A phenological time series of the date of budburst of birch (Betula pendula) was constructed using data from the years 1907-1950. Data for the years 1908, 1910-1915 and 1930 are missing. The phenological observations were made at Saarijärvi (62 degrees 42' N, 25 degrees 44' E) in Central Finland. Different models describing the rate of development in the spring were analyzed using temperature data from the city of Jyväskylä about 60 km to the southeast of Saarijärvi. The models based on temperature sum gave more accurate predictions of the date of budburst than the model based on time. Differences between different development rates seemed to be negligible when computing the temperature sum.