By David Walther, UC Master Gardener of Butte County, March 2, 2018.
Mulch is good for the soil, and thus good for your plants for a number of reasons. It jump-starts the useful work done by microscopic organisms (this is often referred to as microbial activity); retains moisture, thus preventing plants' roots from drying out; and deters weeds by providing a barrier between the soil and the sun.
Mulching is also a time-saver for the gardener – it takes a fraction of the time to put down mulch that it would take to weed later on. And using mulch to control weeds reduces the use of expensive herbicides.
Natural inorganic mulches include gravel, pebbles, and crushed stone. Other inorganic mulches are plastic, cardboard, and even pieces of old carpet. The mulch you use will depend upon the job you want the mulch to perform and how you want it to look.
Any ground that needs enrichment, such as flower or vegetable beds you will be planting later in the season, will benefit from an application of organic mulch. Mulching around shrubs, trees, annuals, and perennials will improve the soil that feeds their roots, as well as deter weeds. Large areas that you wish to keep weed-free are also candidates for a thick application of mulch.
To control annual weeds, apply four to six inches of mulch on top of the soil, or even on top of the weeds themselves. To control perennial weeds, an application of 12 to 16 inches of mulch is needed.
The reappearance of weeds in an area that has been mulched is an indication that the mulch has decomposed to such a degree that a new application of mulch is required.
While the benefits of applying mulch are many, mulch can also lead to an increase in mole activity because of the increase in the worms and grubs they feed on – the more alive your soil is, the more it becomes a habitat for other creatures.
Finally, different garden situations require different choices of mulching materials: for example, dryland garden plants (xeriscape plants) such as native California plants, cacti, and succulents, have evolved to flourish in dry conditions and poor soil. The moisture-retentive qualities of organic mulch could be detrimental to their vigor and growth while the use of stone or gravel mulch would be helpful in replicating their natural environment.
Your mulch choices can range from commercial mulches purchased at a landscape center, to bags of organic mulch available at nurseries and home improvement stores, to your own (free!) shredded leaves, grass clippings, and yesterday's paper. The benefits of mulching far outweigh any negatives, so there is no reason not to mulch.
For more information see: Mulches
Questions? Plant problems? Call the Master Gardener Hotline at 530-538-7201 and/or visit our website Hotline page at ucanr.edu/p/49588.