Answer: Soil piled against the trunk can damage the tree by inducing decay in the lower trunk area. While the exterior tissues of the tree's roots are suited to being in contact with moist soil, trunk tissues are not.
Keep soil back away from the trunk. I suggest you give it about a foot of space to allow the trunk and its flared base room to breathe before starting the pile of soil for your planting bed.
Q&A Library Searching Tips