What Is Soil Solarization? How Clear Plastic Heats Garden Soil
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The question what is soil solarization has a short answer. Soil solarization is a method that uses clear plastic and sunlight to heat the top layer of soil. The plastic traps solar heat, and the warmer soil is meant to reduce soil-borne pests and pathogens. The publication describes a process that needs moist soil, a sealed sheet of plastic, and a long treatment period. This article explains how the method works, step by step, using one University of Arizona Extension publication.
Quick answer
- Soil solarization uses the heat transmitted by solar energy through transparent polyethylene to kill soil-borne pathogens.
- The soil is tilled, saturated with water, covered with clear plastic, and left for at least six weeks, ideally 14 weeks between June and September.
- The plastic is removed in late summer or early fall, and the soil is not cultivated afterward.
- Soil solarization does not completely kill soil pathogens, so it gives crop plants a competitive advantage rather than eliminating the pathogens.
What the method targets
The University of Arizona publication explains that soil-borne pathogens and diseases are very difficult to control. It names damping-off diseases, plant parasitic nematodes such as root knot nematodes, and weeds as the kinds of pests that often arrive in soil amendments, transplants, and nursery stock.
The publication also describes why growers used to rely on chemical sterilants. Sterilants such as methyl bromide were once used to sterilize nursery soils and propagation beds. The source says that practice was questionable because it kills all living organisms in soil. It also notes that the tools available to fight these soil-borne pests are being phased out for use by growers. Solarization is presented as an alternative that uses no chemicals.
The publication also notes that the technique is based on extensive research. It states that following a few guidelines will greatly increase success. The guidelines are the subject of the next section.
Step one: prepare the site and the plastic
The materials are simple. The publication says that all you need is a roll of clear polyethylene plastic, and that the best types come in 10-foot wide rolls and contain ultraviolet inhibitors. It also says 2 to 4 mil thickness is most durable.
The site is prepared before the plastic goes down. The first step is to till the soil to a one foot depth, then rake it free of large clods, weeds and plant material. The publication says the site must be level, well-tilled, and smooth, which provides maximum contact between the soil surface and the plastic.
The publication also says to rake the soil free of weeds and plant material during this preparation.
Step two: water, trench, and seal
The second step is watering. The publication says to use a sprinkler or drip-line irrigation to saturate the soil to a depth of 3 feet or more. The source states that this can take hours, but it is essential for successful solarization.
The plastic is then sealed at the edges. The publication says to dig a 6 to 8 inch trench around the plot, then wet the soil again. One edge is placed in a trench and covered tightly with soil. The plastic is stretched taut over the site, and the remaining edges are buried. The publication says the wind must not blow or lift the plastic, because that lets heat and moisture escape.
The site should then be left alone. The source says the plastic should stay in place for at least six weeks, and that the longer the plastic stays in place, the better. For maximum effectiveness, the publication recommends leaving the plastic in place for 14 weeks between June and September.
Why the soil gets hot
The effect is based on heat that builds up in the soil over time. The publication says the effect is obtained from the long-term increase of soil temperature at normal root depths. In some tests in California, the soil was measured at 6 inches deep, and temperatures of tamped soil were 14 to 23 degrees higher than those of uncovered soil. The source notes that this varies with soil texture, moisture, the amount of solar radiation, and other related factors.
The publication also states that root knot nematodes, including eggs, are readily killed when soil temperature exceeds a threshold, which it gives as a temperature held for a set time.
The heat is also not the only factor. The publication says that combining organic amendments with soil solarization has also been shown to improve control of soil borne plant diseases. It explains that volatile compounds released by the breakdown of organic materials enhance the effect beyond that of heat and steam alone.
Step three: remove the plastic and plant
When the treatment period ends, the plastic is removed. The publication says to remove the plastic in late summer or early fall. It also warns against cultivating the soil after removal. The source says that cultivating will bring viable weed seed back into the upper 2 to 3 inches of soil.
Planting follows the removal. The publication says to plant after removing the plastic, or to cover the site with a weed and disease free mulch and plant the following spring.
For related soil care after a bed is cleared, see how to keep garden soil healthy. For a different soil amendment, see how to add compost to flower beds.
What solarization does not do
The publication is clear about the limits. Soil solarization does not completely kill the soil pathogens. It is a technique that can be used to give crop plants a competitive advantage, allowing them to survive and produce a crop in the presence of soil pathogens.
The source describes solarization as a sustainable practice. It says the method is also a sustainable practice and uses no synthetic pesticides. That statement is about the method’s inputs. It does not claim that every pest is removed or that the result will last through a full season without other care.
How we chose
The source was selected because it is a University of Arizona Cooperative Extension publication, which is a non-retail, institutional source. It was opened with HTTP 200 on 2026-10-27. The publication is dated June 22, 2022, and it is adapted from earlier Backyard Gardener publications. A second source, a UF/IFAS publication, was also checked, but its PDF could not be read, so it is not used. This article includes no product recommendations, prices, or ratings.
What we could not verify
The method and its steps are supported by the University of Arizona publication. The article does not give a verified second source for the same steps, because the second PDF could not be read. The article also does not give exact temperature thresholds for each pest, because the extracted text for those figures was garbled.
What the sources do not say
The publication does not say how well solarization works in regions other than Arizona or California, although it describes the research base. It does not give a percentage reduction for any pest. It does not say how solarization affects beneficial soil organisms in detail beyond the statement that methyl bromide kills all living organisms. It also does not give a recommended plastic color or a yield result, so those points are not treated here.
Frequently asked questions
How long does soil solarization take?
The publication says the plastic should stay in place for at least six weeks, and that 14 weeks between June and September is recommended for maximum effectiveness.
What kind of plastic is used?
The publication says clear polyethylene plastic is used, that the best types come in 10-foot wide rolls and contain ultraviolet inhibitors, and that 2 to 4 mil thickness is most durable.
Does solarization kill every soil pathogen?
No. The publication says soil solarization does not completely kill the soil pathogens, and that it gives crop plants a competitive advantage.
Should the soil be tilled after the plastic comes off?
The publication says not to cultivate the soil after removal, because that brings viable weed seed back into the upper 2 to 3 inches of soil.
Is soil solarization a chemical treatment?
The publication describes it as a practice that uses no synthetic pesticides. It contrasts this with methyl bromide, which it describes as a sterilant.
Sources
- University of Arizona Cooperative Extension, “Soil Solarization,” Backyard Gardener #28, https://extension.arizona.edu/sites/extension.arizona.edu/files/attachment/SoilSolarization.pdf (opened 2026-10-27; source dated June 22, 2022). Quotes: “uses the heat transmitted by solar energy through transparent polyethylene to kill soil-borne pathogens” and “It is a technique that can be used to give crop plants a competitive advantage, allowing them to survive and produce a crop in the presence of soil pathogens.”
