Plants are flowering a month earlier in the UK as the climate heats up, a study has found.
The researchers examined 420,000 recorded dates of first flowering for more than 400 species, dating to 1793. The average date for the first blooms was about 12 May up to 1986, but since then the date has been pushed forward to 16 April.
Herbaceous plants saw the biggest advance, producing flowers an average of 32 days earlier. Trees blossomed 14 days sooner and shrubs advanced by 10 days. The researchers think faster-reproducing herbaceous plants can more easily adapt to the warming climate.
During the most recent recorded year, 2019, spring arrived 42 days earlier than the pre-1986 average. The difference between flowering times in the north of the UK, above Stoke-on-Trent, and the south has shrunk from nine days before 1986 to four days afterwards. In the period after the mid-80s there has been accelerated global heating caused by fossil-fuel-burning and other human activities.
“The results are truly alarming, because of the ecological risks associated with earlier flowering times,” said Prof Ulf Büntgen, at the University of Cambridge, who led the research. “When plants flower too early, a late frost can kill them – a phenomenon that most gardeners will have experienced at some point.”
But the even bigger risk is “ecological mismatch”, he said, when plants and hibernating or migrating insects, birds and other wildlife are no longer synchronised. “That can lead species to collapse if they can’t adapt quickly enough.” Such mismatches are already being seen, for example, between orchids and bees and great tit chicks and their crucial caterpillar food.
The researchers concluded that if plants in the UK continued to flower earlier, and climatic extremes increased further, then “biological, ecological and agricultural systems will be at an unprecedented risk”. Farmers could suffer, for example, if fruit trees were flowering early and then a late frost killed the entire crop.
While it could be difficult for people to see the significance of average global warming of 1C, Büntgen said, plants flowering a month early showed a clear change. “It is not abstract anymore. We all have a feeling for a month, particularly if you have to wait for your next salary payment,” he said.
The study, published in the Proceedings of the Royal Society B journal, examined first flowering dates from the Channel Islands to Shetland, and from Northern Ireland to Suffolk. The study did not highlight specific species, because the data available for individual plants were very variable over time and in geographic coverage. “You have to be very cautious,” said Büntgen.
But the study did note that there were now hundreds of records of snowdrops and hellebores flowering before New Year’s Day and dozens of such records for primroses, winter aconites and lesser celandines.
The scientists found that the earlier flowering dates correlated strongly with the average daily maximum temperature from January to April, which rose from 7.8C from 1952-1986 to 8.9C from 1987-2019. The researchers did not find a link with the length of the day, but they said this cue used by plants might become important if average flowering dates are brought forward into February in the future.
The researchers found that their data showed flowering had advanced by 15 days more than the UK Spring Index used by the government, which includes only two species, hawthorn and horse chestnut.
John David, at the Royal Horticultural Society, said: “The main focus of this study is on native plants and so we don’t yet have a clear picture of the full impact of these changes on garden plants, but we would expect a similar pattern and have seen indications of this in our own gardens, such as the apple flowering times in our orchard at Wisley.”
“The UK’s 30 million gardeners have an important role to play therefore in supporting our native biodiversity and counteracting the effects of any mismatch between flowering times and dependent wildlife. Our Plants for Bugs research, for example, shows what plants you can grow that will support biodiversity all year round.”
The flowering date records used in the study come from Nature’s Calendar, a collection of 3.5m observations of seasonal change and maintained by the Woodland Trust and the Centre for Ecology and Hydrology. Many of the records are provided by citizen scientists and anyone in the UK can submit data.
“It is the world’s largest dataset and it’s superb, but it’s not enough,” said Büntgen, who called for more people to participate. “Updating the dataset every year is extremely valuable, because this is the only tool we have to really understand how climate is affecting our ecosystems. This is not something we can model.”
