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What are frost dates?
There are two types of frost date that are used for gardening. The first frost date is the date in which you can expect to get the first frost of the year, with the last frost date being the expected time that you will receive the last frost of that year. By year I'm referring to the gardening year, starting with the first frost date and carrying on for an entire year until that time comes again.
Your last frost date will typically be in spring, and your first frost date in late autumn or early winter. The website I use, linked at the bottom of the post, gives it in a 10-day range, and should be used more as a rough guide than an exact number. For example, my first frost in 2024 was a month before I would usually expect it, and as of writing this post I still shouldn't have had a frost. It's a good idea to have your rough frost dates in mind, but keep an eye on the weather forecast, even as early as a month or two out from those dates.
You can get a ground frost when the air temperature is as high as 4°C (39°F), although that's usually only on a clear night and will only be a light frost, meaning only the ground will freeze and not the air. But this can still cause problems for plants, so you'll want to look out for temperatures just above 0°C (32°F) when looking for frosts.
Planning with the last frost
The last frost date typically signals the date from which you can plant seedlings out. Some plants, such as spinach and peas are fine to be planted out when it's still cold, however most summer crops can be killed by a frost, so it's best to wait until you won't get any more frosts for the year. If you want to plant out before the last frost in your area, there are multiple ways in which you can. Cold frames, greenhouses, and polytunnels can help here, as can growing inside.
I will be referencing the last frosts a lot more than the first frosts in other posts, as the last frost of the year is relevant to planting out a lot of seedlings or sowing seeds outside. It's a good idea to note down your expected range for first and last frosts, as it's easier to reference frost dates for plantings because it will be different for everyone.
Planning with the first frost
The first frost date typically signals the date at which the 'summer' growing period is over. Some plants, such as carrots, parsnips, and some brassicas, can be grown or stay in the ground during winter, but many common plants won't survive, or do very well, when the weather gets that cold, so you'll want to be harvesting most things before this time of year.
When are your frost dates?
Frost dates can vary quite a lot within the UK, and much more country to country. Some places in the UK may not even get a frost for the entire year, others might be frost-free from late February onwards, giving them very long growing windows. April is the most common month in the UK for people to be getting their last frost, whereas those with the shortest growing window may have to wait until late June for the frosts to leave for summer.
The earliest first frosts in the UK, for those that can expect a frost, should come at the start of October. However this year much of the UK had frosts in early October. The last 10 days of November and first 10 days of December are the most common within the UK, with some places not getting a frost until January.
You can find below links to maps of the first and last frost dates for the UK, the maps that I personally use.
https://www.plantmaps.com/interactive-united-kingdom-last-frost-date-map.php
https://www.plantmaps.com/interactive-united-kingdom-first-frost-date-map.php
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