Thermal-Time Hazard Models of Seven Weed Species Germinability following Heat Treatment

Determining the amount of heat units required to kill weed seeds is a crucial aspect for the success of weed control through soil solarization. Lab experiments were designed to determine the duration of exposure for weed seeds that is required to suppress germination at temperatures (40, 45, 50, 55, and 60 °C) in the range of those typically achieved during soil solarization in California. The species tested were annual sowthistle (<i>Sonchus oleraceus</i> L.), bristly oxtongue (<i>Picris echioides</i> L.), nettleleaf goosefoot (<i>Chenopodium murale</i> L.), redroot pigweed (<i>Amaranthus retroflexus</i> L.), common purslane (<i>Portulaca oleracea</i> L.), little mallow (<i>Malva parviflora</i> L.), and redstem filaree (<i>Erodium cicutarium</i> L.). Germination tests were performed to assess the germinability of the weed seeds. The germination suppression by the lab-simulated solarization temperatures differed among the species based on their seasonality. The cool-season annuals <i>S. oleraceus</i> and <i>P. echioides</i> were more susceptible to the heat treatments than the warm-season annuals <i>P. oleracea</i>, <i>A. retroflexus</i>, and <i>C. murale</i>. The hard-seeded weed species <i>M. parviflora</i> and <i>E. cicutarium</i> were the least susceptible to the heat treatments. The germination rates of <i>S. oleraceus</i>, <i>P. echioides</i>, and <i>C. murale</i> were reduced at all of the temperatures that were tested. The germination rates for <i>A. retroflexus</i> and <i>M. parviflora</i> were not affected by temperatures below 40 °C. The germination rates for <i>P. oleracea</i> were not affected by temperature below 45 °C and the germination of <i>E. cicutarium</i> was not affected by any of the temperatures that were tested. The duration (hours) of exposure and percent of germination suppression of the weed seeds were used to create thermal-time hazard models for weed seeds using logistic regression.