The effect of temperature on viability of imbibed weed seeds

Imbibed seed of 10 common arable weeds were placed in trays in initially moist soil and, after imbibing for 2h, heated in ovens/incubators set to 31^oC, 42^oC, 56^oC, 75^oC or 100^oC for 0.5, 1, 2, 4, 8 or 16 days or at 102^oC, 155^oC, 204^oC or 262^oC for 0.5, 1, 2, 5, 7.5 or 10 min. After heating, seeds were incubated for 28 days at 10/20^oC or 20/30^oC on a 12 h dark/light regime, depending on species, and germination recorded. At the lower temperatures, germination of all species was prevented by temperatures of 75^oC or higher for periods of 0.5 days or more. Germination was lower after treatment at 56^oC than at 31^oC or 42^oC for all species except Rumex obtusifolius. The maximum temperature required to prevent germination varied among species and was of greater importance than the duration of heating. Germination was variable with duration of heating. At the higher temperatures, there was very little germination of any species after heating at 204^oC for 7.5 min or 262^oC for 5 min or more. Seeds were greatly buffered from the air temperature by 3 mm of soil, throughout the shorter duration of heating. The average temperature of the soil, over the 10 min heating required to prevent over 90% germination, varied among species and ranged from 48^oC for Avena fatua to 65^oC for R. obtusifolius. This work implies that composting systems maintained at 65^oC are unlikely to provide an efficient method of weed control. Recommendations for improvement of the laboratory technique are suggested.