Despite the tremendous need, there are currently no preemergence herbicides that are organic-compliant. Steam injected into the soil such that the soil temperatures reach >140°F for 15-20 minutes will kill weed seed in the soil. The effect of this reduction in the seedbank viability results weed control in the treated area that persists for several weeks or months, similar to the effects of a preemergence herbicide.
Two studies were conducted at the USDA Hartnell Farm at Salinas, CA during July to September 2020. Steam was applied to raised beds using a custom-built steam injector. Prior to seeding lettuce, steam was applied in a 4-inch wide band to a depth of 3 inches deep. The steam was supplied by a SF-20 Sioux steam generator at approximately 6 to 9 PSI. Soil temperatures in the treated zone were monitored overnight with Hobo temperature monitors. Treatments tested were steam, steam plus 1% w/w peroxide, and nontreated. Treatments were replicated 4 times and arranged in a randomized complete block design. Peroxide was included as it releases heat when injected into soil, i.e., an exothermic reaction, based on the premise that it supplements the heat released by steam. Data collected were soil temperatures following steaming, weed control, hand weeding times, diseased plant counts, and lettuce yields. Trial 1 was initiated July 1 and harvested August 31, 2020. Trial 2 was initiated July 21 and harvested September 25, 2020. Soil temperature intervals >140°F in the top 4 inches of soil for steam were 88 and 67 minutes for trials 1 and 2, respectively. Similarly, soil temperatures >140°F in the steam + peroxide treatment were 76 and 80 minutes for trials 1 and 2, respectively. In trial 1 steam and steam + peroxide resulted in 90% and 92% weed control relative to the nontreated. In trial 2 steam and steam + peroxide resulted in 66% and 84% weed control, respectively. Steam alone reduced hand weeding times 22% to 35% compared to the nontreated, and steam + peroxide reduced hand weeding times 36% to 40% compared to the nontreated. There were no differences in numbers of plants with lettuce drop (Sclerotinia minor) in trial 1. Compared to the nontreated, lettuce drop incidence in trial 2 was 66% and 59% lower in steam and steam + peroxide treatments, respectively. Lettuce plant diameters in trial 1 found that lettuce grown on steamed soil was 16% larger, but there were no differences in trial 2. Lettuce plants grown on steam + peroxide treated soil were 12% and 23% larger in trials 1 and 2, respectively. There were no treatment effects on yields in either trial 1 or 2.
We are not aware of any commercial scale applicators for lettuce that apply steam in a band to the lettuce bed. However, we are working with Dr. Mark Siemens, an agricultural engineer with the University of Arizona to design a commercial scale applicator. Mark has a prototype applicator built and will be demonstrating it during the winter season in the Yuma Valley. It appears that the design for a steam applicator is not complicated and is well within the capabilities of machine shops in California to build. — By Nelly Guerra & Steven Fennimore, UC Cooperative Extension