The answer to 'why is my mulch dead trees' is that dead or decaying mulch can actually cause harm to your trees. Mulch that is too thick, old, or made from poor materials can deprive tree roots of oxygen and prevent water absorption, leading to tree decline and death.
Common causes of dead mulch include using too much mulch (more than 2-4 inches deep), letting mulch sit for too long without replenishing, or using low-quality mulch made from diseased or chemically-treated wood. Symptoms of dead mulch include mold growth, foul odors, and wilting or yellowing tree leaves.