Since most mulch materials are applied for weed control, it is important that they be in place before weeds have emerged from the soil. If, however, the application is delayed until weeds are observed, effective weed control is still possible. Delaying application of mulch also allows the soil to warm slightly, which aids in active root development. If the mulch is applied too early in the spring, soil temperatures will remain cool and root growth will be slow.
The settled depth of the mulch layer is also an important factor in its effectiveness around plants. To function properly, a mulch layer should be from 2 to 3 inches deep. This depth will easily smother young weed seedlings, prevent evaporation of soil water, and allow water penetration to the soil below. Thinner layers will not be adequate. Thicker layers tend to waste mulch and are no more effective in most cases.
Mulch material applied to the proper depth can work to a plant's advantage, but it must not contact the stem or trunk of the plant. Organic mulch materials absorb water, and when moist mulch comes in contact with a plant's stem, it creates conditions favorable for the decomposition of the bark on the plant. The bark on most woody plants must be dry or at least able to dry off rapidly after it becomes wet. Continued moisture on the trunk can be fatal to the plant