Image: Wikimedia.

From a water conservation perspective alone, our trees need a good layer of mulch. But there are many more reasons to mulch, according to research by James Downer, Farm Advisor with the Cooperative Extension in Ventura County, California:

• Mulch provides nitrogen, potassium and phosphorus.
• A serendipitous accident in one of Downer’s studies revealed that mulch changes soil structure so that mulched soils are able to absorb more water than un-mulched soils.
• And, most astonishingly, mulch provides habitat for beneficial fungi that repel the dreaded root rot organism Phytophthora cinnamomi.

Downer is also a mulch myth buster:

• Adding a layer of mulch does not rob soil of nitrogen.
• And Eucalyptus mulch? Not a problem.

His recommendation is to apply a layer of six inches of mulch made from chips approximately an inch in length. This application will settle to around 3 to 4 inches. Why is this the optimal amount? According to his research, more mulch risks leaching nitrogen into rivers, streams and oceans. Less mulch does not give you the benefits.

There are other caveats and subtleties to mulching. For those details check out Downer’s handy pdf, Mulch Effect on Trees.