Soil Solarization: a Method of Killing Weeds and Diseases Using the Sun

by Matt Gibson

Soil temperature gardening techniques, or soil solarization, is the process of utilizing heat energy from the sun, to heat up the soil to levels that will reduce the effects of soilborne diseases and naturally eliminate pests, weeds and other soil problems. Soil solarization can be an effective option for treating your garden’s soil for a multitude of issues. Soil solarization works well in any type of garden, be it vegetable, flower, or herb intensive, and can help treat in-ground garden soil and raised beds alike.

The basic premise behind soil solarization is to lay out plastic sheeting over the plot of land that you are attempting to purge. The plastic sheeting uses the sun’s heat to raise the soil temperature to a level that will kill weeds, pests, and diseases. When executed correctly, the soil should reach temperatures of 120 F (49 C) or higher, which easily wipes out any soil-borne diseases or garden pests that have been plaguing your gardening efforts.

Selecting the Best Plastic Sheeting For Solarization

One common mistake that gardeners make when attempting solarization is using the wrong type of plastic. Black plastic seems like the wisest choice, as the color black is known to draw the sun’s rays. However, black plastic does not sufficiently heat the soil underneath as well as clear plastic, which allows sunlight to pass through more easily, more efficiently heating the soil underneath. Using clear plastic is essential to heat retention. Thin plastic sheeting is also recommended, no more than 1 to 2 millimeters thick, as it allows the sunlight to penetrate the plastic more easily in order to heat up the soil underneath.

What Time of Year Is Best?

Though soil solarization can be effective for six to eight weeks in the spring prior to planting, or in the fall just after harvesting, the practice is most effective during the heat of the summer, when the soil receives the maximum amount of exposure to sunlight. Summer solarization is the most effective for killing weed seeds and soil pathogens that are lurking deep within the soil.  

Length of Time and Location

Unfortunately, the shortest amount of time necessary for the solarization process to work is six to eight weeks. This amount of time is also recommended for areas in the deep south (or other extremely hot regions) during the hottest months of the year, in a year with a lot of sunny days. In the mid to upper south, gardeners should increase the duration of the project to around three months (June, July, and August), and in more northern parts of the country (or colder climate areas), solarization will only be effective if the plastic stays in plays for the entire season in a full-sun location during an especially hot summer. It is also recommended in cooler climate areas, that two sheets of plastic are used instead of one. The extra layer of plastic will help to heat the soil more by trapping warm air between the two layers.

Additional Benefits of Soil Solarization

Aside from reducing harmful soil organisms, such as nematodes, insects, weeds, and soil-borne diseases, soil solarization can improve the overall quality of your soil immensely. The heating process actually increases the numbers of many beneficial soil organisms that will help combat disease and pest infestation in subsequent growing seasons. The process also makes several important nutrients more available to your plants, such as nitrogen, calcium, and potassium. Research has shown that solarization leads to increased vegetable yields and more vigorous, naturally resistant plants. For these reasons, many gardeners in warm climate regions have started to solarize their land every three to four years, alternating which plots of land will spend the summer underneath plastic each season.

How To Solarize Garden Beds

Step One – Prepare The Beds – Till the garden area that you are wanting to solarize, removing any debris, such as rocks, compacted soil clots, roots, and anything else that doesn’t belong. Rake the area until you have a smooth and level surface. Next, water the area until the soil is about 70% saturated at least two feet below the surface. Preferably, the soil should be moist but not soaked for the solarization process, but there will be some time for the water to seep deeper into the ground as the next steps are completed. Moisture will help heat penetrate the ground easier, and moist soil issues are better suited to solarization in a moist environment.

Step Two – Dig A Trench Along The Edges – Before breaking out the plastic sheeting, dig a trench along the outer edges of the garden bed that you are planning to solarize. The trench should be about one foot wide and eight to 12 inches deep. Take your time digging your trench, as the more even and well made it is, the better it will work and the more even your plastic sheeting will lay out and stretch across the top of the garden bed, which will keep less air pockets from rising to the top. If you don’t feel like digging a trench, or if doing so is not feasible in your garden setup, you can always gather a lot of heavy stones to hold the plastic down on the edges. The trench will help you lay out your plastic more evenly, however, and is strongly recommended.

Step Three – Laying Out The Plastic – Once the trench has been dug and the garden bed has been tilled and raked smooth, check the moisture level again to see if the soil is still moist at least two feet down. If it is, then you are ready to lay the plastic sheeting down over the soil. Cover the entire garden area that you are solarizing with thin, clear, plastic sheeting. Large rolls of plastic sheeting can be found at most home and garden centers. Place the edges of the plastic sheeting into the trench you dug and backfill with previously excavated soil from the trench digging, keeping the plastic pulled as tight as possible during the process. The closer the plastic is to the soil itself will depend on how tightly the plastic is pulled across the area as well as the precision and smoothness of the leveling that you were able to obtain when preparing the bed. The closeness of the plastic to the topsoil is important because it keeps air bubbles and air pockets from forming along the top. Excess air can keep the soil from reaching temperatures that are high enough to solarize the soil effectively.

Step Four – Wait Six to Eight Weeks – Unfortunately, there is no getting around the long waiting period that solarization requires. If you become impatient and decide to uncover the soil and plant your crops before six to eight weeks time has elapsed, then all of your work has been for naught. Once six to eight weeks have passed, remove the plastic, water the soil, work in some compost or organic matter and decide what you want to grow in your newly improved and solarized garden bed.

Step Five – Plant Right Away – Planting and growing a new group of crops in your freshly solarized beds is important for multiple reasons. Firstly, you will avoid re-infestations of weed seeds that might find their way back to your new and improved bed. Secondly, you will begin to rebuild organic matter levels and fix the nitrogen and other nutrients for subsequent growing seasons.

Videos About Soil Solarization

Need to get rid of the weeds in your garden and want to give solarization a try? This video teaches you how to use solarization to attack the weeds in your garden beds and features a doctoral student who specializes in solarization techniques:

Want to try solarization but worried that you live in a region that is not warm enough for solarization to be effective? This video shows you how to solarize the soil in the pacific northwestern United States:

If you are thinking of trying your hand at soil solarization to control nematodes and diseases in your garden beds, this short film is full of tips and tricks for how to do so successfully:

Want to save some money on potting soil, or revitalize old potting soil to use in your garden beds? This short tutorial shows you how to use solarization to recycle and reuse old potting soil so that you can put it back to use:

Want to Learn More About Soil Solarization?

Gardening Know How covers How to Solarize Garden Beds

Gardens Alive covers Solarize Garden Woes Away

Grow Network covers How to Solarize Garden Soil

HGTV covers Soil Solarization How-To

The Spruce covers Kill Weeds with Soil Solarization