Mulching around trees is generally a sound practice when it is applied correctly: a moderate layer of organic mulch can conserve moisture, suppress weeds, moderate soil temperature, and protect the trunk from equipment damage. It is best avoided on poorly drained sites, around trunks with decay or disease, or when you cannot keep the material a few inches away from the bark. For valuable, mature, or stressed trees, getting advice from a certified arborist can help you choose the right material and placement.