Steam: Sustainable management for weeds, soil pests

SALINAS, Calif. -- Steam treatment of soil offers growers a viable alternative to chemicals and a money-saver for organic farmers. It also benefits farm laborers by reducing their exposure to potential harm, said graduate student Erika Escalona. She is assessing the impacts on weeds, soil-borne disease and the soil biome of steam treatments used to disinfest lettuce and spinach fields in the Salinas Valley. 

She works with Steve Fennimore, a professor of Cooperative Extension in the UC Davis Department of Plant Sciences, and she presented her research during the poster session at the recent Automated Technology Field Day here.

Steam injected into the ground is hot enough to kill most weed seeds in the soil after cultivation, making it a sustainable alternative to both chemical herbicides and hand-weeding, Escalona said. With temperature above 160 degrees Fahrenheit for 10 to 20 minutes, steam also kills pathogens such as Fusarium, Pythium and Sclerotinia -- all fungi that grow in the soil. They can cause plants to rot from the roots and die, causing farmers to lose production.

“It’s a balance,” Escalona said. The steam can’t be too hot, or it could kill things living in the soil that help the plants. “At 70 degrees Celsius, we’re finding good recovery of the soil microbiome after 30 days, including the beneficial organisms.”

Steam treatment was used as early as the 1880s, mostly in greenhouses but also in fields, to battle weeds and disease, Escalona said. It fell out of favor when chemical pesticides became more prevalent in the 20^th century. 

Previous studies have shown steam treatments reduced weeds and boosted lettuce yields. In addition, steaming appears to reduce the fungus Fusarium; and the tiny balls, or microsclerotia, that allow fungus to survive in the soil, according to Fennimore’s research.

“It also offers a tool for organic farming to deal with pests, because right now they don’t have a lot of options,” Escalona said.

Steam has a personal connection

Steam also would benefit the health of agricultural workers by avoiding exposure to potentially harmful chemicals. For Escalona, that’s personal: Friends and parents of kids she went to school with work in the fields where lettuce, spinach and strawberries grow. Her father manages a vineyard, and she spent summers there tending the vines.

“We should think about the people working the land and about the land itself. It’s connected with the whole aspect of agriculture,” said Escalona, who hails from nearby Watsonville.

“I’ve grown up with a majority of my community being agricultural workers,” she added. “So the aspect of being able to minimize the exposure they could have to pesticides is very satisfying.”