Spring 2023

Mulch: It's Got You Covered

by Anne Sutherland 

It’s hard not to get excited about mulch.  Mulch is one of the best things you can do for your plants and… you can get it for free.  Free sources include your own yard waste such as grass clippings, shredded fallen leaves or wood chips from a reliable tree service.  UC Master Gardeners prefer organic mulches because they are part of the carbon cycle and have benefits that rock, weed barriers and plastic lack.

Temperature Regulation

It’s hard to imagine a long, hot, dry summer in the middle of this atmospheric river.  But that is the nature of our Mediterranean climate.  Mulch helps regulate soil temperature by insulating the soil, which helps to protect plants from extreme temperatures and keeps the soil cooler in the summer and warmer in the winter. Your plants will thank you for it.  

Hot, dry summers also mean wildfire risk.  If the soil underneath is properly irrigated, composted wood chips tend to smolder rather than flame up. Shredded rubber, pine needles, shredded cedar bark and eucalyptus are particularly flammable. Use only non-flammable mulch 0’-5’, low-flammability mulch 5’-30’, and large wood chips 30’-100’ away from structures. It’s OK to use composted wood chips in modest batches but keep them at least 30’ away from structures.

Erosion Control

Mulch reduces the impact of raindrops on soil, thereby reducing erosion and improving water percolation into the soil.

Weed Control

Mulch also reduces the number of weeds and makes the few that get through easy to pull up.  

Pest Control

Mulch helps suppress soil-borne diseases and makes a nice home for beneficial organisms, like earthworms digging dead surface material into the body of the soil. Some types of organic mulches, such as cedar chips, can help to repel certain types of pests, such as termites and ants. 

Soil Improvement

Organic mulches will return minerals and nutrients back to your plants. Organic mulches also break down over time, adding organic matter to the soil and improving soil structure, which helps to create a healthy growing environment for plants.

Wood chips about 1” in length work well for landscape plants and are less likely to blow away than shredded bark.  Gravel and rock are good for weed control and most succulents but provide no soil or microbe nourishment and can heat up the soil.  Mulches that break down quickly, such as rice straw, grass clippings, or shredded leaves, are best used for vegetable gardens.  Make sure these garden mulches are not matted down so the soil can get air and water.  Finally, do not count on mulch preventing frost damage.  You still need to protect frost-sensitive plants such as citrus.

A few cautionary notes:  Get to know your mulch supplier!  Be sure it is not from diseased plants nor sprayed with herbicides or insecticides.  Free mulch may have unwanted debris in it such as plastic or treated wood – be sure to specify what you don’t want when getting it from an outside source.  Keep a few inches clear around the base of the plant so it doesn’t stay damp and get crown rot. Unless you are mulching with mature compost, mixing mulch with the soil will use up nitrogen before releasing it when it breaks down later, so just put it on top.  Finally, dyed mulch may contain unwanted chemicals and retain heat.

Overall, using organic mulches is an excellent way to improve the health of your plants and soil, and using organic mulches is a sustainable choice that supports the health of the environment.

References and Resources:

Mulches for landscape plants: https://anrcatalog.ucanr.edu/pdf/8672.pdf

Mulches for vegetable gardens: https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=24048

Mulch and fire in California: Mulch - Fire in California

Short video by Missy Gable on the Importance of Mulch