Knowing the emergence periodicities of individual weed species and how tillage and eco-regional field location cluster affects them can assist producers in more accurately timing control methods. Emergence periodicities of wild oat ('Avena fatua' L.) and green foxtail (' Setaria viridis' L. Beauv.) were measured in 1999 and 2000. The survey examined 34 fields, which were under conventional- or zero-tillage. The fields were grouped into 4 eco-regional field location clusters based on field proximity to each other, soil type, soil moisture deficit and environmental conditions. Seedling emergence was tracked in the post-seeding (1999 and 2000) and pre- and post-seeding (2000) periods. Determination of the variation of emergence periodicities in different weed species due to tillage system and eco-regional field location clusters effects is the first step in developing predictive emergence models. Simple environmental conditions such as air temperature and precipitation can be used to assist producers in determining more optimal timing for weed control or crop scouting. (Abstract shortened by UMI.)