Mulching is the one technique, beside proper planting and watering, that can help provide optimum growing conditions for your tree, and help insure it a long and healthy life.
Mulching is placing a circle of either organic materials (like pine bark) or inorganic materials (like landscape fabric and rocks) on the ground around plants. In the case of a tree, the mulch should be from 2-6" deep and reach to tree’s drip line, but should not be against the trunk. The mulch around a newly planted tree, regardless of the drip line, should cover an area at least four times the area of the root ball.
Mulch, whether organic or inorganic, benefits the tree in many ways, it: reduces soil erosion, conserves soil moisture, restricts soil cracking, impedes weed growth and protects the trunk from mowers and weed-whackers. A ground cover plant (like ajuga or vinca vine) can also serve as mulch, but it can compete with the tree for water and nutrients. Organic mulches, like composted bark or leaves, are the very best mulches; they provide extra benefits, such as reducing soil compaction, protecting the roots from traffic, improving soil structure and fertility, and moderating soil temperatures. Indeed, organic mulch can sharply increase the rate of tree growth in the first few years after planting.
A word of caution: organic mulches that have not been composted, if applied too thickly, can deplete soil nitrogen as they break down (fresh grass clippings, for example), or can be toxic (like fresh sawdust and bark). Also, any mulch that rests against the trunk of the tree, or is too deeply applied, can increase the incidence of diseases and insect damage. Fabric mulches can increase soil temperatures, and the fabric should be slit so that air and water can move more freely through the fabric. Also weeds can be difficult to extract from landscape fabric mulches. As mulch, black plastic, no matter what is placed on top of it, creates more problems than it resolves. Avoid it.
Remember that the natural habitat of most trees is in the forest, and the only way we can even attempt to recreate a forest floor environment in the city is to mulch. For more information about the benefits of mulching trees, contact your County Extension Office.