Frost Dates

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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:

Year LatestAF EarliestAF LatestGF<3 EarliestGF<3
2010 22-Apr-10 25-Oct-10 13-May-10 17-Oct-10
2011 19-Mar-11 17-Dec-11 04-May-11 20-Oct-11
2012 15-Apr-12 14-Oct-12 16-May-12 22-Sep-12
2013 07-Apr-13 15-Nov-13 25-May-13 04-Nov-13
2014 03-May-14 06-Nov-14 03-May-14 12-Oct-14
2015 13-Apr-15 22-Nov-15 28-Apr-15 12-Oct-15
2016 30-Apr-16 08-Nov-16 15-May-16 10-Oct-16
2017 27-Apr-17 30-Oct-17 10-May-17 30-Oct-17
2018 21-Mar-18 29-Oct-18 01-May-18 25-Sep-18
2019 14-Apr-19 09-Nov-19 12-May-19 02-Oct-19
2020 14-May-20 04-Nov-20 15-May-20 28-Sep-20

AF = air frost     GF = ground/ grass frost
Latest means the last frosts in the spring
Earliest means the first frosts in the autumn

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.

LatestAF EarliestAF LatestGF EarliestGF
NOT SAFE      
Mar 1-10 1      
Mar 11-20 2      
Mar 21-31 7      
Apr 1-10 11      
Apr 11-20 half safe 25   NOT SAFE  
Apr 21-30 37   4  
May 1-10 52   15  
May 11-20 60   half safe 30  
May 21-31 62   48  
Jun 1-10 SAFE   58  
Jun 11-20     64  
Jun 21-30     SAFE  
Jul 1-10        
Jul 11-20        
Jul 21-31        
Aug 1-10        
Aug 11-20        
Aug 21-31       SAFE
Sep 1-10   SAFE   2
Sep 11-20   1   8
Sep 21-31   3   21
Oct 1-10   9   half safe 40
Oct 11-20   22   54
Oct 21-31   half safe 37   60
Nov 1-10   55   64
Nov 11-20   59   NOT SAFE
Nov 21-30   62    
Dec 1-10   64    
Dec 11-20   NOT SAFE