Ask The Arborist-landscape beds around mature trees
By following a few important guidelines, you can safely landscape around an established tree.
Avoid soil build up against the bark of the tree. Raising the soil level by just a few inches can cause the trunk to rot. This will cause the tree to decline and eventually die. If possible, try to avoid adding soil to any area underneath the tree canopy. If you must add soil, use the smallest amount possible, taking care to keep it a few feet away from the trunk. Mounding soil on top of woody roots will have a similar effect to piling it against the bark. The wood protecting the root will rot, potentially cutting off a large source of water and nutrients to the tree.
When it comes time to plant, dig with care! Avoid damaging the roots as much as possible. Keep in mind that by severing a root close to the trunk, you are also cutting off many tiny feeder roots that are connected to the one root that is cut.
Using about 2-3” of mulch will not hurt the tree. However, do not add mulch right up against the trunk. It is important to keep good airflow around the base of the tree.