The roots of plants need a constant supply of oxygen at all times. Overmulching kills the roots of shallow-rooted plants by suffocation.
Symptoms of too much mulch include chlorotic foliage (symptoms often resemble iron deficiency), abnormally small leaves, poor growth and dieback of older branches. Disease organisms that are active under conditions of low oxygen and excessive moisture can become active and attack the roots. Sometimes the old root system will be rotted as the plant tries to send out new roots into the mulch layer. Excessive amounts of mulch applied around tree trunks can lead to cankers on susceptible species