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Thursday - April 20, 2006

From: Kansas City, KS
Region: Midwest
Topic: Trees
Title: Inadvisability of mounding earth around tree in ring
Answered by: Joe Marcus


We have a mimosa tree in our backyard. My husband wants to build a tree ring around it. My mother told me that some trees will die from having soil built up around it like that. Will a tree ring kill our mimosa tree?


This is one of the more controversial topics in arboriculture. Some arborists swear by tree rings, others say they're useless and, worse, detrimental to the trees. I take a moderate stand on this. In my opinion, tree rings are useful for two purposes: providing a temporary aid to watering a tree after transplanting and protecting the crown of the tree from lawn equipment.

The old donut watering dike that we've all seen for many, many years has recently morphed into a mound of earth around the base of the tree. I have seen these as much as 1 foot in depth! These mounds do no good and actually can do great harm. Soil and mulch mounded against the trunk of a tree can harbor both harmful insects and pathogens and provide the perfect environment for them to gain entry to the base of a tree. Moreover, some trees seem to have some critical gas exchange happening at the base and certainly depend on oxygen at the roots. A thick mound of soil over the root ball greatly diminishes the volume of gases available that are critical to the health of the tree.

Watering rings (the donut mounds) should be built outside the hole dug for the tree, not on top of the root ball as they almost always are constructed. They're really not necessary if the tree owner will spend six minutes watering their newly transplanted tree at low flow rather than two minutes with the hose gushing at full pressure. Little sprinklers running at low pressure do a great job of watering new trees.

Some people maintain rings or bare earth or loose mulch around their trees as a way to protect them from mowers and weed trimmers. This is in general a good idea as mechanical damage to the base of a tree is often the source of serious problems far into the future. However, care should be exercised not to mound soil against the trunk of the tree or to smother the roots. For small trees, I like to use curved clay tiles or bricks as a physical barrier around the base of the tree. Grass can grow right up to (and into) the barrier which I can mow up to. If grass or weeds pop up between the barrier and the tree it's a simple matter to remove the tiles or bricks, pull or trim the grass and replace the barrier. If the barrier is not placed directly in contact with the base of the tree insects and diseases won't have any better chance of getting started than if it weren't there at all.


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