Mulch is anything that will facilitate the improvement of the micro organisms in the soil. Much has been misleading in mulch applications. I would like to set some straight. First let us focus on trees. Anything with the word FRESH in front of it is NOT GOOD. The relationship fresh material has with the soil leaves us with the statement: "letting these materials sit for at least one year is best." That is fresh chips, fresh manure and so on. After one year, these things are fine in the right amounts to be used as mulch. Tree trimmings are very good if applied after they have sat for at least one year. Some places sell so called "mulch" that is nothing more than just painted fresh chips (Not all). Mulch is too thick when roots start growing into it. When roots are growing in the mulch, several continuous hot dry days can lead to root death. Mulch is very beneficial for trees. Start by applying 3 to 4 inches(7 to 10 cm.) and inspect the mulch several times a year for root growth. Keep mulch at least
6 inches(15 cm.) away from the trunk to reduce chances of rodent injury and infection by pathogens. Excessive amounts of mulch may disrupt soil moisture and aeration. It is best to keep adding small amounts of composted material over time. The latter are clips from 100 TREE MYTHS by DR. ALEX L. SHIGOMulch "should" extend to the drip line and placing it on top of the grass (FLAT) rather then removing the grass is better. We can do more damage by removing the grass now that it is there. We are over grassed as a nation and grass will kill a tree. Only extending the much out to four feet surly is better than no mulch at all! If we take "dead" wood out of your trees, we can chip it up and place it right back around your trees. Fallen leaves are great mulch. Bark mulch should be avoided. For much more on the topic: