How do scientists know spring flowers are blooming much earlier than they did years ago? By looking at meticulous records kept by American naturalists Henry David Thoreau and Wisconsin’s own Aldo Leopold.
In a study published last week, researchers from Boston and Harvard universities and the University of Wisconsin-Madison said some species are blooming almost a full month earlier.
“In 2012, the warmest spring on record in Wisconsin, plants bloomed on average nearly a month earlier than they did just 67 years earlier, when Leopold made his last entry in his records,” UW-Madison said in a statement announcing its findings.
La Crosse was no exception, said Bluff Country Master Gardener Marilyn Rebarchek, who tends the International Riverside Friendship Gardens in Riverside Park but was not part of the study.
“Last year, I noticed an early bloom on the spring shrubs and perennials. They were about five weeks early.”
But La Crosse County Ag Agent Steve Huntzicker said we can’t judge our growing season by a single year, especially 2012.
Last year’s weather was so out of whack with what is normal, he said, including temperatures in the 60s in March.
Apples bloomed early only to be hit by frost. Plants from deciduous trees to strawberries were thrown off by an unusually early thaw, late frost and summer drought.
The study used Thoreau’s records of 32 native plant species in Concord, Mass., gathered between 1852 and 1858, and data of flowering times for 23 species in southern Wisconsin compiled by Leopold, between 1935 and 1945.
“These historical records provide a snapshot in time and a baseline of sorts, against which we can compare more recent records from the period in which climate change has accelerated,” said Stan Temple, co-author of the study and professor emeritus of wildlife ecology at UW-Madison.
Two examples of early blooming today when matched up against the naturalists’ work show the dramatic differences in this age of global warming.
Leopold observed the black cherry tree blooming in southern Wisconsin in 1942, noting the first blooms on May 31, when the mean temperature was 48 degrees.
In 2012, the mean spring temperature in southern Wisconsin was 54 degrees, and black cherry trees were blooming as early as May 6.
Leopold also observed bloodroot blooming on April 12 in 1942, when the same wildflower started blooming on March 17 in 2012.
The work has implications for predicting plant responses to changing climate, essential for plants such as fruit trees which are highly susceptible to the vagaries of climate and weather.
“Earlier blooming exposes plants to a greater risk of experiencing cold snaps that can damage blossoms and prevent fruiting,” Temple said.
Huntzicker said it’s too soon to change recommendations on planting dates.
“We would have to see more of a trend and then adapting to that if necessary,” he said. “I don’t know that it changes it right now. Last year was not a typical anything. I don’t know that it’s dramatically changing our start dates.”
It may not be quite as scientific, but Rebarchek, the master gardner, said she’s noticed some obvious changes in bloom time in her own garden.
She has tree peonies blooming earlier than they did five to 10 years ago.
“Typically, they were the first or second week in June. Now they’ve moved up a couple of weeks for sure.”
Other plants that are blooming earlier in her garden are coneflowers and black-eyed Susans, which used to be the stars of the fall garden.
“I’ve noticed a difference of a few weeks. They’re really blooming in the summer now, in August,” she said. “And we’re supposed to get them for fall color. Those are the two that stand out the most for me.”