Summary:Emergence ofAvena fatuaandA. sterilisssp.ludovicianainfesting winter cereals during two years and at two sites in Aragon began after sowing in late October and continued for 23 weeks, with 75% of seedlings appearing in the first 9 weeks. The start of emergence was associated with a fall in minimum air temperature to below 9°C and a maximum of less than 20°C. Soil moisture was not limiting, and during winter flushes of seedlings tended to be associated with rises in mean temperature. In contrast with results from other latitudes,A. fatuaemerged mainly in autumn at the same time asA. sterilisssp. ludovician