He taught himself to recognize hundreds of local plants, placing specimens in his well-worn straw hat. "When some whom I visited were evidently surprised at its dilapidated look, as I deposited it on their front entry table," he wrote, "I assured them it was not so much my hat as my botany-box."
The earliest blossoms and other signs of spring especially fascinated Thoreau. "I often visited a particular plant four or five miles distant, half a dozen times within a fortnight, that I might know exactly when it opened," he wrote. The author Louisa May Alcott, a Concord resident, remembered that the writer "used to come smiling up to his neighbors, to announce that the bluebirds had arrived, with as much interest in the fact as other men take in messages by the Atlantic cable."
Thoreau organized his eight years of botanical notes into detailed monthly charts, listing the first flowering dates for several hundred species. After his death, the dozens of pages of charts were scattered to libraries and collectors, forgotten by all but his most ardent students. Thoreau's data finally found a champion in Bradley Dean, an independent scholar, who supported his research on Thoreau with a trickle of fellowships and grants. Dean, who died in 2006, tracked down every page of Thoreau's charts, collecting a full set of copies at his home in rural New Hampshire.
Primack, 57, lean and sharp featured, had spent decades researching tropical forests in Malaysia, Central America and elsewhere before turning to his own backyard in 2002. Like Thoreau, he was interested in springtime, but his motivations went beyond a simple love for the season: Primack wanted to study how the natural world was responding to global warming. "Over the coming decades, we're likely to see a lot of significant changes caused by global warming—more and more extinctions, for example—but we can't measure most of those things yet," he says. "Bird migrations and flowering times are the best indicators we have that natural communities are starting to change."
Primack began searching for natural-history records from Massachusetts, talking to bird-watchers and amateur botanists. Through a former student, he learned that Thoreau, of all people, had collected exactly the sort of data he was looking for. In 2003, Primack called Dean to ask about his collection of Thoreau's charts. Dean, not at all surprised, said he'd expected that scientists would one day come looking for Thoreau's data.
Dean wasn't the first person to take an interest in Thoreau's record keeping. Sixteen years after Thoreau's death, an enigmatic Concord shopkeeper named Alfred Hosmer decided to continue Thoreau's botanical project. In 1878, and then consistently from 1888 until 1902, he recorded the first flowering dates of more than 700 species in the Concord area. A bachelor, Hosmer spent his Sundays exploring meadows, swamps and even the town dump. "Fred is...better informed about Thoreau's haunts than any man living or dead," wrote his friend Samuel Jones. "I, poor miserable I, admire Thoreau; Fred lives him!" Like Thoreau, Hosmer turned his field notes into hand-lettered tables, sometimes pressing a leaf or flower between the pages. He died in 1903, leaving no explanation for his dedication.
Primack, joined by his doctoral student Miller-Rushing, now had detailed reports on Concord's flora from Thoreau and Hosmer, and it was time to compare the past with the present.
It's not easy to collaborate with dead botanists. Thoreau's penmanship was atrocious, and he used antiquated botanical names. Using the research of an amateur botanist and Thoreau admirer named Ray Angelo, Primack and Miller-Rushing deciphered Thoreau's and Hosmer's tables.
During their first year of fieldwork, in 2003, Primack and Miller-Rushing searched the sunniest, warmest corners of Concord, just as Thoreau had, looking for the first blooms. They found a place on the campus of the private Middlesex School where flowers turned up especially early. They talked a local farmer into allowing them to survey his fields. They walked the railroad tracks behind the site of Thoreau's cabin at Walden Pond.
When Primack found the season's first blue violet bloom on the gravel railroad bed, he was so absorbed that he failed to hear a construction truck approaching on the rails. The driver pulled up just 20 yards from the surprised researcher and angrily demanded that he explain himself. Primack quickly made clear he was no saboteur, but a botanist, and vowed to be more cautious. But as Thoreau himself surely would have, Primack and Miller-Rushing continued to inspect the tracks for flowers, paying for their persistence with a few run-ins with local police.