Here in Ohio the most popular type of mulch that people use is shredded hardwood bark mulch, which is a byproduct of the timber industry. When they haul the logs into the sawmill the first thing they do is debark them. Years ago the bark was a huge problem for the mills because there didn't seem to be a useful purpose for it, until people realized the hidden benefits that it held. Still to this day, the bark is a headache for the saw mills, and they don't always understand how to properly handle it.

They like to pile it as high as they can so it takes up less space in their yard. The mulch really tends to back up during the winter months because there is little demand for it. In order for the mills to pile the mulch high, they literally have to drive the large front end loaders up onto the pile. Of course the weight of these large machines compacts the mulch in the pile, and this can become a huge problem for you or I if we happen to get some mulch that has been stacked too high, and compacted too tightly.

When the trees are first debarked the mulch is fairly fresh, and needs to decompose before we dare use it around our plants. The decomposition process requires oxygen and air flow into the pile. When the mulch is compacted too tight, this air flow cannot take place, and as the mulch continues to decompose it becomes extremely hot as the organic matter ferments. Sometimes the extreme heat combined with the inability to release the heat can cause the pile to burst into flame through spontaneous combustion.

In other cases the mulch heats up, cannot release the gas, and the mulch actually becomes toxic. When this occurs the mulch develops an overbearing odor that will take your breath away as you dig into the pile. When you spread this toxic mulch around your plants the gas it contains is released, and this gas can and will burn your plants.

It has happened to me twice. Once at my own house, and once on a job I was doing for a customer. This toxic mulch is very potent. We spilled a little mulch in the foliage of a Dwarf Alberta Spruce that we were mulching around, and just a few minutes later brushed the mulch out of the plant. The next day my customer noticed that one side of the plant was all brown. The mulch had only been there for a matter of minutes.

Not only did I have to replace the Dwarf Alberta Spruce, but the mulch also damaged at least 10 other plants that I had to replace. I once saw where somebody ordered a truck load of mulch, had it dumped in their driveway, and as the toxic mulch slid out of the dump truck onto the asphalt the toxic gas that was released settled on the lawn next to the driveway.

The gas, not the mulch, turned the grass brown next to the mulch pile.

This same person spread several yards of the mulch around their house before they realized the problem, and it ruined many of their plants.

Now here's the hard part; trying to explain to you how to identify toxic mulch. It has a very strong odor that will take your breath away. But then again almost all mulch has a powerful odor. This is very different than your typical mulch smell, but I can't explain it any better than that.

The mulch looks perfectly normal, maybe a little darker in color than usual. If you suspect a problem with the mulch you have, take a couple of shovels full, and place it around an inexpensive plant. Maybe just a couple of flowers. When doing this test use mulch from inside the mulch pile and not from the edges. The mulch on the edge of the pile has more than likely released most of the toxic gas that it may have held.

If after 24 hours the test plants are okay, the mulch should be fine. The purpose of this article is not to induce panic at the mulch yard, but toxic mulch can do serious damage. At my house it burned the leaves right off some of the plants in my landscape, and burned the grass next to the bed all the way around the house. It looked like somebody had taken a torch and burned the grass back about 2” all the way around the bed. If I hadn't seen it with my own eyes I wouldn't have believed it.