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August 5, 2005

Madera farmer praises the benefits of soil solarization

Tom Willey is spreading the word about solarization and how this inexpensive, chemical-free approach killed the weeds plaguing his 75-acre organic farm in the Central Valley.

   Richard Molinar, UC Small Farm Program advisor for Fresno County

Richard Molinar, UC Small Farm Program advisor for Fresno County.
Photo by Stephanie Klunk

In July 2005, Willey spoke to nearly 30 people at a workshop on solarization sponsored by UCCE at his farm in Madera. Jim Stapleton, plant pathologist and IPM advisor for the UC Statewide IPM Program, and Richard Molinar, UC Small Farm Program advisor for Fresno County, joined him for the presentation.

Soil solarization works like a greenhouse to trap the sun's heat to raise temperatures that kill insects, plant diseases, weed seeds, nematodes, and soil pathogens. The process has become a widespread and growing practice for organic growers, home gardeners, and other users. Stapleton has published several technical articles describing the science behind the technique and also guides for end users who would like to use solarization in their own gardens or farms.

Soil solarization is a perfect fit for small-scale specialty crop growers because specialty crops have few labeled pesticides, due to the high development and support costs and low returns for the manufacturers. Also, many of the farms are located next to urban areas, and the use of pesticides and fumigants is further restricted. Stapleton and Molinar, along with IPM weed ecologist Anil Shrestha and other UC personnel, published an article earlier this year which describes the benefits of solarization for weed management on organic and limited-scale farming operations.

Master gardeners, pesticide applicators, farm advisors, and small-scale growers rode a tractor-trailer out to Willey's fields to see for themselves how solarization works. Participants watched a tractor stretch the clear plastic across the field. After several days of sunshine, soil temperatures rise to as high as 165 degrees Fahrenheit at the surface. It takes four to six weeks of sunny weather to pasteurize the soil. The larger the solarized area, the more heat is generated and maintained, and the longer lasting the effects.

Willey has been farming in Madera since 1980 and earned his organic certification in 1987. He and his wife, denesse, employ 55 staff to tend everything from Asian turnips to rutabagas.

Willey says cheeseweed, or little mallow, was threatening his crops, and hand weeding it was "like painting the Golden Gate Bridge?a never-ending job." Cheeseweed is a winter annual that forms dense bushes and can grow 4 feet high. It has flowers that are held in clusters, and the fruit resembles a miniature wheel of cheese. The pervasive weed was choking his winter seedlings, and he had to act fast.

"We launched ourselves into solarization," says Willey. "There's a real learning curve to it. You can't buy it and pour it out of a can to make it work."

Simple steps are to level the soil and be sure it is free of weeds, debris, or large clods. Water the soil thoroughly, and use clear polyethylene plastic that is 0.6 to 2 millimeters thick and ultraviolet (UV) treated. Cover the area; making sure it is airtight, with no holes.

"Solarization is a knowledge-based, rather than product-based, soil disinfestation method," says Stapleton. "Users are largely missing the benefit of having trained consultants for their particular geographic locations. To address that problem, we collaborated with the Kearney Agricultural Center Geographic Information Systems unit to create statewide air-temperature maps to help users determine the suitability of their area for solarization."

Users may access the maps at the UC Kearney Research and Extension Center (REC) Web site.

Stapleton recommends using solarization in the Central Valley from June through August when weeds are more easily controlled. He suggests using a wide soil bed that is at least 20 to 24 inches wide across the top, with furrows on both sides of the bed. To reduce the number of furrows or trenches, Willey uses beds that are 52 inches wide. This minimizes the amount of land that is nonproductive.

Willey had been spending $2,000 to $3,000 an acre to hand weed his farm. Using solarization on one of his four fields every four years has reduced that amount to $400 an acre, including the cost of the plastic and labor.

The results of a field study with parsley, along with the two farm experiments with strawberries, showed that solarization effectively controls weeds in fall and spring specialty and organic crops in the San Joaquin Valley. The research has provided guidelines and technical support for growers in a wide variety of specialty crops.

"During solarization, helpful microorganisms living in the soil benefit," says Stapleton. "Soil that has been solarized allows plants to draw on the nutrients, especially nitrogen, calcium, and magnesium, more readily. Seeds germinate more quickly. Plants grow faster and stronger, often maturing earlier with substantially higher yields than in soil that isn't solarized."

However, there are exceptions. Stapleton says that certain weeds, such as yellow and purple nutsedge, are not consistently controlled by solarization.

The advantages of solarization include ease of use by the grower, low treatment costs, and no hazards to the grower, workers or public, which is important for farms close to urban areas. Solarization is acceptable for use in organic production, and no permits or pesticide reporting is required. Growers also have the option of leaving the plastic in place after treatment as a bed mulch to further justify the cost.

Solarization must be timed soon after the spring harvest, but before planting for the next crop, so one of the disadvantages for San Joaquin Valley growers is that the land will be idle for four to six weeks during the summer. To avoid losing a growing season, growers can rotate their crops to take advantage of the land before and after treatment. Maintenance and getting rid of the plastic are also important considerations. An easy way to collect the plastic is to roll it onto a telephone cable spool.

During the last eight years, Stapleton and Molinar have been conducting weed research on soil solarization at the Kearney REC in Parlier and on farms in the surrounding San Joaquin Valley. Following the Madera workshop at Tom Willey?s farm, they conducted another workshop in the Imperial Valley in August. Growers in the hot desert valleys of southern California are using solarization for weed control more extensively than farmers in the Central Valley. They also have a longer window of opportunity to apply solarization?the main production season is in the winter and many fields are empty during the summer months, due to the very high temperatures.

Learn more about solarization from the UC IPM Web site.


High-resolution image (400KB) "Richard Molinar, UC Small Farm Program advisor for Fresno County, punches a thermometer into the soil during a soil solarization demonstration by the University of California Cooperative Extension at Tom Willey's organic farm in Madera." Photo credit: Courtesy of UC Statewide IPM Program, Stephanie Klunk. Photos are for use with this release only. All other uses see Legal Notices.

Statewide air temperature maps for solarization suitability

Soil Solarization - A Nonpesticidal Method for Controlling Diseases, Nematodes, and Weeds (PDF* 1.8MB)


Stephanie Klunk, Communications Specialist
UC Statewide IPM Program
(530) 754-6724

James Stapleton, Plant Pathologist
UC Statewide IPM Program
Central Valley
(559) 646-6536

Richard Molinar, Farm Advisor
UCCE Fresno
(559) 456-7555

*You need Adobe Acrobat Reader version 4 or later to view or print this PDF. If this software is not installed on your computer, you can download a free copy of Acrobat Reader

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