Fritter and Fritter have observed first flowering dates of 557 British plant species over the past 47 years at a single location in south-central England. They find that the first flowering dates of the subset of 385 species for which data were available for more than half of the years were, on average 4.5 days earlier in 1991-2000 compared to 1954-1990. One species now flowers 55 days earlier than 50 years ago. Ninety-four (24%) of the species now flower later than in the middle of the 20th century, with one introduced shrub flowering 36 days later. This study corroborates a study from Washington, DC that showed a mean retreat (earlier in spring) of the first flowering date of 2.4 days over 30 years.
Reference
Fitter, A. H., and R. S. R. Fitter, 2002: Rapid changes in flowering time in British plants. Science, 296, 1689-1691.