July 17, 2022
By David Goldstein
As local gardeners and farmers look for ways to keep their fruit trees alive while meeting water conservation goals, they can consider the water savings gained by applying organic mulch, as documented in an influential 1999 University of California study.
The study’s findings and recommendations have gained relevance today as water supplies tighten and watering restrictions take effect during the severe drought.
The two-year demonstration project, “Use of Yard Trimmings and Compost on Citrus and Avocados,” was conducted by a team of scientists that included Ben Faber and Jim Downer, who still work in Ventura as farm advisors with the UC Cooperative Extension.
At the time of the research, farmers and gardeners already knew compost builds soil structure and promotes plant health, while mulch suppresses weeds, moderates soil temperature, prevents erosion and reduces evaporation.
The UC study systematically examined the effects of these organic treatments by applying varying levels of mulch and compost to groups of trees at ranches in Oxnard, Somis, Ojai, and other locations. Each location also had control groups of trees with no application of organics.
The organic treatments suppressed Phytophthora root rot and were associated with water savings as well as increased root growth, improved plant nutrition and weed suppression.
The study recommended 1/3 to 1 cubic yard of yard trimmings applied around trees two or three times during the first eight years after planting, to enhance root development and fight plant disease. The water-saving benefits of mulch also resulted in a warning: “Moisture concentrations in mulched soils must be monitored to prevent overwatering.”
While the addition of organic matter was highly recommended around avocado trees, it was recommended for commercial citrus groves only in areas of sandy soil without sufficient organic matter, because “in sandy soil, mulches can improve soil nutrition and moisture-holding capacity.” In areas with other soil types, the cost of applying mulch around citrus trees was less likely to be justified by the value of the water savings or other benefits.
For your own garden, the first decision on adding organic matter might be whether to use a surface mulch or incorporate compost into the soil.
Compost is made from decomposed plants, grass and other vegetative matter converted by microorganisms into the texture of soil. Gardeners and farmers usually use it as an amendment, working it into the dirt to help plants grow.
In contrast, mulch is generally made from woody plant material, reduced to a uniform small size by a grinder. Mulch is designed to slowly decompose after placement on top of soil.
Rather than promoting growth, the main purposes of mulch are to reduce erosion, suppress weed growth, moderate soil temperature, and save water by retaining soil moisture. Like compost, mulch also eventually recycles nutrients back into the soil, but it takes longer and serves other purposes first. However, if too much mulch mixes into the soil before degrading, it can deplete nitrogen and reduce plant growth.
While compost is worked into the soil, generally at a ratio of 70% topsoil to 30% compost, mulch is meant to be placed on top of dirt. The recommended depth for mulch generally varies from one inch for cosmetic purposes, to three inches for moderating soil temperature, to six inches for suppression of weeds between tree rows in an orchard.
As mulch compacts and decomposes, you should periodically add more to maintain the desired thickness. Most experts recommend keeping mulch away from actual contact with the plant stems and tree trunks. Mulch piled against plants or trees can cause crown or trunk rot.
You can buy mulch locally at many of the same businesses that sell compost: garden and home improvement stores and some equipment rental yards. Processors of yard trimmings and other organics also sell mulch and compost. They include Agromin’s sites at Ormond Beach and in Oxnard, Peach Hill near Moorpark and American Soils near Simi Valley.
Sometimes you can get lucky and receive free mulch from public, private or utility tree trimming crews working in your area. If you see tree trimmers feeding branches into a grinder that shoots mulch into the back of a truck, you can approach cautiously and ask them to dump a load on your front yard. It saves them a trip to the compost or mulch processing site.
David Goldstein, an environmental resource analyst with the Ventura County Public Works Agency, can be reached at (805) 658-4312 or firstname.lastname@example.org