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.