The most common threshold for a freeze is 32 °F (0 °C). This is the level used by the National Weather Service to mark the end (or beginning) of the growing season. Not all vegetation is as sensitive to the cold though, and may survive a 32 °F freeze.
While wheat can survive sub-freezing temperatures, it becomes vulnerable to damage around 24 °F.
The level of damage is affected by many factors (exposure, wind, etc.), but wheat that is jointed is especially susceptible to long durations below 24 °F.
Duration and Lowest Temperatures
Since each crop is unique, the lowest temperature and duration below freezing can help producers and gardeners assess conditions for their crop.