The term “mulch volcanos” has become common: “When you mulch trees, you want to avoid making mulch volcanos.” The volcanic picture of a little cone of mulch around a tree is a good one, but I think that sometimes the better a term is at creating a picture, the worse a term is for prompting thought. So let’s discuss the bigger picture for the why’s and why not’s for creating mulch volcanos.
Why do people build mulch volcanos? Maybe some people heard that trees should be mulched, so they thought, “If some mulch is good, more must be better.” Or maybe they figured out that the 3-D look of a cone shows off their red mulch job better. But the best reason for piling a small amount of mulch around the trunk of a tree is to protect it from lawn mowers and trimmers.
Look at the unmulched photo. There is a chunk of bark missing from mower damage, and there is bark damaged by a trimmer. Missing bark is a major problem on a tree trunk. Aside from inviting decay and structure weakness, missing bark means missing meals. In contrast to your flesh-and-bone protected esophagus, just underneath the dead bark of a tree is its phloem (food transport) and xylem (water transport) system. When you break bark off a tree, and you feel slimy wood, you are actually touching the tree’s digestive organs. If the tree’s diet ends due to loss of bark, it either suffers or doesn’t live. This might be why that tree you planted lingers but never really seemed to grow.
So it is wise to use mulch to fend off the mowers. But let’s not replace one problem with another. Pull the mulch back from the trunk on that old mulch volcano of yours. What does the trunk look like? The bark is wet and slimy, right? Where do we want wet and slimy? Not on the trunk—look up, trunks are designed to contact air and be dry—no, we want the roots to be wet. (Slimy might mean that decay has already begun). So keep the mulch away from the trunk, and keep both the mowers and the decayers away. It is not only the mowers that can damage the bark and diet of a tree.
Now what about mulch? Mulch does replicate a forest floor with its cool, moisture-conserving, air-encouraging environment. And, studies have shown that a properly mulched tree out grows trees grown in turf or on bare soil. But does an 18” diameter mulch ring help a tree grow? Not unless we are talking about a seedling. Remember the tree’s feeder roots are at least as far from the tree as its drip-line.
So mulch your trees. Use three inches of natural mulch, and keep it two inches away from trunk contact.
Joshua Arp is an ISA-certified municipal specialist, Clarks Summit’s municipal arborist and an operator of an organic lawn and landscape maintenance business. Reach him at firstname.lastname@example.org.