Q.Frost Dates & Planting Timing
What does 50% frost probability mean?
A.
50% frost probability is the median date — half of historical years had their last spring frost on or before this date, half had their last frost after. It's the coin-flip date. For unprotected high-value crops, use the more conservative bound (10% probability — the date 9 of 10 years are already past their last frost).
The 50% date is the single most-cited number in older frost-date references, but it ignores the actual variability. Two locations with the same 50% date can have very different ranges — one might be tightly clustered (low risk) and one spread out (high risk). The full probability table shows the spread.