ccwood,
It does depend on the variety a little bit, for my coldframe I have an old thermostat and two small heating rods. My thermostat is set at 44/45 degrees. I have to supplement with an old blanket covering the whole top and most of the sides if the temp is going to get below 40 degrees.
So with an enclosure like your unheated greenhouse 40 degrees plus should be maintained.

I have 8 plants in the ground since mid February in zone 10. Low temps went down to 39 but typically we are upper 40s low, 70s high at this point with a lot of sun. All the plants are happy and growing.

If your prevailing conditions in the greenhouse will be favorable, with a few odd days here and there that drop down below 40 but above freeze, your plants will be just fine if they get enough sun to warm up during the day.

If the forecast calls for a few cold nights, just light a candle in the greenhouse before going to bed. It can increase the temperature in an enclosed area by up to 20 degrees. The Inuit in Alaska did this in their igloos.

cirtes,

How big an enclosed space, and how big a candle?

A Roman Candle?

They are tropical plants. Just keep that in mind. I think 50 degrees is a good minimum.

In the San Jose, CA area, I go from garage to garden on April 15th. Tax day is an easy one to remember. At that point the night time lows are typically in the 50's. I've found that putting them out earlier has never done any good, and occasionally can have very bad results.

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I am in zone 9, and have 5-6 varieties of tomatoes growing in an outside garden. I have had them uncovered as low as 34 degrees, and they are all fine. If it drops to 32 or below, they will die. I had a whole crop die like that a year ago.

DarJones,
This is really helpful! Thanks!

Good info - bumping

O" this thread is old, but the subject is valid.

Just this season, in late May, we had several night that the temps dipped down to 39, 41, 42, .. and many nights in HI 40s. This was , of course out of the norm. But the tomatoes survived without any protection but cucumbers died and eggplants stunted. So tomatoes are pretty tolerant of cold, more than most people think. But the thing is that they will not grow much, until the lows get around 48F+.

Late May we had nights under 50, days almost 90, of course June a foot of rain, sporadic 90 degree days, heat wave (90+) last week plus dry July - so blossoms dropping. Now raining buckets, could get up to 3" today! I am beginning to think we won't get any ripe tomatoes at all this year, unless weather continues warmer than usual and frost holds off well into October.

Hi have a small greenhouse in Va. Beach. Seen something on E How about growing tomatoes in the winter they have a good idea of putting plants on shelf about 3 ft off the ground and putting hardwood mulch on ground and water it once a week. It will decay and give off heat going to try it this year sounds like it will work. Just changed our heating in house to heat pump now have 100 gls. of heating oil tiring to figure out a way to use it to heat my greenhouse without killing me or my plants any ideas folks. ty gussmo

What you are talking about will help if there is enough nitrogen in the mulch.
I have a small greenhouse attached to the workshop and use an old solar hot water panel with a water heater in the line for backup.
The key for tomatoes is to keep the soil warm enough. They will survive air temperatures near freezing. Of course, if frost develops on the leaves, as stated above, they are history. Very dark clear nights can also be a killer because the leaves will radiate too much heat.
Several of us are working on a truly freeze tolerant tomato and are making some progress. Darrel, who posted above, has done the most work and we owe him big time.
Lee

Soil temperature is also a factor. But in early spring (past LF date) soil temp should be much higher than night lows. Average ground temp (few feet below) range 55F to 65F. But when the air temps are in 40s or lower, the soil will cool off. This is especially the case with narrow raised beds, that cool down deeper than flat ground bed. To help this situation plastic covers can be useful for a while.

We have also tried circulating warm water through tubing in the soil. A 4x8 solar panel will easily heat a 10x20 foot bed. Some of our earliest tomatoes go out in those areas. Landscape cloth really raises the soil temperature as well.
Here is one of our fields the winter before last. The tomato plants are underneath the cones.
Lee