For corn the threshold temperature is 50Ã‚Â°F. For peas the threshold temperature is 40Ã‚Â°F.
The baseline for field crops like wheat, rye, etc. have been determined by experimenting, probably in growth chambers - the average 24-hour minimum temperature needed for growth.
It is an "all other things being equal" sort of measurement. Drought and that sort of thing would make a difference. The Weather Service begins keeping track on March 1st, I believe. So, any warm days in March or April when your warm-season plants are not out in the garden - wouldn't count. Of course, plants can be set out too early and their growth retarded.