Knowledge of the environmental and climatic factors influencing the demographic success of weed species will improve understanding of current and future weed invasions. The objective of this study was to quantify the potential sources of regional-scale variation in the demographic parameters of giant ragweed (Ambrosia trifida) and common sunflower (Helianthus annuus). To accomplish this objective, field experiments were conducted across 18 site-years for giant ragweed, and 16 site-years for common sunflower between 2006 and 2008 throughout the north central region of the USA. Giant ragweed and common sunflower were planted following the soybean phase of corn – soybean rotations, and demographic parameters (winter seed survival, summer seed survival, seedling recruitment, seedling survival to reproductive maturity, and fecundity) were measured in intra- and interspecific competitive environments. Environmental data (e.g., elevation, daily air temperature, and precipitation) were collected within each site-year. Seed and seedling recruitment and survival were most influenced by location, suggesting that soil and average climate conditions were important predictors of survival. However, interplant competition from a corn crop reduced fecundity relative to giant ragweed and common sunflower monoculture. Partial least squares regression (PLSR) indicated that the overall demographic success of both giant ragweed and common sunflower were most influenced by growing degree days base 2° C (GDD₂), though the relationship with GDD₂ was negative for giant ragweed and positive for common sunflower. The first PLSR components, both characterized by growing degree days, explained 65.7% and 77.3% of the variation in the demographic success of giant ragweed and common sunflower, respectively; the second PLSR components, both characterized by precipitation, explained 17.0% and 5.5% of the variation, respectively. Demographic success of both species was negatively related with precipitation.  The apparent influence of growing degree days and precipitation is important in understanding and predicting the future distribution and performance of these species in response to climate change.