"Sea of red... mulch, that is" by WSDOT is licensed under CC BY-NC-ND 2.0.

 Benefits of Mulch

Mulch is one of the gardener's most useful tools: applied correctly, it suppresses weeds, improves water retention in soil, and keeps the soil temperature down. But does the kind of mulch matter? And do you tie up nitrogen when you use high carbon materials like wood chips as mulch? 

Color matters

First of all, UC researcher Janet Hartin writes, don't use black dyed wood chips as mulch. Hartin, a resident of Coachella Valley, found that black dyed mulch, along with synthetic lawn and asphalt, can superheat surfaces up to 65 degrees hotter than the surfaces of plants in a study she conducted in the valley over the months May through August. 

To till in or not? 

Second, what about the carbon to nitrogen ratio of your mulch? This becomes an issue when mulch is tilled into the soil. Tilling in high carbon to nitrogen ratio materials like wood chips will tie up nitrogen in the soil leading to stunted plant growth. As the population and activity of the decomposers in the soil increase, they consume carbon and nitrogen, competing directly with garden plants for limited supplies of nitrogen. However, leaving mulch on top to slowly decay over time will not have this effect. 

If you are tilling in your mulch, UC Riverside researchers (Pittenger 584) found that mixing in high nitrogen materials like grass clippings in with high carbon wood chips offset any temporary nitrogen tie up. 

Tilling in green manures like alfalfa straw will supply nitrogen and has been found to be especially valuable to citrus trees. 

Mulch and pests

Other things to consider when mulching are pests. UC Riverside researchers found that voles and gophers preferred trees with mulch. Pill bugs may find a home in leaf litter. Monitor plants and be sure to keep mulch away from stems.      

A few notes on proper application

When applying mulch to trees, be sure to keep the mulch from touching the tree trunk and keep from piling too much on top of the root ball. Roots can grow up into the mulch and girdle the trunk. For other plants, keep mulch 4 to 6 inches away from stems. Apply 2 to 4 inches deep for maximum benefits. 

Work Cited

Pittenger, Dennis R. California Master Gardener Handbook. Edited by Dennis R. Pittenger, University of California Division of Agriculture and Natural Resources, 2015.