Mulch can kill trees–that sounds kind of extreme, doesn’t it? But it caught your attention, right? I suppose it would be more accurate to say improper mulching can kill trees, but as I look around me, improper mulching of trees seems to be the standard practice, the only kind of mulching around trees that’s being done.
The previous owners of our new home appear to have paid a lot of attention to detail and tried to rebuild and remodel everything correctly, so the house would endure. They very considerately left behind a paper detailing the house’s history, in which they say that this oak tree was planted in 1885, the same year the house was built:You can see that the trunk comes straight out of the ground, like a telephone pole. Compare that to this tree growing naturally in the woods of our old home: While the previous owners took great pains when remodeling the house, they apparently didn’t realize that the way they mulched the trees would eventually kill them. I’m sure they wanted the trees to live as long as possible, especially a tree so intimately connected to the history of the house itself.
How does mulch harm trees?
When you put mulch around the trunk of a tree and cover the root flare, you are covering up parts of the tree that were meant to be exposed to air. The excess moisture can cause rot, but the trunk of the tree is supposed to have access to air, and by piling up mulch against the bark you are preventing this. If the mulch is left there long enough, the tree will try to compensate by growing adventitious roots. Unfortunately, these same roots can wind up wrapping around the tree and girdling it. Figure 11 on this article written by a tree preservation expert illustrates this girdling in progress. The tree winds up killing itself. So we have three ways improper mulching can kill a tree:
In addition, the stress of improper mulching could make the tree more vulnerable to a secondary problem, such as an insect infestation.
Uh-oh. How do I fix this?
You need to remove the excess mulch from around the tree. I’ve started doing that with the oak that I mentioned previously.Just about every tree on the landscaped part of the property has this problem. I can even see some landscape fabric peeking out of the mulch around one tree. It’s going to be a big job, but at least the decomposed mulch from around the trees will make a good soil amendment for the native clay that the shrubs and perennials are planted in. If your trees have been buried for a long time, you might want to visit the resources listed below, and then consult an arborist in your area.
So what’s the right way to mulch a tree?
According to Marianne Ophardt, a Washington State Cooperative Extension Agent writing for the Mid-Columbia Community Forestry Council, you should keep the mulch “six inches away from the trunks of young trees and one foot away from the trunks of older, mature trees.” Ideally, the mulch will be in a 2 to 4 inch layer extending out to the drip line, because lawn grass releases chemicals that inhibit the growth of trees. Not many homeowners with large trees want such vast expanses of mulch. Trees can grow in lawns, but more slowly. The most important thing is to avoid burying the tree flare.
Why does this happen?
I can understand why homeowners over-mulch their trees. They’ve heard that mulch is good for plants in general. They know that string trimmers can damage the trunk. But they want to have their lawn looking neat without getting off their riding lawn mowers if at all possible. It seems like the way to accomplish that is to mulch right up to the trunk, and mow right up to where the mulch and grass meet. I guess people spend so little time nowadays around naturally growing trees that the lack of a tree flare doesn’t bother them, or they think it doesn’t matter.
But I have seen plantings of trees around businesses, obviously hired out, that have “mulch volcanoes” around them. You would think a professional landscaper would know better. Could it be that their clients actually prefer the volcano look, and insist on it? What do you think?