Microwave Soil Heating for Controlling Ryegrass Seed Germination

Several studies have evaluated the effectiveness of microwave soil heating to control a range of weed species. Ryegrass species are problematic weeds of cropping systems across many of the major crop production regions of Australia. This study determined the effect of irradiation time, soil moisture, and distance into the soil from the irradiated surface on the efficacy of microwave soil heating in reducing the germination of both perennial (Lolium perenne) and annual (Lolium rigidum) ryegrass seeds. The study used a horn antenna to apply the microwave energy to the soil. Temperature increased linearly with heating time in dry sand, although the rate of temperature increase depended on distance from the irradiated surface. The rate of temperature increase was highest at 2 cm depth (˜5.6°C min-1) and lowest at 10 cm depth (˜2.6°C min-1). The germination of seeds in the dry sand was unaffected until the microwave treatment time reached 12 min. Even after 12 min of heating in dry sand, only seeds that were close to the irradiated surface were affected, with the germination percentage of seeds on the surface dropping to 2% of the control treatment's germination response. There was no effect on seeds that were 10 cm from the irradiated surface. The rate of temperature increase in wet sand was more complex. The temperature in the first 2 cm of sand rose rapidly (˜18.3°C min-1 at 2 cm) during the first 4 min of heating and then stabilized between 80°C and 90°C. As the heating time continued beyond 4 min, the temperature at other locations farther from the irradiated surface also rose and stabilized between 80°C and 90°C; however, the heating rate was slower than near the surface (˜7.8°C min-1 at 10 cm). Heating in the wet sand reduced seed germination at all depths to less than 2.5% of the control treatment's germination response after 8 min of microwave treatment. Seeds that were within 5 cm of the irradiated surface of the wet sand were all killed (0% germination) after only 4 min of microwave heating.