The research reveals that each 1C rise in temperature has seen flowers appearing five days earlier and suggests that even after the cold winter, long-term changes are happening because of climate change.
It shows long-term changes in temperature, with the greatest in the past 25 years. Flowers are coming out between two and 12 days earlier in the past quarter century than in any previous 25-year period.
The new study used almost 400,000 records of annual first-flowering dates for more than 400 species of plant such as hawthorn at sites across the UK.
Records stretching back to 1753 were used to create a 250-year index of the first flowering dates of the 405 species.
The report was released by the UK Phenology Network, which since 1998 has been run as the Nature's Calendar project by the Woodland Trust and the Centre for Ecology and Hydrology.
Woodland Trust conservation adviser Richard Smithers said: "We used the plant data set because it's the best set. You could expand it across all species and all countries to come up with a global index," he said.
There are now 40,000 people across the UK registered as recorder.
Records dating from 1875-1947 were part of a project run by the Royal Meteorological Society.