Lowest Temperature Tomato Plants Can Withstand In Greenhouse

soulreaverMarch 26, 2012

I have a dilemma this year. I have a small business and I grow a few thousand plants in my greenhouse a year to sell.

My propane heater is not up and running this year so I have resorted to using an electric heater. I have an airtight greenhouse that is 10x20x8 that has greenhouse plastic on it.

I need to start growing my plants in the next week or so but I am worried about temperatures. I can keep the greenhouse at about 45-50 if the temp drops to the low 20s other than that the temp will stay in the 50s at night.

When I first started growing plants 5 years ago in an unheated greenhouse my tomato seedlings in flats survived in the greenhouse unheated to temps in the low 40s without any damage. If I start to grow flats of tomatoes now, will they survive if we get a fluke night in the 20s and the greenhouse temp is a steady 45-50? The greenhouse I use has no drafts it is air tight but I am investing hundreds of man hours in seeding plants and if I lost all my plants I would be in big trouble.

I know that plants outside cannot withstand the same low temps as protected plants. I have with my own experience seen my tomato plants take temps in the 40s when I was just a hobby grower with no real damage and I have read in these forums that people have had no problem with tomato plants surviving 40 degree nights when protected from wind in a greenhouse. The day time temps are always in the upper 80s to low 90s without any heat now unless it is cloudy outside. Even then the temp doesn't drop below 65-70.

I am just looking for advice from anyone that has experience with greenhouse tomato plants and dealing with temps in the 40-50 degree range. This isn't something I expect to happen everyday, it might occur 3-4 more times between now and May, I am just looking for advice about this situation, thanks.

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timmy1(6a ri)

They will be fine with 40-50 now and then...I like to run mine about 64. This keeps them growing.

So, the answer is Yes. As long as they don't freeze.

Is it the textbook right way to do it? No. You should have adequate BTU's to maintain 64 with whatever mother nature throws your way.

    Bookmark   March 26, 2012 at 10:19PM
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digdirt2(6b-7a No.Cent. AR HZ8 Sun-35)

45 is the usual recommendation for minimum allowed as long as it isn't for a long period of time. 50 would be better but I know the problems you face myself and I can live with 45. Any lower than that and I start covering them.

One thing that does help IF you have the room is to tent over the plants with a second layer of plastic (not touching) - a tent inside the greenhouse - and stick a couple of incandescent lights inside the tent. Good for an additional 20 degrees or so on the really cold nites.


    Bookmark   March 26, 2012 at 10:20PM
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poppa(z5 MA)

I'll let you know tomorrow... Temps outside epected could be between 19-24F. I have no heat or power in my GH (22' x 50') I've covered my flats with an extra sheet of plastic after placing them on the ground. I am hoping the ground will release heat to the plastic. I am thinking that if i kept them on the benches there wouldn't be any residual heat to help them. It was sunny today so i kept it shut hoping that would help the soil temps to hold a bit more heat. got quite warm in there! Maybe a bit too warm...

I also have a 1500 gallon open water tank that i use for some fish and i floated some styrofoam on top and set as many flats of marigolds in there as i could. Covered the tank with styrofoam. The last time the temps dropped to the high teens i lost about 80% of my marigold seedlings.

Hope i can sleep tonight!


    Bookmark   March 26, 2012 at 10:25PM
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mytime(3/4 Alaska)

About May 1 I get tired of dragging all my plants in and out of the garage every day, so move them down to the greenhouse. For the next 30 days, I feel fortunate to keep the temp above 40 every night. My tomato plants suffer no ill effects.

    Bookmark   March 26, 2012 at 11:13PM
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Mytime, snowmobile trailer, roll em in and out... just a thought.

    Bookmark   March 27, 2012 at 1:05AM
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The tomatoes should survive 32F. I use 600 litres of water to moderate tempertures in my unheated greenhouse and the only effect is to prevent below 32F temps. Ironically, the only tomatoes I have lost were sub-arctic-plentys. They aren't happy but they live. They do better if they are bigger. I'm starting mine indoors and putting them in the greenhouse at the first potting up. Using the dense planting approach I have 64 varieties planted and ~150 growing plants in two trays. The two trays could handle 300 plants; but I only have room for 50 in the garden. :)

I'm going to cheat and supplement with an electric heater.

    Bookmark   March 27, 2012 at 10:54AM
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poppa(z5 MA)

So the overnight temps reached 23F last night. No damage to any of my tomatoes (big boy, celebrity, Super sweet 100's). Actually no damage to any plants in my unheated GH. I left some trays of marigilds out and they all were fine. Whew!

I did have some asparagus spears coming up in the garden which froze.

    Bookmark   March 27, 2012 at 12:09PM
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Thanks for the posts guys. Poppa glad to hear your stuff survived. I am in Rhode Island not far from you and timmy. Had the same frost last night and ran a 1500 watt heater all night and tested temps throughout. It kept the temp 10 degrees warmer when it was 35 out but once the temp dipped to 28 it just couldn't sustain the temp more than 3 degrees at 31.

