Key points are not available for this paper at this time.
A. H. Fitter, R. S. R. Fitter, I. T. B. Harris, M. H. Williamson, Relationships Between First Flowering Date and Temperature in the Flora of a Locality in Central England, Functional Ecology, Vol. 9, No. 1 (Feb., 1995), pp. 55-60
Fitter et al. (Wed,) studied this question.