[Kristen]

Below 10C Tomatoes stop metabolising Phosphorus - the leaf veins turn purple and the leaves take on a purplish tint. I presume this is an evolved defence mechanism, but it takes the plant some time to recover from this "stall" perhaps again an evolved trait so that the plant doesn't resume growth only to find another cold night follows.

So the plant takes some time to recover and is of course stressed in the meantime.  People tell me that their plants recover and it makes no difference.  I've never tried growing them side-by-side, some cold stressed and some not, and I'm pretty sure the people who tell me that their plants were find didn't try half-and-half either.  I expect that on average plants recover.  However, my view is that it is better not to let the plants get into that state (barring some unforeseen very late frost perhaps) and it is better to sow-later to avoid the outcome where they will definitely be cold.

Stressed plants are more likely to succumb to disease later in the season.

[Steveharford]

Interesting Kristen, I have read that they dont put on any growth whilst below 10 deg. But as i said, mine have been lower during the night and so far, are showing no purple leaves. In fact they look very healthy, and they have been down to 2.2. Maybe the odd drop in temp is bearable, but prolonged cold does the main damage? I do this every year and experience similar temperature variations without any perceivable damage. Perhaps there are other factors which contribute.....

[Beetroot Queen]

Mine have been in the porch, hot during the day and freezing during the night, they have today moved to the cold frame as they are getting too big.

Will see what happens now

[Sparkyrog]

while GH has shown minimum of 0.1 Toms are standing in propagator with sides on but top off , they seem fine 

(perhaps I shouldn't say that)

[Yana]

I'm going to chance it for a few nights but temps due to drop this weekend. 

[mumofstig]

Clear skies after hot days often give the coldest nights - cloudy days/nights are often warmer. Strange but true 

[Kristen]

Maybe the odd drop in temp is bearable, but prolonged cold does the main damage?

I think that is very likely ... it will take time for cold air to reach into the leaf / impact its chemical processes.  On a cold night at this time of year temperature falls steadily from, say, midnight until dawn and then as the sun comes up it rises sharply, particularly in a greenhouse where the indoor temperature lags behind the outdoor, so the length of time the plant is "cold" is relatively short.  That is in contrast with earlier in the year where the temperature falls (to several degrees below zero) and then stays there for several hours during the night!

I do this every year and experience similar temperature variations without any perceivable damage. Perhaps there are other factors which contribute.....

My view is that without a controlled experiment, side-by-side, it is not possible to determine what the actual damage (if any) is.  People do it, even getting purple leaves, the plants recover and all seems well. People repeat the cold-shock in future years ...

What I don't know is whether this makes any material damage.  Less yield, less sweet / less flavour, sufficiently stressed that the plants are weakened against disease - maybe they would survive a brief infection of blight if they had not been cold stressed earlier in the season? ... that type of thing. In many years the plants won't be subjected to blight (well, not here at least!) so difficult to say if they would be more susceptible or not.

So in the absence of any evidence that it might/might not be detrimental I take the view of avoiding cold-stressing my Tomatoes as I absolutely hate putting huge amounts of time and effort into growing to then have failures / poor performance ... I would prefer to use my time more productively! so I avoid cold-stress, I plant late to avoid having to mollycoddle on cold nights (e.g. I plant all-but-a-few Potatoes late to avoiding having to protect the whole crop against frost, and so on)

[Steveharford]

Yes I fully agree,  so I think next year I will repeat the process with 50% of them, then repeat say 1 month or so later with the remaining 50%, which should grow on unchecked. They do grow fast when conditions are good so it may be a bit of an eye opener.

[Headgardener22]

Comparing plants is difficult even so.

For the last few years I've been keeping a record of the crops of different types of tomatoes from different plants. Even with exactly the same treatment and side by side in the polytunnel, the crop on two same variety plants has varied by up to 50%. So how on earth would you decide whether its "coldshock" that makes a difference or just something in the particular plant?

[Willow_Warren]

I still go by the 10°C rule for most tender plants.

The problem I have is that I want to start hardening off my plants and give them more light on these lovely sunny days... but... I leave for work at about 7.30 in the morning.  At that hour it can still be frosty outside, it certainly was on Monday so my plants stayed indoors for the whole day!  I could pop home at lunchtime but to be honest it's not really worth the drive.

Today the morning was a bit warmer, about 7°C so I chanced my tomato seedlings in the greenhouse for the whole day!  To be honest I think the issue is work, it is perhaps getting in the way of my gardening...

Hannah

[JayG]

I think this issue may be getting too complicated! 

All the evidence suggests that tomato plants will suffer to some extent if exposed to temperatures below 10C - the best plan is surely to try to make sure that doesn't happen to yours, but if it does, try to make sure it doesn't happen again.