UC IPM Makes It Happen
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.
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
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. Jim 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.
solarization is a perfect fit for small-scale specialty crop growers
because specialty crops have few labeled pesticides, due
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. Jim and Richard, 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
Master gardeners, pesticide applicators, farm advisors,
growers rode a tractor-trailer out to Willey's fields to see for
themselves how solarization works. Participants watched a tractor
clear plastic across the field. After several days of sunshine,
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
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
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 Jim. "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.
recommends using solarization in the Central Valley from
June through August when weeds are more easily controlled.
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
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
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
in fall and spring specialty and organic crops in the
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
Jim. "Soil that has been solarized allows plants to draw on
the nutrients, especially nitrogen, calcium, and magnesium, more
Seeds germinate more quickly. Plants grow faster and stronger, often
maturing earlier with substantially higher yields than in soil that
However, there are exceptions. Jim
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,
or public, which
is important for farms close to urban areas. Solarization
is acceptable for use in organic production, and no permits
is required. Growers also have the option of leaving
the plastic in place after treatment as a bed mulch
Solarization must be timed soon after the spring
harvest, but before planting for the next crop, so one of
Joaquin Valley growers is that the land will be idle for
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
During the last eight years, Jim and Richard 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