Sometimes we run across bizarre things in Ohio landscapes that simply defy explanation. Such was the case yesterday when I came across "tree moats" (sometimes called "mulch moats") in a park near my home in the southwest part of the state. I've encountered this bizarre practice before and fail to fathom the logic.
As their name implies, tree moats are created by excavating a moat-like ring around trees or shrubs at about the dripline, or slightly beyond. This is done with an edger or a shovel. In either case, there always appears to be a concerted effort to sever roots growing beyond the excavation zone.
If a shovel was used, the excavated turf and soil is often piled over the root zone rather than carting it away. Of course, this can serve as a great foundation to create truly noteworthy mulch volcanos (more on this later).
What's Wrong with This Picture?
Plants don't have a cardiovascular system to ship oxygen to the roots. The root cells acquire oxygen directly from their environment. Piling soil on top of the root zone blocks oxygen from infiltrating the soil.
If the primary root system dies, certain trees have the capacity to form a "secondary" root system (even "tertiary") from buds located on the main stem. However, this elevated root system growing into a mound of soil is subject to a number of tree-debilitating issues including moisture stress. Mounding soil enhances soil drying and the slopes can shed rainwater reducing water infiltration.
When roots reach the edge of the mound, they may turn back towards the main stem; the roots can't grow into thin air despite needing oxygen! Eventually, these roots encircle the tree trunk and merge with the stem tissue. As these errant roots increase girth, they gradually girdle the trunk and restrict vascular flow. Thus, they are known as "stem-girdling roots."
Cutting roots that cross the tree moats may harken back to the days when we thought that tree roots are confined to an area within the dripline. We also envisioned tree root systems as being mirror images of the canopy with the roots extending deep into the soil.
Of course, research has taught us that a tree's root system actually looks like a giant Frisbee. Taproots are rarely present because oxygen levels decline with soil depth. Over 80% of the "feeder roots" are in the upper 6 – 8 in. of the soil; that's where the highest level of oxygen is found. The horizontal spread of a tree's root system is 2.5 – 3.0 times the crown spread with more than 60% of the roots growing outside the dripline. Obviously, cutting the roots at the dripline is a recipe for a tree health disaster.
The Great Cover-Up
Tree moats and so-called "volcano mulch" often go hand-in-hand; perhaps to hide the first offense. Volcano mulch is so-named because of its sculpted resemblance to a stratovolcano; like Mount St. Helens before it blew its top.
Frankly, I fail to understand why in the name of all that is horticulturally holey do we continue to see mulch piled around tree trunks to stratospheric heights? What is the appeal? And, why can't we stamp out these mulch monstrosities despite years of educational efforts?
Volcano mulch does not kill trees outright; if it did, people wouldn't do it. Instead, it produces many of the same subtle, long-term, ill-effects I've described with soil mounded over a tree's root system during tree moat excavation.
Although bark mulch is at first light and airy, it will ultimately compact as it degrades to interfere with oxygen reaching tree root cells. Trees respond by growing roots into the mulch; however, the roots can become exposed as the mulch further degrades.
As with the soil mounded onto the root system during tree moat excavation, volcano mulch can also cause roots to turn back towards the main stem to encircle the tree trunk. In fact, it is common to see stem-girdling roots associated with volcano mulch, particularly with maples.
As the mulch decomposes and dries out, it will eventually start to repel water; it becomes hydrophobic. You can observe hydrophobicity of dry organic matter when you try to moisten a bag of dry peat moss. Of course, water repellency ultimately causes infiltrating roots to dehydrate.
The deleterious nature of tree moats and volcano mulch are not immediately apparent. While moisture starvation and vascular strangulation can ultimately kill a tree, along the way they produce tree stress. This can induce trees to drop their defenses against infestations by opportunistic insect pests such as native borers or infections by plant pathogens. Of course, the pests and diseases get blamed if a tree succumbs, not the bizarre horticultural practices that set the tree's demise into motion in the first place.