Over the past 30 years, the length of the freeze-free period has increased. This increase is expected to continue into the century and extend by at least 19 days.
Recent Changes in Frost Days and the Frost-Free Season in the United States. (2002). Bulletin of the American Meteorological Society, 83(9), 1327–1332. doi:10.1175/1520-0477(2002)083<1327:rcifda>2.3.co;2
,Kunkel, K.E, L.E. Stevens, S.E. Stevens, L. Sun, E. Janssen, D. Wuebbles, J. Rennells, A. DeGaetano, J. G. Dobson, 2013: Regional Climate Trends and Scenarios for the U.S. National Climate Assessment. Part 1. Climate of the Northeast U.S., NOAA Technical Report NESDIS 142-1, 78 pp.