Timing of weed seedling emergence relative to the crop is a critical factor determining the outcome of weed crop competition. Field and laboratory studies were conducted (1) to determine how tillage and presence or absence of corn affected soil temperature and moisture and, subsequently, common lambsquarters seedling emergence and density in the field, (2) to determine the interaction of soil moisture and temperature on common lambsquarters seed germination and shoot and radicle elongation, and (3) using the information from steps (1) and (2) to construct a mechanistically based model of common lambsquarters seedling emergence in the field. The effect of tillage systems (no-till, chisel, and moldboard plow) and the presence or absence of seedling corn on soil temperature, moisture and subsequently, the emergence phenology and density of common lambsquarters were studied from 1993 to 1995 at Elora and Woodstock. Elora's soil was a Guelph series Loam; Woodstock's soil was a Guelph loam. Tillage rarely affected the phenology of common lambsquarters seedling emergence; only in 1995 were effects of tillage systems observed. The presence or absence of seedling corn did not affect soil temperatures, soil moisture, or common lambsquarters seedling emergence phenologies and densities. Cumulative densities of common lambsquarters seedlings however, were influenced by tillage and environmental conditions. Laboratory studies on common lambsquarters seed germination and shoot and radicle elongation demonstrated that the process of germination was described by the interaction of temperature and water potential. Rate of seedling shoot and radicle elongation was described as a function of temperature only. A new mathematical model was developed describing the process of seed germination and shoot and radicle elongation of common lambsquarters seedlings in terms of hydrothermal time and temperature respectively. Parameters for the model (cardinal temperatures and base water potential) were determined by probit analysis. Probit analysis was used to account for variation in cardinal temperatures and base water potentials within the population. This is the first hydrothermal time model to describe the phenology of weed seed germination using a single curve, generated from the relationship of temperature and water potential. A new mechanistically based model predicting common lambsquarters seedling emergence based upon parameters developed from field and laboratory studies was developed. This model incorporated hydrothermal time, thermal time and a soil temperature model and successfully simulated seedling emergence phenology of common lambsquarters across locations, years and tillage systems (r$\sp2$ = 0.85, P = 0.0001, N = 91) using independent data sets. Predictions of lambsquarters emergence, coupled with ecophysiological crop models, will improve predictions of crop yield losses due to common lambsquarters competition. This model also will be useful as a decision support tool for weed management decisions.