Pinning down precise air and tissue temperatures in a standing alfalfa crop, and then predicting the impact on growth (or death) is not an exact science. The air temperature reported on the weather report or on your local thermometer may not be what the alfalfa crop is experiencing. Topography of the site (cold air "flows" into low-lying areas), wind, and the moderating influence of the warmer soil mass greatly influence the microclimates in the standing alfalfa canopy and from site to site in the field. My descriptions here should be considered subjective, with an appropriate amount of acceptable variability that reflects real-world conditions.
Iowa State University
Barnhart, Stephen K., "Frost/freeze on alfalfa" (2005). Integrated Crop Management News. 1381.
The Iowa State University Digital Repository provides access to Integrated Crop Management News for historical purposes only. Users are hereby notified that the content may be inaccurate, out of date, incomplete and/or may not meet the needs and requirements of the user. Users should make their own assessment of the information and whether it is suitable for their intended purpose. For current information on integrated crop management from Iowa State University Extension and Outreach, please visit https://crops.extension.iastate.edu/.