Henry Thoreau and Phenology

Henry Thoreau walked in Concord for several hours every day looking carefully at the plant life around him. He took notes on the different species of grasses, wildflowers, shrubs, and trees he encountered—when their leaves emerged, when they flowered, and when their leaves turned color. This science is called phenology.

Thoreau wrote these observations (among other thoughts) into his journal each night. In this way he assembled years of observation on hundreds of species. In 1860, he began to extract these observations from his journal and to compile them into charts. The charts compiled the observations of many individual species over a number of years, allowing for comparison of flowering times and other phenomena from year to year.

In 2003, Boston University biologist Richard Primack learned about Thoreau’s data and recognized immediately that it was exactly what he was seeking for his research: an extensive, site-specific, accurate set of historical observations that he could compare with present-day observations at the same location. This comparison, building on the observations of Thoreau from the 1850s, shows that the climate does not just change from season to season; it changes in measurable ways over decades and centuries.

Henry Thoreau provided the engineer Henry Francis Walling with the survey information on Walden Pond and White Pond used in making this map.

To Learn More

See a page from Thoreau’s Journal, 8 October 1852, recounting how a loon engaged Thoreau in a cat-and-mouse chase on Walden Pond. The manuscript journals (1837-61) of Henry David Thoreau are part of the collection of The Morgan Library and Museum in New York City.

Read “Uncovering, Collecting, and Analyzing Records to Investigate the Ecological Impacts of Climate Change: A Template from Thoreau’s Concord” by Richard Primack and Abraham Miller-Rushing, published in BioScience, February 2012.

Explore Ray Angelo’s “Vascular Flora of Concord, Massachusetts,” perhaps the most complete list that has been compiled to date of the vascular plant species that have been found growing wild in Concord.

Read “Drivers of leaf-out phenology and their implications for species invasions: insights from Thoreau’s Concord” by Caroline Polgar, Amanda Gallinat, and Richard Primack, published in New Phytologist, November 2013.