Should you install a landscape plastic or fabric prior to mulching?

Many people ask about the installation of a fabric or plastic prior to mulching. These are viable options but they have some disadvantages that by far outweigh their benefits.
Black plastic provides excellent annual-weed control. However, the surface does not hold mulch well, especially on a slope. Heavy rains will wash off the mulch, exposing the plastic and detracting from the appearance of a landscape. This allows sunlight to reach the plastic. In time, the material will break down and allow weeds to emerge. Black plastic also impedes water infiltration, and prevents gas and oxygen exchange with the soil below. The plastic barrier also prevents the mulch from breaking down and incorporating into the soil profile.
Landscape fabrics, on the other hand, allow water to move through the material to reach plant roots below the fabric and allow for exchange of gases such as oxygen and carbon dioxide. Therefore, landscape fabrics are better choices for long-term weed control than the plastics. However, the fabrics have the same limitations as the plastics. The surface does not hold mulch well, especially on a slope and the barrier prevents the mulch from breaking down and mixing with the soil below.
If you are considering using a decorative gravel of some sort, it is absolutely essential to install a layer of fabric or plastic, prior to installing the gravel. If you do not, weeds will become a huge problem over time. The gravel can mix with the soil below and create a nightmare if you ever decide to change the planting area to a mulch bed.