In his meticulous notes on the natural history of Concord, Massachusetts, Henry David Thoreau records the first open flowers of highbush blueberry on May 11, 1853. If he were to look for the first blueberry flowers in Concord today, mid-May would be too late. In the 160 years since Thoreau’s writings, warming temperatures have pushed blueberry flowering three weeks earlier, and in 2012, following a winter and spring of record-breaking warmth, blueberries began flowering on April 1—six weeks earlier than in Thoreau’s time. The climate around Thoreau’s beloved Walden Pond is changing, with visible ecological consequences.