There is no single morning when a city becomes permanently safe from frost. There is a range of historical dates, and a choice about how much risk to accept.
Start with the temperature threshold
Our guides use NOAA’s 32°F (0°C) air-temperature freeze dates. Gardeners often call these frost dates. Actual frost on a plant depends on its surface temperature, moisture, wind and exposure; the air reading at a weather station cannot describe every leaf.
What 50%, 30% and 10% mean
For spring, the percentage describes the historical probability of a 32°F freeze on or after the date. Choosing 10% moves the spring date later than choosing 50%. For fall, it describes the probability of a freeze on or before the date; choosing 10% moves the autumn planning boundary earlier.
The days between two selected boundaries are a planning interval. They are not a promise of freeze-free conditions or a joint probability for the entire season.
Your garden has its own microclimate
A sheltered wall, a low pocket that collects cold air, or a change in elevation can alter conditions. Check the station distance shown on each guide. Use local extension advice and the weather forecast to make the final decision.
NOAA frost-date resources
NOAA: When to expect your last spring freeze
NOAA: U.S. Climate Normals