III. Types of Solarization

A. Whole-Field Treatment

The bulk of this article has addressed the application of plastic mulch to an entire field or growing site. In sunny and hot regions, solarization of fields has shown encouraging results.

B. Row or Bed Solarization

Research suggests that covering only individual planting rows (as is commonly performed on raised beds) may encourage weed growth in the borders of solarized areas. Covering strips of soil gives more opportunities for weed encroachment into treated beds. In addition, rhizomes of grasses are able to grow beneath the plastic at bed margins and flourish in beds when the mulch is removed (McSorley, unpublished data). Strip mulching may also be less effective than entire-field mulching because of cooling effects and pathogen reinfestation from the surrounding non-treated soil (Katan et al., 1976; Katan el al., 1980). However, row solarization is effective on raised beds and for short-season crops.

If row solarization is used, bed orientation may have an effect on the heating potential. It has been observed by McSorley and McGovern (2003, unpublished data) that soil temperatures in rows running north to south will heat to a significantly greater level than will rows that run east to west. This is due to the angle of solar radiation during sunrise and sunset and its intense contact with the bed sides. Although bed sides are never (or rarely) shaded when they coincide with the path of the sun, east-west oriented beds do not get the kind of intense radiation needed to sufficiently raise soil temperature. Heat units are usually accumulated to a higher level at the west end of the bed due to stronger light intensity in the afternoon. On the other hand, edges in a north-south oriented bed will be shaded during a portion of the day, but will receive direct intense radiation during several hours of the day as well.

C. Disinfestation of Container Media

Solarization has been shown to be an effective and inexpensive 00method for disinfesting container media. In fact, it is believed to be more effective than using steam. Fungi present in a pile of potting medium (Pythium myriotylum, Phytophthora nicotianae var. nicotianae, and Sclerotium rolfsii) were killed within a week of solarization (under 50 mm PE film) (Duff and Barnaart, 1992). Potting mix can be solarized in clear plastic bags for three to four weeks to eliminate Meloidogyne incognita, Phytophthora crytogea, Fusarium oxysporum, and Rhizoctonia solani from the medium. Solarization can be particularly useful for organic media, and in cases where fumigation is inconvenient or undesired. In addition, this treatment of container media allows for recycling of planting media.

D. Greenhouse Solarization

Use of solarization in greenhouses is very common. Closed greenhouses are able to reach far higher temperatures than field soil. As with traditional outdoor solarization, this technique is most efficiently employed during the hottest months of the year.

E. Postplant Situations

The most popular way to use solarization is as a pre-plant treatment since the high soil temperatures achieved may damage plant roots. However, using clear mulch in a year-round situation in established pistachio tree groves, olive orchards, and apple orchards has given good control of Verticillium wilt (pistachios and olives) and Rosellinia necratrix (root rot in apple orchards) (Asworth and Gaona, 1982; Tjamos et al., 1991; Stzejnberg et al., 1987).

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