Planting a tree too deep is one of the most common reasons for tree decline. Trees have above and below ground tissues and if you expose either of them to the wrong environment it will lead to adverse effects. The transition zone between the above ground and below ground tree parts is known as the root or trunk flare; it is the area where the first roots emerge from the trunk of the tree. This area usually bulges out creating a flare close to where these first roots are attached. It is this transition zone that is critical to expose to the air and not to bury when planting a tree.
Planting a tree too deep is detrimental to the future success of that tree for a few key reasons. First, above ground portions of the tree are susceptible to decay by microorganisms (mainly fungi) when they are buried in the soil and kept moist. If you expose a root flare on a tree that has been buried too deep you will often find soggy, peeling bark that is susceptible to penetration by these decay organisms. This can lead to dieback of the entire tree if the decay organisms find their way into the interior wood of the tree.
Another problem comes from the root flare not being exposed to air, preventing it from “breathing” properly. All parts of the tree need to exchange gasses with the environment, namely oxygen and carbon dioxide, in order to function properly (even roots breath and require access to gas exchange, a topic that will be covered in more detail at a later time). The trunk of a tree has little pores called lenticels that facilitate this gas exchange. If covered by soil, gas exchange is greatly reduced and can lead to the decay of the trunk and overall tree decline.
Lastly, a common result of planting a tree too deep is the development of girdling roots. These are roots that wrap around some or all of the trunk. As the tree thickens over the years, the roots will begin to cut into the bark and outer tissue of the trunk. This will prevent the transport of water and nutrients throughout the tree, often leading to the death of the tree. These girdling roots develp because most roots like to be close to the surface of the soil, so when a tree is planted too deep they grow up towards the soil surface and can get in the way of the growing trunk.
I hope this convinced you to expose the root flares on your trees! If an urban tree looks unhealthy this is the first thing that we should look at. Exposing the flare is easiest to do when planting, but it may still be possible on mature trees by gently digging the soil off of the root flare (or by using an air excavator). Proper planting and care for a young tree will lead to a mature tree that is much healthier and that requires less maintenance.