Why do we mulch our trees and planters? Not only does it decrease water needs, but it helps manage weeds, which is enough for me to do it to everything on my property. There are many options for mulch that are effective and relatively cheap. The benefits of organic mulch heavily outweigh its inorganic counterpart due to the following reasons:
Mulch insulates the soil, which in turn helps the roots with varying temperatures. Plants that are on the edge of your zone might be saved from a good mulching.
With organic materials being part of your mulch you gain the benefit of having your mulch provide nutrients. As the mulch decomposes it will release nitrogen, phosphorus, and potassium into the soil. If you use compost as a mulch you can also add several micro-nutrients and trace minerals that commercial fertilizers are often lacking.
3. Free or Low Cost
Plenty of organic mulches can be obtained completely free of cost. Grass clippings, straw, leaves, compost, saw dust, rice hulls, bark chips, hay, straw, shredded paper or newspaper, and even living mulch such as perennial can all be used as organic mulch. If you can't obtain some of these materials for free they are not as expensive as their inorganic buddies.
4. Reuse Discarded Materials
Think of how many things that I just listed that you've previously thrown away at some point in your life. These materials are frequently thrown away and loaded to a dump. Just the leaves and grass that your yard might produce can be turned into organic mulch instead of purchasing up to $30 worth of bagged rubber mulch from a big box store.
5. Condition Soil for Water Retention and Root Development
As the soil is conditioned through organic mulching it increases in capacity to hold water. This has a profound effect on root growth as it encourages roots to grow deeper and extend farther. Additionally this can help prevent erosion by doing so.
While I wouldn't discourage using inorganic mulch for your yard over no mulch, there are plenty of reasons that show organic mulch is the best for your plants and trees.