It's easier to judge when it's something you've grown yourself, as you have a better feel for your local weather conditions, and also the bits of your garden that are suitable for putting out young plants overnight. When in doubt, better to wait. Plants bought in tend to be forced/sown earlier, and need much longer as they will have even more growth than later sowings, and are therefore more susceptible. Often, plants will just sit there and sulk [ even hardy annuals can do that] so there's nothing to be gained by doing it too quickly.
Anything below + 3 degrees carries a frost risk, but so does wet, cold windy weather, and the big differences between daytime and night time temps. Let plants get a few extra days to toughen up, and they'll benefit from it.
It's a place where beautiful isn't enough of a word....
I live in west central Scotland - not where that photo is...
A lot of mine are out in their trays as they have outgrown my little grow house but I have not planted them out yet. At night I can put a layer of fleece over them and so far no casualties. Official date for last frosts where I live is second week in May so nearly there.
I agree with @Fairygirl about wet, cold windy weather having a detrimental effect on some plants - especially those with big leaves, like courgettes, and climbers like runner beans. One year in the 1990s in rural Northumberland our last frost was on June 1st, and the first of the autumn was September 4th. That was not a good year for growing tender veg.
Since 2019 I've lived in east Clare, in the west of Ireland.
Hardiness isn't just about freezing temps. Many plants, especially young, juicy babies, can't cope with less than 5C and will be shocked by less than 10C. They need careful hardening off by day and protection at night till they are sturdy enough to cope and grow once put out.
In exposed areas and higher altitudes that means waiting till mid to late May at the earliest and then watching the forecast for clear skies at night and/or winds that can chill as well as heavy rain or hail which can also wipe out young plants beyond recovery.
Vendée - 20kms from Atlantic coast.
"The price good men (and women) pay for indifference to public affairs is to be ruled by evil men (and women)."
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Plants bought in tend to be forced/sown earlier, and need much longer as they will have even more growth than later sowings, and are therefore more susceptible.
Often, plants will just sit there and sulk [ even hardy annuals can do that] so there's nothing to be gained by doing it too quickly.
Anything below + 3 degrees carries a frost risk, but so does wet, cold windy weather, and the big differences between daytime and night time temps. Let plants get a few extra days to toughen up, and they'll benefit from it.
I live in west central Scotland - not where that photo is...
One year in the 1990s in rural Northumberland our last frost was on June 1st, and the first of the autumn was September 4th. That was not a good year for growing tender veg.
In exposed areas and higher altitudes that means waiting till mid to late May at the earliest and then watching the forecast for clear skies at night and/or winds that can chill as well as heavy rain or hail which can also wipe out young plants beyond recovery.