Home › How to read frost dates

How to read frost dates

"Average last frost" doesn't mean frost is over. It's the date with a 50/50 chance of another freeze afterwards.

The probabilities

For each weather station, NOAA's 1991–2020 climate normals give the dates by which the last spring freeze (32°F) has happened in 10%, 20% … 90% of years. On our city pages:

• 50% date: the median. Half of springs have a later freeze. Fine for hardy crops.
• 10% date: only 1 spring in 10 has a freeze after this. A cautious date for tomatoes, peppers, basil and other tender plants.
• 90% date: 9 springs in 10 still have a freeze after this. Too early for anything tender.

32°F vs 28°F

A light frost (32°F, 0°C) kills tender plants. A hard freeze (28°F, −2°C) also damages hardy vegetables and fruit blossoms. City pages show both.

Your garden isn't the airport

The data comes from official stations, mostly at airports. Cold air drains into low ground, so valley gardens can freeze when the airport doesn't. City centres and south-facing walls run warmer. Treat the dates as a starting point and check the 10-day forecast before you plant out.