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Time for a little detective work

August 11, 2011
By Stephanie Bethke-DeJaeghere , Marshall Independent

Part of volunteering as a Master Gardener is playing sleuth - figuring out a mystery. This is sometimes true, especially when introduced to a problem that you have not seen before and the home gardener has never experienced before either.

The damage symptoms had me as a loss. The leaves were scorched or bleached looking on many of the plants surrounding a home landscape area. There were a few annual plants that had been also planted in this area which were dead.

At first look, it seemed that it was caused from our weather (a lot of rain, then no rain equals funny looking plants) or it could have been from someone spraying an herbicide around the edge of the plants. However, the home owner said they watered when necessary and did not use any herbicides lately around the plants nor did they use a lawn service. The same with the neighbors who were in close proximity of the property.

This left me going back to the internet, from which, all things can be found. A return to the home owner's garden showed that we were on the right track. The answer was sour mulch. After some time searching and then finding some of the symptoms that fit sour mulch, it was a definite.

Mulch materials range from all sorts of things: rock, cut grass, bark, straw, you know it. We all know that mulching has many, many benefits which include suppressing weeds (my favorite), keeping moisture in and making things look good. We generally use anywhere from 3 inches to 6 inches of mulch to control or improve the area that is being mulched.

Hardwood bark mulch can cause something called sour mulch. Most of the time, this is not a problem but occasionally, such as what happened to the home owner that I visited, they were not so lucky.

When a hardwood bark mulch pile becomes too large or too wet, the pile moves from being an aerobic (with oxygen) to an anaerobic (without oxygen) pile. There are small organisms that favor this type of environment and thus, they produce waste products that include acetic acid, methanol, ammonia gas and hydrogen sulfide. Toxic vapors are released from the mulch causing the plants to become injured. Low growing plants, annuals planted in the mulch and low growing shrubs tend to be harmed most often. The biggest indicator of if your mulch is sour is smell it. Yep, if it smells bad, it probably is bad.

So, what did the home owner do? Well, the mulch was moved away from the plants in question and the remaining mulch was spread out, and raked or turned once a day for several days. This helps to vent any toxic gases. After a few days, the mulch was replaced. They were eventually going to use the remainder of a pile of mulch for another garden but before they used what was left, the spread it out and turned it for a few days in order to make sure that it was not going to burn their plants. Incidentally, it is only hardwood bark and not pine bark that causes this to happen.

Good mulch should have a good fragrance.

For more information on gardening, you can reach me at



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