Implementation of an integrated weed management system requires prediction of the impact of weed competition on crop yield. Predicting outcomes of weed competition is complicated by genetic and environmental variation across years, locations and management. Mechanistic models have the potential to account for this variability. Weed phenological development is an essential component of such models. Growth cabinet studies were conducted to characterize common ragweed's phenological response to temperature, photoperiod and irradiance. Ragweed development occurred over a temperature range of 8.0 to 31.7C, and this response to temperature was best characterized using a nonlinear function. A maximum leaf appearance rate of 1.02 leaves per day occurred at 31.7C. Ragweed has a short juvenile phase, during which it was not sensitive to photoperiod. Following this juvenile phase, sensitivity to photoperiod was constant and continued until pistillate flowers were observed. Photoperiods of 14 hours or less were optimal and resulted in maximum rates of development. Irradiance level affected ragweed phenological development only when combined with the additional stress of very low temperatures. Temperature and photoperiod responses derived from the above growth cabinet studies were assessed using phonological development data from a study of common ragweed grown in monoculture at Woodstock, Ontario under field conditions in 1994 and 1995. Photothermal time explained the appearance of phenological events and leaf number of common ragweed emerging at different times under field conditions. Estimated dates of phenological events of common ragweed were within 4 days of recorded values. Interactions between photoperiod and temperature did not need to be considered. Common ragweed seedling density did not influence phenological development indicating that factors affecting ragweed growth do not impact common ragweed phenology. It was shown, however that common ragweed phenological status will impact growth parameters, such as leaf area development, biomass partitioning, and total biomass. Finally, a mechanistic model for ragweed growth and development based on the generic plant model CROPSIM was developed. Adaptations to the algorithms and parameterization of CROPSIM's development routine was done using the photothermal time concept developed above. Adaptations and parameterization of the growth routine were made based on data from field studies using a single source ragweed grown in monoculture and from the literature. The resulting model accounted for the influence of varying environmental conditions across years, density and emergence timing on leaf number, leaf area, leaf weight, height, and biomass accumulation. Deviations between simulated and measured values generally fell within +/-25%, the range considered to be acceptable. Deviations greater than +/-25% tended to be associated with ragweed growth shortly after emergence, particularly when temperature and moisture extremes occurred during this time period. Sensitivity of a multi-species competition model to larger deviations at early stages of weed growth will need to be examined and future versions of the CROPSIM model may need to include more detailed algorithms for upper soil surface layer temperature and moisture conditions, and improved germination and emergence algorithms to reduce these deviations.