PHOTOTHERMAL TIME DESCRIBES COMMON RAGWEED (AMBROSIA-ARTEMISIIFOLIA L.) PHENOLOGICAL DEVELOPMENT AND GROWTH

The ability to predict weed phenological development under field condi tions is fundamental to the development of mechanistic weed-crop compe tition models. We studied how phenological development of common ragwe ed grown under field conditions could be explained using temperature a nd photoperiod responses derived from growth room experiments. We also determined the relationship between phenological development and comm on ragweed leaf area, dry matter production, and partitioning. Phenolo gical development of common ragweed emerging at: different rimes in th e field was described by photothermal time based on temperature and ph otoperiod responses derived from growth room experiments. Estimated da tes of phenological events of common ragweed were within 4 d of record ed values. Common ragweed seedling density did not influence phenologi cal development. Common ragweed leaf area development, biomass partiti oning, and total biomass were related to photothermal time accumulatio n. The results of this study are consistent with our hypothesis that p henological development is a major factor influencing the outcome of w eed-crop competition. Results obtained from this study can be incorpor ated into a mechanistic model of weed-crop competition.