Article 3 of 12 from Trees: Dispelling Myths & Misinformation, prepared by the arborists of the Southwest Division of Bartlett Tree Experts as a community education initiative.
You’ve seen them: trees whose bases look like telephone poles in the ground because no trunk flare is visible. That’s good, right? Because then the trunk flare and buttress roots, which we’ve already learned provide stability, will have the extra support of soil coverage to pack them down and keep them secure. This sounds logical, but it’s wrong. The root crown needs to be visible. Let’s look at why.
Nature designs roots to tolerate the moist, dark conditions underground. The trunk is not specialized to do this, and the root crown (or collar) is part of the trunk. This brings us to the importance of phloem cells that exist inside the bark layer. They transport food (photosynthates) produced in the foliage to other parts of the tree, including the roots. When the root crown is buried, oxygen and carbon dioxide can’t easily move in and out of the phloem layer. Over time, this will kill phloem cells, which won’t be able to deliver photosynthates to roots; eventually those roots will have difficulty absorbing water and elements that other cells send back up the tree.
It doesn’t stop there. This condition encourages disease-causing fungi to infect the tree. The resulting decay attracts borers and other secondary invaders.
Commonly, a root collar becomes buried as a result of construction activities or because the tree was planted too deep. A qualified arborist can determine whether a root crown excavation (with a proper tool) is feasible to relieve the condition.
While we’re on the topic of conditions that harm the tree base, girdling roots and “volcano mulch” are other culprits. Girdling roots are regrettably common, mainly due to growing practices that encourage roots to circle inside their containers. Often, we see “volcano mulch” on newly planted trees. It’s mulch that’s piled cone-like against the trunk, and it’s a terrible practice. We’ll learn more about these in later articles.
Now that we know more about the general habit of root growth and that the root crown should be exposed, we’re better prepared to understand the principles of tree planting.