The growing season is quoted as a number of days for a given location, and it drives planting decisions, variety selection and irrigation planning. The definition behind that number is narrower than most readers assume.
What the freeze-free period measures
The conventional definition runs from the last spring date on which the temperature drops to freezing to the first such date in autumn.
Everything between those two dates counts, regardless of whether conditions during that stretch were actually favorable for growth.
The measurement is taken at standard instrument height above short grass, which matters because air near the ground on a calm clear night can be several degrees colder than the reading.
Why probability replaces a single date
No location has a fixed last freeze. The date varies widely from year to year, and quoting a single average would leave roughly half of years falling on the wrong side of it.
Published figures therefore give dates tied to a stated likelihood, such as the date after which a freeze occurs only rarely.
Growers choose which threshold to plant against based on what a loss would cost, which is why a commercial operation and a home garden reasonably use different dates.
How growing degree days add what freeze dates omit
A frost-free stretch of cool weather supports far less development than the same stretch of warmth, and the freeze-free count cannot distinguish them.
Growing degree days accumulate the amount by which each day's average temperature exceeds a base value below which a given crop makes no progress.
Because the base differs by species, the same weather yields different accumulations for different crops, and maturity is predicted from the accumulated total rather than from the calendar.
Why the season is lengthening unevenly
Freeze-free periods have extended across much of the country, with the change weighted more toward earlier spring dates than later autumn ones.
That asymmetry matters, because an early start exposes emerged buds and seedlings to a late freeze that a shorter, later season would have avoided.
A longer season measured in days therefore does not automatically translate into a longer usable season, and in some crops the added risk outweighs the added time.
What the number cannot tell you
Rainfall distribution, day length, soil temperature and summer heat stress all constrain what can be grown, and none of them appear in a freeze-free count.
Local terrain matters as much as regional climate, since cold air drains downhill at night and a low field can freeze while a nearby slope does not.
The figure is best treated as one boundary condition among several rather than as a description of what a site can support.