Gardening: Mulch can be good for trees - or a disaster

Nancy Szerlag
Special to The Detroit News

It’s a sad fact that most of the trees planted in American landscapes today will probably be dead in 10 to 20 years. 

Arborists recommend adding no more then 3 to 3 1/2 inches of mulch over the planting area. Too much mulch can cause moisture issues.

And the cause of many of these losses is simply because of improper use of mulch – layering it on too thickly and banking it up against the trunk of the tree. Garden writers, horticulturists and plant lovers have been writing about this for years, but sadly the message isn’t getting out. 

There is no doubt that when properly applied, organic mulches, such as wood chips, bark chips, or tub ground yard waste, can be of great benefit to trees. But used improperly, mulch can lead to their early demise.

We know that mulch holds moisture in the soil and acts as a weed barrier. A nice layer of organic material not only looks tidy, it reduces a tree’s need to compete with weeds and grass for water and nutrients.

Mulch also helps to stabilize soil temperatures and increases the beneficial microbial activity below ground. Mulch protects the surface of the soil, keeping it loose and friable and it’s Mother Nature’s way of fertilizing the tree. And a ring of mulch has saved many a tree from an out-of-control, string trimmer toting lawn cowboy. The damage inflicted by these machines cutting into the bark of trees probably runs into to billions every year.

Thinking if a little bit is good a lot is better, a mistake folks make is piling it on too deep and banking it up next to the bark of the tree. Deep mulch holds so much moisture, tree roots grow right up into it. In the hot dry dog days of summer, these mulch mountains dry out and those all-important feeder roots at the base of the tree die. That can be deadly for a young tree. 

Arborists recommend adding no more then 3 to 3 1/2  inches of mulch over the planting area. Here’s a quick tip on how to get it right. Set a few pop cans on the ground around the tree and spread enough mulch to bury them three-quarters deep in the material and you’ll be right on the money. Needless to say, beer cans work, too. 

Another serious problem associated with mulch mountains occurs when the mulch comes in contact with the tree bark. Most tree bark will rot if it remains wet over time. The mulch can also become a conduit for pests and diseases. So leave a blank space of 3 inches or more between the tree trunk and the mulch layer. 


Nancy Szerlag is a master gardener and a Metro Detroit freelance writer. Her column appears Fridays in Homestyle. To ask her a question go to and click on Ask Nancy.