Frost Dates, Forecasts and What Actually Kills Plants

A last frost date is a probability, not a deadline. It is the average of past years, which means roughly half of them were later. Treating it as the day it is safe to plant out is the single most expensive misreading in the gardening year, because the plants you lose are the tender ones you raised from February.

Air frost, ground frost, grass frost — three different things

Forecasts use these interchangeably and they are not the same. The Met Office defines an air frost as the air temperature falling below freezing, measured “at a height of at least one metre above the ground”.¹ A ground frost is ice forming on ground or objects whose surface is below freezing.¹ That measurement height matters: an official air frost is read at least 1 m up, while your seedlings are at 0 m.¹

The one that catches gardeners is the third. A grass frost is, in the Met Office’s words, “an unofficial type of ground frost” that “can occur when other surfaces — such as concrete or road surfaces — don’t experience a frost”.¹ And critically: “It is possible for a grass frost to occur in late spring or even early summer, when the risk of more widespread frosts has disappeared, and is something that gardeners in particular need to be aware of.”¹

What to do with that

Use the average date to plan and the actual forecast to decide. Work out your own local average first, then watch the week before you commit anything tender, and keep fleece within reach for a fortnight after. The cost of covering a bed for one night is nothing against replacing a tray of plants.

The three tools here, and when each one helps

• Frost date calculator — your local average first and last frost from your postcode, as a planning figure.
• Garden weather forecast — what is actually coming this week, which is the part that decides.
• Frost protection guide — what to do on the night, and what is worth covering.

Sources and checking notes

Every figure here was read off the source named, on 19 September 2026.

1. Met Office — What is frost and ice? weather.metoffice.gov.uk. Checked 19 September 2026: air frost is “the air temperature being below the freezing point of water at a height of at least one metre above the ground”; a grass frost is “an unofficial type of ground frost” and “It is possible for a grass frost to occur in late spring or even early summer, when the risk of more widespread frosts has disappeared, and is something that gardeners in particular need to be aware of.” The page states no temperature threshold specific to grass frost, so none is quoted here.