Filling a Gully Around Mature Douglas Firs: Why Rocks and Fill Will Kill Them

A homeowner wants to fill a gully up to 6 feet deep around 50‑year‑old Douglas firs, using large rocks and dirt. The expert explains that tree roots need oxygen; fill deeper than 6 inches (especially clay) suffocates roots. Rocks create the same problem. Even a retaining wall 1 foot from the trunk cuts ~50% of roots, and machinery compacts the rest. The trees will die. No safe engineering solution exists for that much fill.