Estimation of Blooming Dates of Rhododendron kaempferi, Wisteria floribunda and Hydrangea macrophylla

Blooming dates of Rhododendron kaempferi, Wisteria floribunda and Hydrangea macrophylla are estimated by accumulating so called DTS (the number of days transformed to standard temperature). The estimated blooming dates are the day when the accumulated value of DTS at the station under consideration reaches the predetermined value. The day from which accumulation starts (Ds) is determined by means of a sort of trial and error approach, by varying both the starting date and the temperature characteristic Ea, which is used to convert air temperature to DTS. Both the most appropriate Ds and Ea differ from station to station. However, in this study as well as other previous studies, the value Ea to each species is assumed to be a constant to all stations. In this study, the values of Ea are, 54.4kJmol-1 for R. kaempferi and W. floribunda, and 33.5kJmol-1 for H. macrophylla. The blooming dates are computed at 20 stations for R. kaempferi and W. floribunda, and at 9 stations for H. macrophylla. It is shown that despite the large range of starting dates for accumulation of DTS, i.e., 8-70th day from January 1st, the root mean square errors (RMSE) for R. kaempferi fall within a relatively small range, i.e., 1-4 days in Tohoku and Kanto districts, and 2-6 days in warm regions of western Japan. It appears that the larger errors in estimated blooming dates in warm regions are due to imcomplete transition of the rest break in unusually warm winters. The starting dates for accumulation of DTS for W. floribunda and H. macrophylla are relatively late, i.e., 40-60th day, except for Tohoku district. Ds for W. floribunda almost coincides with the time when the bud break occurrs. Fairly good results are obtained for blooming dates of W. floribunda with RMSE ranging 1-5 days. However, rather poor results are obtained in estimating blooming dates of H. macrophylla for which range of RMSE are found to be 3 to 7 days.