What Does Last Frost Date Mean? 10%, 50% and 90% Explained
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The three probability levels
NOAA's climate normals give each date at three levels. NC State Extension describes them as the 10% level (low risk), the 50% level (medium risk) and the 90% level (high risk). In spring, for the last freeze:
- 90% date. There is a 90% chance a freeze still happens on or after this date. It is the early date. Planting tender crops here is the risky choice.
- 50% date. The middle: a freeze after this date is about as likely as not. NOAA's Climate.gov last-freeze map shows this date. It is also the date extension guides count from.
- 10% date. Only a 10% chance of a freeze on or after it. It is the late, cautious date.
In fall the labels flip. The 10% first-freeze date is the early one (only a 10% chance a freeze has already happened), and the 90% date is the late one.
At that Ohio station, a gardener who plants tomatoes on April 18 is betting on a coin flip. Waiting for May 5 cuts the chance of a later freeze to 10%.
The three temperatures
NOAA reports three thresholds, and NC State Extension names them this way:
| Threshold | Name | What it means |
|---|---|---|
| 36 °F | Frost | NC State says there is no single frost threshold, but 36 °F is the temperature NOAA's tables associate with frost |
| 32 °F | Freeze | A freeze happens when air temperature falls below 32 °F |
| 28 °F | Hard freeze | A hard freeze happens when air temperature dips below 28 °F |
The same Ohio station, same level of risk (the 50% date):
| 36 °F | 32 °F | 28 °F | |
|---|---|---|---|
| Last spring date, 50% | Apr 29 | Apr 18 | Apr 5 |
Higher thresholds are reached later in spring, because a cold night that stops at 34 °F counts as a 36 °F event but not a 32 °F one. The last 36 °F frost comes about 11 days after the last 32 °F freeze here.
Which date should I use?
- For counting seed-starting weeks: the 50% date at 32 °F, because the extension guidance is written relative to the "average" last frost. The seed starting calculator does this.
- For frost-tender plants such as tomatoes and peppers: look at the later 10% date, as NC State's guidance for cautious decisions suggests. Climate.gov also warns that the 50% date may be too risky for tender plants.
- For fall: the 10% first-freeze date is the earliest date to plan around. If you want to protect a crop, mark that date.
No date is a guarantee. Climate.gov says the date varies from year to year with the weather, and you only know the last freeze after it has passed. Check your local forecast in the weeks around your date.
Why a station is not your yard
The numbers are for a weather station. A garden at a different elevation, on a slope, or in a warmer city can have different dates. The frost date tool shows each station's elevation so you can judge how well it fits your site.
Sources
- NC State Extension, Interpreting Freeze/Frost Probabilities from the National Centers for Environmental Information (Nelson, El-Khoury and Boyette, AG-873).
- NOAA Climate.gov, Last Freeze map.
- NOAA National Centers for Environmental Information, U.S. Climate Normals 1991–2020, station Port Columbus International Airport, Ohio.