Is this a viable tool for your operation?

Photos by Jim Stapleton, Copyright by The University of California Board of Regents.Inset Photo by Richard Molinar, Copyright by The University of California Board of Regents.
Jim Stapleton travels a lot talking about soil solarization, and his Web site is a wealth of information. Top, Solarization plastic is used commonly now in the deserts of California and the Central Valley, especially for control of weeds on spring mix salad crops.

Jim Stapleton calls it “predictable success.” That’s the goal for the scientists and growers using plastic sheets to heat up the soil and kill weeds and pathogens: to be able to use soil solarization in situations where it can be reasonably certain to do its job.

That point has arrived here in California for certain procedures. Solarization has been tested for over a decade and has found a niche among vegetable growers. It’s not widespread usage—Stapleton guestimates that up to 10,000 acres of leafy vegetables are being treated this way this year—but commercial growers have refined techniques and materials to the point where they can rely on them.

Stapleton is a plant pathologist in the University of California’s Cooperative Extension statewide IPM program, and has been for 20 years. During that period he has seen the methodology mature. That’s partly because of the simplicity of the idea. What could be more obvious than using the sun to kill pesky organisms? The popularity is partly attributed to the loss of or reluctance to use fumigants that once did the job, but now are considered volatile organic compounds that pollute the atmosphere. There are drawbacks to the system, but there are also new developments being tested.

Solarization has a long, documented history. It was used in the early 1900s in California’s Imperial Valley, and in other countries such as Russia. Some of that was soil heating under glass. Modern advances were made in the fields of Israel and the greenhouses of Japan. The technique still finds its most widespread use in greenhouses around the world, where summer heat in closed spaces can cook the soil, and more and more in urban gardening settings. It can also be used in orchards and to sterilize containers in nurseries. There’s even an International Workgroup on Soil Solarization, and Stapleton is on the Advisory Committee.

Basically, what the process has evolved to is the use of plastic films spread over either planting beds or the entire field, with the soil moistened prior to application of the plastic to maximize the “disinfestation.” Plastic can be laid by hand, or, in large operations, by tractor-mounted, multirow applicators now common and easy to find. Clear plastic, rather than colored, gives the best heating results. Many pests, from nematodes to fungi, can be killed this way, but the most common use today in California is to kill weeds and weed seeds in the hot summer months.

“Growers of organic row crops are the big users,” Stapleton says, because the operation suits such crops as spring mix salad plantings where the lettuce and other varieties are harvested by mowing and have zero tolerance for weeds. Being organic, the crops can’t be sprayed.

Multiple tractor-mounted applicators have brought down the price of installation of plastic film, which is sold by the pound and varies with the price of oil.

Because the plastic must be applied in the heat of summer, over a period of anywhere from two to six weeks, spring mix growers treat it as part of their crop rotation. They can plant crops in the fall and winter without fear of weeds germinating, harvest the last crop in the spring and treat with plastic again the next summer. Growers like it also, Stapleton says, because it eliminates the chemicals and requisite paperwork.

As more information becomes available through research such as Stapleton’s and his colleagues’ around the state, this type of weed control becomes more of a “predictable success” story. UC farm advisors help growers try new methods, and Stapleton maintains a solarization Web site from the Kearney Agricultural Center in Parlier. It is a wealth of information, with publications, a seed thermal death guideline for six species of weeds, temperature maps and a list of plastic suppliers. That site is . Don’t bother looking for purple nutsedge death statistics; that’s a species that is resistant to heat.

The killing of other soil-borne diseases and pathogens with solarization is less of an exact science, with some species being more resistant to heat or having the ability to move deeper into the soil. Verticillium wilt, for example, is a disease that can be controlled effectively, while nematodes are more of a guessing game.

“If nematodes are your main problem you can probably expect control down to 12 inches, maybe more, maybe less,” Stapleton says, depending on conditions. Nematodes may reinfest the field, but a grower might grow crops for a season or two before they become a problem again. There’s a definite learning curve, and growers in the southern desert and the Central Valley who are successful have years of experience and have sorted out just what they can solarize and how.”

This is sort of old history in the solarization business. There are some exciting new tests going on around the state this summer that could open up new areas of use. One of those is a test on coastal vegetable ground to see whether steam injection can be combined with plastic to kill organisms in cooler climates.

“That’s so brand-new that we don’t have any information yet,” Stapleton says, but it could be very important if proved successful on crops such as strawberries and cut flowers. The steam will simulate the solarization process.”

Another method being tested is one that has already shown effectiveness in some circumstances. That is in combining an application of pesticides beneath the plastic to get a better and deeper kill of hardy pest organisms. Stapleton’s colleague Tom Turini is testing dazomet and metam sodium under plastic for such hardy pests as vine decline fungus, which is resistant to simple solarization. This is closely related to the already-tested use of biofumigation, which is the use of plastic over certain natural amendments or crop residue such as disked Brassica to boost the killing power of the heat.

New research is also looking at “double tenting” of plastic around containers with soil in them. Containerized soil solarization is already an internationally established method of sterilizing small amounts of soil, and double tenting might be more effective on pests such as nematodes. This could lead to efficiency in organic agriculture, nursery plantings and in Third World agriculture.

There is also some testing of new technology going on around the world. Stapleton says that although solarization is a “very simple” process, it benefits from new materials and methods every year. The standard plastic products that inhibit UV radiation are still used (thinner films give better solarization), yet now embossed plastic is being tried to see if it will hold tighter to the soil.

Which raises one of the obstacles to solarization with plastic films. It works best when adhering tightly to the soil contours, yet the matter of holding it down in windy areas is an ongoing problem. Several methods of weighing it down along the edges with soil have been tried—including mechanical means—but one of the most effective and simplest, Stapleton says, is the use of a narrow bead of soil dropped on the bed by the plastic-laying crew.

Another inherent problem is that of the soil becoming hot around trees. At one time it was thought that solarization would be a useful tool in orchards, particularly in reducing nematodes or weeds around new tree plantings, but clear plastic actually heated the soil so much that it killed young almond and apricot trees. Stapleton says that black or other colored plastic can be used on most tree and vine crops without killing them, but it less efficacious in killing pests and the practice has declined.

Crop management is also made difficult when there is plastic in the field, and that can be a problem for both scheduling and for cultivation or other types of activities that could damage the plastic and reduce effectiveness (it should be patched if torn). In areas such as the Central Coast, where year-round vegetable production is the norm, solarization could take a serious bite out of the growing season.

Finally, the plastic ends up being junk that must be removed from the fields. Stapleton notes that there are some recycling centers in California for plastic mulches, but none are near the desert areas where solarization is common. Remedying this problem could go a long way toward promoting the technique.

On the upside, Stapleton points out that the cost of plastic is “a fraction” of the cost of fumigants. “The price can fluctuate widely,” he says of plastic, which is sold by the pound and follows the price of oil. The cost of plastic alone, without installation, is currently about $250 to $300 per acre in bed-only applications.

Price varies also depending on the acreage being treated. Discounts apply to large quantities, and mechanical installation over large areas brings the price down per acre. Stapleton notes that rising oil prices may drive plastic prices up, but there are also biomass-based plastics coming to the market that may offset that.

Don Dale is a freelance writer and a frequent contributor. He resides in Altadena, Calif.