UC Davis

Despite the overwhelming need for weed control in organic crops, there are currently no preemergence herbicides that are organic-compliant. As an alternative, steam injected into the soil, in which soil temperatures reach >70 °C for 15-20 minutes will kill weed seed in the soil. The effect of this reduction in the seedbank viability results in control of weeds in the treated zone that can persist for several weeks or months, leaving the steamed soil area pasteurized. The effect of steam pasteurization on weeds seeds is similar to a preemergence herbicide. In my study, steam was applied alone and combined with hydrogen peroxide to create an exothermic reaction in the soil to kill weed seed and inoculum of Sclerotinia minor and Pythium spp. Replicated field trials in carrot, lettuce, and spinach were conducted using two types of banded steam applicators in 2020 and 2021. Data collected were soil temperatures after steam application, weed control, hand weeding times, diseased plant counts, and lettuce yields. Post-steam soil temperature intervals >70 °C in the top 4 inches of soil ranged from 67 to 176 minutes. Soil temperatures >70 °C in the steam + peroxide treatment were 76 and 80 minutes for trials 1 and 2, respectively. Steam and steam + peroxide reduced weed densities by 68-100% and reduced hand weeding times by 23-91%. The reduction of S. minor after steaming in all trials ranged from 77-94%. The reduction of Pythium spp. colonies after treatment in all trials ranged from 89-98% compared with the nontreated control. The lettuce head diameters in all trials ranged from 12-24% larger compared with the nontreated control. Carrots grown in the steamed treated soil had a 10% greater root diameter than the untreated control. There were treatment effects on marketable yields in trial 4.

Keywords: Steam, soil disinfestation, lettuce, carrot, spinach, soil-borne diseases