What kind of mulch?

Shredded hardwood bark, a byproduct of the timber industry, is one of the most popular mulches. Upon their arrival, the first step done in the sawmill is debarking the logs. The mill then repurposes this byproduct as mulch. This was not always the case years ago since they seemed to have no idea what to do with it. It was a huge problem for them until people realized the hidden benefits bark held later on.

How mulch becomes toxic

Though they may have figured out a use for bark, mills do not always understand how to handle mulch efficiently. In order to save on space, they like to pile mulch as high as they can get it. This is especially evident during the winter months when there is lower demand. In order to achieve this, mills use large front-end loaders to make mulch pills. This, in reality, can become a huge problem since they have to drive the loader unto the pile itself. A process that often results in the mulch becoming too tightly compacted by the weight of the loader.


Before using freshly debarked mulch around plants it needs to go through a decomposition process. In order for decay to occur this process requires enough airflow and oxygen. Instead, heavily compacted mulch restricts or halts this airflow from taking place.

In some cases, as the mulch continues the decomposing process it becomes extremely hot due to the fermentation of the organic matter. This inability to release heat, combined with the extreme heat conditions can result in the pile igniting due to spontaneous combustion.

Mulch can actually become toxic as a result of this heat buildup, and the failure to release such gases. When this happens, the mulch immits an overbearing stench, making it extremely hard to breathe as you dig into it. Furthermore, it releases these toxic gas during the application, resulting in your plant being burnt.

Identifying toxic mulch

Identifying toxic mulch by the scent alone may not always be reliable, since all mulches tend to have a strong odor. Correspondingly, toxic mulch may even appear perfectly normal, with the exception of perhaps being darker in color. If you suspect a problem, it is best to err on the side of caution by testing a couple of shovels full on an inexpensive plant.

For best results, always use mulch from the center of the pile when doing this test and not from the edges. The reason for this is that the mulch on the corner may have already released most of the toxic gases. If after a full day there is no damage to the plant, the mulch should be fine to use.

The purpose of the article is not to discourage the purchase of mulch from the mulch yard, but to bring awareness.  Toxic mulch is real and can cause permanent damage, so it is best to take precautions when using mulch especially in delicate areas.