Effect of Temperature and Soil Moisture on Time of Emergence of Tomatoes and Four Weed Species

The relative times to emergence of seedlings of tomato (Lycopersicon esculentum L.) and four weed species were compared at five alternating temperatures and five levels of available soil moisture in a growth chamber. Weed seeds were sown either on the soil surface or 1.5 cm below it. A cumulative distribution function was used to describe the time required for emergence under a given set of conditions. Time to 50% emergence, base temperatures and accumulated degree days required for 50% emergence were calculated for each species. In general, total emergence decreased as soil moisture decreased, but the species differed in the optimum temperature for emergence. Time to 50% emergence decreased with increasing temperature and increased slightly with decreasing soil moisture. Base temperatures and thermal times required for 50% emergence varied among species, but were relatively insensitive to soil moisture above a critical level. Differences in relative times to emergence of crop and weeds in response to temperature can be used to suggest optimal crop sowing dates and to estimate potential crop yield losses.Key words: Germination, Solanum ptycanthum, Amaranthus powellii, Setaria viridis, Chenopodium album, Lycopersicon esculentum