Time and Temperature Requirements for Thermal Death of Seeds of Yellow Starthistle

Solarization is a method of soil disinfestation that relies upon high temperatures to reduce populations of pest organisms in the soil (Stapleton and De Vay 1986). Soil temperatures above 45 C have been found to reduce emergence of seeds of annual weed species (Horowitz et al. 1983, Peachey et al. 2001). While solarization has promise as a potential alternative to soil fumigation, specific guidelines for treatment are lacking (Stapleton et al. 2000). Time and temperature requirements for thermal death have already been determined for seeds of six weed species (annual sowthistle, barnyardgrass, black nightshade, common purslane, London rocket, and tumble pigweed) at temperatures reached during solarization (Dahlquist et al. 2007). We determined the time required for thermal death for seeds of two additional weed species, yellow starthistle (Centaurea solstitialis L.) and black mustard (Brassica nigra L.), at 50 C. Seeds were placed in organdy bags and allowed to imbibe water at room temperature for 2 hours before heat treatment. Seed bags were placed in 0.47-L mason-type canning jars filled with 30-grit silica sand wetted to field capacity (10.4 % moisture) with deionized water and maintained at 50 C in a water bath. Six jars per species were maintained at 50 C, and three jars were kept at room temperature as controls. In each experiment, three to four bags of seeds were placed i