Temperature for tomatoes under cover?

I’ve been growing tomatoes for 25 years, and have found that they don’t need to be kept above 10°C at all times as is often stated. If that was the case, we couldn’t grow them in the UK, except in heated greenhouses. For example last year on one occasion, there was an overnight temperature low of 3°C at the end of June in my poly tunnel. There are always numerous occasions throughout the summer when it dips comfortably below 10°C. This is in SW Wales, which is reasonably mild. I have always assumed that it’s because they can tolerate some cooler temperatures, especially in the summer when the nights are shorter and it can’t have been very cold for very long. What I never become comfortable with is how early to plant them out. Our last frost date is officially early-mid May, although it’s almost always in mid-late April. I’m not sure how young tomato plants will tolerate temperatures which are above freezing, but still cold, when the nights are still reasonably long. I try to help them by not tying in the supporting strings just yet, so that I can put fleece or a bucket over them for the night if the met offices hits us with an unexpected colder snap. Any thoughts on your experience with spring planting out? How early do you risk it?

 

Hi @Emerion I'm also in Carmarthenshire and accidentally left some tomato seedlings in the unheated section of my greenhouse after pricking out a few weeks ago. I noticed that they have been very slow and showing stress type colouration, compared to those in the heated section. So I've moved them back into the heated section (only just above freezing) and they're all going to stay there for a couple more weeks as I think the nights are still too nippy - particularly on the rare days we get clear skies.
Even in the frost free section, I lost an expensive petunia plant (only bought for its name!). I think it was too close to the louvres which always let some air in even when closed - and a couple of weeks ago we had frost overnight.

 

I've always taken the advice about minimum temps. for tomatoes as relating to the average temperature rather than a brief minimum below 10c.

A friend grows his in a tiny unheated room at the back of his house on a bench opposite a single south-facing window.
Once germinated indoors in January they stay in the little cold room until he plants them in his greenhouse - which he done last week.
He's used the same method for 30+ years.
The plants are about 10" - really stout and with plenty of leaves.
They look better plants than the ones I sowed on 10 March which are about the same height.
As he's not given them any heat they've grown slowly with a very small internode gap.

 

This is very interesting @Pete8. When you say opposite a south facing window, do you mean right next to it or a few feet away? Annoyingly we have one tiny south facing window in a bathroom, so it’s not an option for me, but I would have liked to try this. It sounds a restful way of doing it. Also, does cold mean well below 10°C?

 

I guess the room is about 8ft x 5ft ish with a single window and accessed from the original kitchen door to the garden. So it's a small brick built lean-to with a solid roof.
It is on the back of the house which faces south and he keeps the plants against the brick wall of the house on a bench opposite the window that is about 5ft away.
So that wall will warm up a little in the day and act as a storage heater overnight.
The house hasn't been occupied for a few years, so it has only minimal heating and there's no heating in the little room. So the temperature probably does fall below 10c I'd imagine, especially in late Jan and Feb.
I've never tried that method, but he and his partner are well-reputed locally as being very successful fruit and veg gardeners.
They're complete luddites! no mobile phone, no computer and no TV, and they're such a lovely couple dedicated to their gardens (they each have a house)

 

Thanks @Pete8. If and when we move house, I hope I remember that method if I have a south facing window to spare. Just shows that there’s always more than one way with veg growing.

 

Because we are having to replace our polytunnel with another we have had to rethink where plants that should be there are going to go. Tomatoes have always been in the tunnel and never had any problems with growth or fruiting. It has no additional heat apart from (when) the sun shines. We give them no extra protection so they have to just go with it.
So this year they have been planted outside in a make shift tunnel. We have had many cold days and nights but they are growing. OK they aren't producing flower trusses yet but then I sowed late.