So poppa would you assume that it appears that plants like tomatoes suffer wind chill similarly to human beings? It seems like a plant that is not stuck in cold plus wind can survive just the cold if protected.

    Bookmark   March 27, 2012 at 12:49PM
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mytime(3/4 Alaska)

Jon, the snowmobile (or as we call them up here...snowmachine) trailer is a great idea, except that ours is too large for the garage. I do use garden carts, the grandkids wagon, and shelving on wheels, but even that reaches a point where it's just too much as the plants get potted up in size.

    Bookmark   March 27, 2012 at 12:56PM
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poppa(z5 MA)

soulreaver: While i don't have heat or power i do have a few things that work in my favor.

First off, my gh is pretty good size, 22 x 50 inside. The size helps to moderate temps.

Second, i have a good size tank in there, 1500 gallons, and that has also helped to moderate temps. Prior to installing the tank (Thirdly, i have built a greenhouse within a greenhouse. I have 2 foot dead air space on all sides and 3 feet in the roof. My goal was to build a "Solaroof" design where the dead air space is filled with soap bubbles at night to act as insulation. I have not really needed it as i grow (not just harvest) a number of cool weather crops in there all winter long. Tomatoes and potatoes need a couple of more degrees and if i decide i want to grow them i will need to add the insulation. As it is my tomatoes will bear fruit until December when temps fall below freezing. I start my tomatoes in march and have only a few scares. It does get quite damp and i would likely have molding issue with tomatoes anyway. It literallyr rains inside when the temps drop outside.

Last year I actually had a citurs tree survive all winter in there, though it did die back to the roots. Again, if i wanted oranges in winter i may need to add the foam insulation, but i'd have to run power out there and i have no plans to do so quite yet.


    Bookmark   March 27, 2012 at 3:09PM
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Poppa I only have 1 lay of insulation and it is a 500 dollar greenhouse plastic 6 mil. But my temps are definitely 5-7 degrees higher than your temps on most nights and I have a red brick foundation on the greenhouse which absorbs quite a bit of heat during the day so I hope those two things combined with the fact that I will have 2 1500 watt heaters running on the colder nights would be enough to keep my greenhouse above 45 from this point on.

I am also wondering if once I have 150 flats in the greenhouse all watered during the heat of the day if the soilless potting mix of peat and vermiculite soaked with water in in the 150 flats will contribute to thermal warming during the nite.

Also I have 6 105 watt heating mats which will be used for seed germination and those will be running during the night as well. All these little heat sources, the mats, the 2 1500 watt heaters, the brick foundation, the 150 flats you think this will be enough to combat a 25 degree night here and there? I know there are alot of variables that I am putting fourth but like you just posted I have all these little things that might make a difference going on in my greenhouse too.

    Bookmark   March 27, 2012 at 4:20PM
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timmy1(6a ri)

If your greenhouse is 22x50', I would just get a thermostatically controlled kerosene bullet heater in there for the cold nights.

    Bookmark   March 27, 2012 at 10:11PM
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I have a 7x10' x7' tall greenhouse that is built against the house, I live South of Boston MA
I actually dug down into the ground, so the front is only 5' out of the ground
It faces South, no heat....it has a "cement" foundation maybe 12" deep
I also have a 55g drum of water that stays in the GH to help moderate temps
I dug down inside maybe 2' & put some rigid foam around the outside perimeter
Usually the end of Feb is the last time it dips below freezing
I put 3 TM plants in as a test, I wanted to see what would happen
March 16th Temp dipped to 31.1, 2 of the 3 died
March 22nd Temp dipped to 31.6, 3rd plant still alive
I'm past the last freeze now & just moved 60 plants from my sm indoor greenhouse to the outside greenhouse
I bury the 3-4" pots in the soil...Daytime temps go up to 55-65
Today it hit 74 with a nice sunny warm day

    Bookmark   April 1, 2014 at 8:03PM
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seysonn(8a WA/HZ 1)

Tomatoes can survive temperatures 34F and higher. I have had tomatoes planted that stayed at 38F low without protection.

Remember that when they say low will be, eg, 38F it is not going to be all night long. Most Often the forecasted low will last no more than 2 hours. If the day before the high was 55F and the following day will be also 55F, then the lowest between those highs will last just couple of hours. BUT when they issue a frost warning, that is different. In that case a cold freezing front will be moving in with high winds that will cool off the air and ground much faster and will last longer too.

    Bookmark   April 1, 2014 at 8:30PM
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Well, the thing to remember also, they might survive but you are not doing your tomatoes any favors if they are sitting in cold trays. There has to be some middle ground to provide not only survival but growth and minimizing stress of dropping to low temps is something to remember. I have a hoophouse, large tarp like plastic dome, it is a bit of a game. If I only have occasional dip into nights low I can get away with wrapping electrical blankets over shelves for the night. If I have days and nights on low side for prolonged times there is no reason to stress seedlings so much.

    Bookmark   April 3, 2014 at 12:19PM
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