Since the late 20th century, the freeze-free season has gradually increased. The last spring freeze has been occurring earlier and the first fall freeze has been occurring later. This increase is expected to continue into the 21st century, with climate models simulating an increase of up to 26 days annually across most of the Midwest.
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, S. D. Hilberg, M. S. Timlin, L. Stoecker, N. E. Westcott, J. G. Dobson, 2013: Regional Climate Trends and Scenarios for the U.S. National Climate Assessment. Part 3. Climate of the Midwest U.S., NOAA Technical Report NESDIS 142-3, 89 pp. ,Wuebbles, D. J., & Hayhoe, K. (2004). Climate Change Projections for the United States Midwest. Mitigation and Adaptation Strategies for Global Change, 9(4), 335–363. doi:10.1023/b:miti.0000038843.73424.de