In the 1960s, the U.S. Department of Agriculture started a network of clone plantings of Syringa vulgaris at agricultural research stations, universities, and home gardens around the Northeast. The idea was to keep track of spring bloom dates and give farmers a heads-up about whether there had been a warmer-than-normal winter. This helped them predict the best spring planting time and warned them if bothersome insects managed to survive through winter. Climate change was not the point, but now those decades of records give "the most reliable data set in the world," according to David W. Wolfe, professor of plant ecology at Cornell University. You won't be surprised to learn that the lilac data find, over the years since the 1960s, an average advance in spring bloom in the Northeast of four to five days.