What a frost date really means
An average last frost date is a coin toss, not a promise. Here is how to read it, and how to play it a little safer.
Last updated · Sources checked on
What the sources say
The average date of the last spring freeze is the date after which the chance that temperatures stay above freezing for the rest of the season becomes higher than the chance of another freeze. The exact date in a given year is never known in advance. For Boulder, Colorado, the chance of a later freeze drops below 50 % after 6 May, but below 10 % only after 18 May.
(source: NOAA Climate.gov)Our explanation
That is why SowClock offers two settings when it knows your town: Average (the median: in about 1 year in 2 the last frost came later) and Careful (in about 1 year in 10 it came later, and for the autumn, in about 1 year in 10 the first frost came earlier). Switch to Careful if you have precious seedlings or a cold garden.
Air frost, ground frost and your plants
What the sources say
Air frost is measured in the air, typically around 1 metre above the ground; ground frost happens when the surface drops below 0 °C even if the air above stays positive. Gardens and crops are most commonly affected by ground or grass frost. Frost is most likely on clear, calm nights, in rural areas and in valleys where cold air pools.
(source: Met Office)What the sources say
The last spring air frost typically occurs two to four weeks before it is safe to plant out tender plants; ground frost is still common after it. The RHS advises planting tender plants out when nights are consistently above 7 °C, and above 10 °C for tomatoes, peppers, basil and cucurbits.
(source: RHS)What the sources say
A screen temperature of 2.2 °C indicates that the temperature at the surface is approaching 0 °C.
(source: Bureau of Meteorology)Our explanation
Our frost dates use the standard measurement: a night counts as frosty when the minimum air temperature at the weather station falls to 0 °C or below. So a ground frost in your garden can still nip plants on a night the station records +1 or +2 °C.
Frost dates are moving, but late frosts still happen
What the sources say
Comparing recent climate averages with earlier periods, the average date of the last spring frost has been shifting earlier, consistent with a warming climate. This does not remove the risk of late frosts, which can become more problematic when warm springs push plants into growth early.
(source: Met Office)What the sources say
UK frost days have declined in recent decades, but spring is variable and late frosts still occur: April 2021 recorded the most April air frosts in at least 60 years.
(source: RHS)Good to know: the limits
- Averages describe past weather (1991-2020 for SowClock) and do not predict a given year.
- Your garden may be colder or warmer than the station: height, slope, nearby water, walls and town heat all count. (source: Utah State University Extension; Bureau of Meteorology)
- Frost protection helps: heavier grades of fleece give about 2 °C of protection. (source: RHS)
Frequently asked questions
If the average last frost has passed, am I safe?
Not quite. By definition, in about half of years the last frost comes after the average date. For tender plants, use a later date, watch the forecast and keep fleece ready.
What is the difference between air frost and ground frost?
Air frost is when the air about a metre or more above the ground falls below 0 °C. Ground frost is when the surface falls below 0 °C even though the air above stays just above freezing. Ground frost can damage tender plants and still happens after the last air frost.
Sources
- NOAA Climate.gov: Interactive map, average date of last spring freeze across the United States (April 2025)
- Met Office: Why frost can still occur in late spring (blog, 11 May 2026)
- RHS: When to plant out tender plants, avoiding late frosts
- Bureau of Meteorology: About frost
- Utah State University Extension: Suggested Vegetable Planting Dates, Grand and San Juan Counties (2024)
Sources checked on 1 November 2026.