Growers are well aware that a late Spring frost can damage plants, especially the young ones they have only just planted. For vegetable growers, they do not want to see their tender young beans, sweet corn, courgettes cut down by a cold night...
So it is useful to be aware when it is safe to plant out the tender subjects in the spring.
We need to know when the air frosts (temp below zero C) stop happening, and the less damaging, but still problematic, ground/grass frosts (which occur when the grass freezes, although the air temperature a few feet higher may be around 3C).
Over the last decade, the dates have looked like this:
AF = air frost GF = ground/ grass
Over the last 65 years, 1956 to 2020, this graph shows when the air frosts of the spring cease... clearly this is spread out over 2 months, likewise when the autumn air frosts begin... again this is likely to happen over 2 months.
So, when is it safe to plant out?
It's a matter of how much risk you are prepared to take, because you'll almost certainly be safe by 11th June, but you might have wasted valuable growing time. At the other extreme, plant before 21st March, and your plants will most likely get frost damaged. Many growers will settle for around 21st May, which is safe around 90% of the years!
Similarly for ground frosts, which will still occur after the air frosts have finished, remember, as the air temperature will still fall below 3C even if it doesn't reach zero...
Another way of looking at this, the left side axis is how many of the 65 years on record will have air and ground frosts by a certain date.
If you don't like pretty graphs, then here is the same presented in a table.
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