I have two Cedar trees growing a slope ( about 30 Degrees) in my back year. Recent rain has started the soil to erode rather quickly around the bases of both trees. I am considering placing a retaining wall to bring the area up to about level with the rest of the yard. This would require me to cover the tree trunk up about three feet on the lover tree. I have heard this is a bad thing to do to a tree. What do you think?
Covering the root zone, also known as critical root zone or crz, is a very bad idea.
Reason is roots require two things, oxygen and moisture. As a result, the roots grow close to surface, the vast majority will be within two feet of the surface, and a cedar will be closer to 18" of surface. Yes, some roots will be lower, but the majority will not.
The second issue, trunk wood and root wood is not the same, by covering the trunk, you expose it to new issues it is not prepared to handle which can lead to rot and pathogen entry. The cedar is better than some in handling this to a small degree, but three feet.
Adding even small amounts of soil within the of a tree will usually decrease the amount of air in the soil around the roots. Air in the soil is vital for the tree's health. Reduction in the soil air leads to increases in gases and chemicals that are toxic to the tree's roots. The symptoms of the injury may appear within a few months or not until a few years after you change the soil grade.
Some species, and cedar is one of them, which is tolerant of some fill over some portion of their roots. But some portion is not all of the root system, and it is not two to three feet depth. Generally, you should avoid covering roots if what you intend to cover them with is soil. While an inch or two of soil atop just a few roots won’t kill a tree, many trees species are very sensitive to soil level changes.
You likely do however need to address the erosion issue. Serious potential issues you need to think about are root coverage, trunk coverage ( which is similar to issues created with volcano mulching ), soil compaction and potential root damage if digging in root areas.
What is diameter of the cedars, how close to area where you are putting in wall. Is it possible to step the surfaces, and avoid excessive soil over root zone? Do you have photograph of area?