They can probally survive to about 34* and not have permanat damage (they will be stunted for awhile though) if temps only go as low as 45* they will probally just grow at a snails pace until it warms up. They will probally be perfectly fine with temps 50* and up.

Just my guess though.

Last year, my tomato seedlings (in my shed) were exposed to 25 degrees one night. They looked pretty rough afterward, but bounced back in no time.

EG

Mine are currently setting between 40deg at night and 70deg during the day until I open the doors. They are growing fine now, but I question how low and how long they can go when I have to move them.

Thanks "grow_anything" for bringing this up.

I had the same concern and question, cause being I cannot accommodate big pots/plants inside. But I have a cold frame.
So then, when night lows climb over 32F, I can transfer my seedlings to cold frame.Obviouly, the temperature in the cold frame will stay several degrees warmer than outside.
Thats GREAT.

organicdan

Thanks for that information. I was really wondering if the temps in the 30s and 40s were all that good for young tomato plants myself. I know there may be times when there's an unexpected and short cold spell, but in that case most of us rush to protect the plants if we've put them outside.

How well the plants survive those cold spells may depend a LOT on their root structure -- they'll do better if it's large and deeply planted. And for several years I've thought that seedlings need to be transplanted when the soil is the correct temperature, not totally dependent on the ambient temp.

Anyway, others will do as they wish, but I wonder if the seedlings just don't thrive if allowed to languish in cooler weather, the kind many of us would want to wear a sweater in!

My passively heated green house regularly went down to 0C last year. Daytime temperatures were in 15-30 C range. The established plants weren't happy, but they survived. I did lose some new (1-2 days) sprouts.

engineeredgarden, How did tmato plants survive down to 25* this winter in my cold frame I lost 3 out four lettuce plants when the temps outside dipped to 7* and inside the cold frame they were only 24*.

Was it 25* outside the shed? (warmer inside) otherwise you have some tough tomato plants.

Our thermostat in the greenhouse is set at 45. At 45 the heater comes on. Plants do fine because their soil cools more slowly, the fan keep the warmer air circulating, and various water heat sumps are around. You could easily incorporate various types of passive heat sumps into your hoop house to help provide protection. I don't know your zone so have no idea how cool it may still get.

But the real problem for us at this time of year is keeping it cool enough. Greenhouses/hoophouses build up heat VERY quickly, especially when the sun is out and the plants can quickly turn leggy or even easily be cooked if not monitored closely. Thus the need for an auto monitoring system unless you want to spend 24 hours a day in there. ;) Thermostat alarm is set at 65 and at that temp the vents open and the exhaust fans come on.

Dave

It depends on the amount of solutes in the water inside the plant, the more the solutes, the lower the temperature before ice crystals form that rupture cell walls. So, 32o F. is safe.

Some unusual tropical plants suddenly die at around 45 o F. but I don't know why. I suspect that precipitates other than ice form inside those plants.

Thanks to all for the input, but I see there is lots of differing opinions, which I did expect. What I see here is from 32F to 50F which is a real wide range. It looks like I will just try to keep the hoop house as warm as possible. I can provide some heat, but it would be costly and complicated do. On the other side I can open doors and windows to keep it cool without much trouble. I am in zone 7 and hope the freezing temps will be gone before it is a real problem.

I can only leave plants in cold frame if nigth low goes to 32F(not 20F, for example), for less than couple oh hours. If cold frame got up to 50F during the day, it will atlest stay above 40F over night. That is enough for safe survivl, to me. Tomaoes are close relative of potatoes. And potatoes can tolerate anything above frost.

Ever hear of "Wall-o-Water"? It is made just to keep tomato seedlings above 32o F..

Yes, the problem is I have 500 plants, and the Wall-o-Water idea would not work to well for me.

What about Christmas lights strung in with the plants? I've never tried it myself, but have heard those are good for protection from cold temps.

daylilyfanatic - It was 25 degrees in the propagation chamber, but the temp outside was 9. I realize that in theory they should've died, but those were some tough little plants.

EG

I don't recall the exact details too well but,
last year I had some plants get down to 18F.

They weren't exactly seedlings. 6-10" tall.
My estimate is they were in sub freeze for at least 4-6 hours. Many plants were crystalized frozen, not just cold.

1/3rd died outright. 1/3rd died a slow painful lingering death later on. the other third eventually recovered.
They recoved extremely slowly. 2 weeks to 2 months.
They lost all their leaves and 30+% of their tops.

A Mighty Few, about 6, seemed unaffected. They lost their leaves but quickly boiunced back.
I wish I could recall which ones they were.
Very foggy memories say the tough guys were Sunset's Red Horizon and Siberia.

In the end it would have been faster to replant
the frozen ones. But the few that did recover from freezing
lived on through the summer and did quite well.

It's funny i came on here to search/ask this same question. My maters are too big to keep inside, so I made a "cold frame" (and I use that term loosely, its basically an old window on supports). The temp outside is 42, temp inside the cold frame is 46. I'm hoping it doesn't go much lower tonight....These guys are about 8 inches high and all in 1 gallon pots if that makes a difference. I got nervous and went out and threw some Xmas lights in there, just in case it might help.

if your plants are in pots and forcast calls for below 40F, you can invert a bigger cardboard box on them in the evening. Put some weight on it so winds won't blow it away.
Usually the nightly lows last only couple of hours or so, not all nights. Just to give you a actual data, tomorrow night our low will be 29F. An hour by hour forecast is as follow:
Hour (temp-F)
2am(35),3am(34), 4am(33), 5am(31),6am(32), 7am(34), 8am(36)...
You can see that temperatures will be around freezing mark just for less than 2 hours. When lows forecast 25F, there will be about 3 hours within freezing range.

ONE MORE THING. At temperatures in the range of 25 to 35, it is not so much the thermometer reading but the winds that do most of cooling and damage. So covering by a box blocks the wind and cooling down under the box will bbe very slow.

Excellent idea! Thanks!