A temperature threshold, starting date, and heat unit total are required for use of degree-days. The three parameters are not independent, the heat unit total depending largely on the threshold used. An iterative solution is presented to estimate these parameters for Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco) bud burst from observations of flush dates and weather station records. The estimates obtained predicted average flush date and first bud burst ± 1 day in 80% of years for a 10-year period for a group of 154 Douglas-fir trees. Date of first and last tree flush were predicted ± 1 day in 50 and 60% of years, respectively. Sensitivity analysis indicates that the threshold and starting date can be varied to a considerable extent without influencing the accuracy of the predictions.