Questions about greenhouse minimum temperatures

I am somewhat confused about greenhouse minimum temperatures and what that equates to as far as plant growth.
I understand that bananas (just an example) need a minimum of 57 degrees but what does that really mean? I know that it will not die below 57 degrees but will freeze and die back below 32 degrees. So does this mean that it will simply not grow at temps below 57 degrees, if this temperature is kept at that constant and not below freezing?
My confusion is that if the plant is subjected to higher temps during the day and fall below the minimum temp at night, then what happens? Does the plant grow during the warmer day temps? Would it go dormant when temps are spiking from 50 degree nights to 90-degree greenhouse day temps?
My concern is spiking temperatures between day and night temps or is this not a concern?
Here is a link to my average temperatures. Except for a few nights during winter, I could grow tropicals outside.
http://www.idcide.com/weather/la/new-iberia.htm
Oldude

I've found my tropicals do best when I keep the nighttime minimum temps in the 55-60 range. I had a couple super cold nights last winter where temps dipped below 40, and everything was fine with a few hours at the colder temperatures, but I definitely wouldn't want every night getting down to those temps. Daytime temps are anywhere between 70 and 95-ish depending on how sunny it is and I've never had that cause problems (except in a couple cases where I missed a few plants when I was watering!) A lot of plants slow down growth in the winter based on day length rather than temperature. And keep in mind too that in the tropical climates these plants are native to, they would probably see warm daytime temps, then nighttime lows around 50 or so, so for many of them you're probably not too far off from the temps they would experience in their natural environment during the winter.

I don't think this answers the question it just confirms our current knowledge !!

the book "bananas you can grow" by Stokes tropicals publishing company
page 21
Greenhouse
The easiest way to grow bananas in cold climates is in a warm greenhouse. The warm, moist conditions in a heated greenhouse are ideal for bananas. Provided that there is enough day length, bananas will fruit regularly in places as far north as Iceland.
- - - -
Temperatures should be kept between 57 degrees and 95 degrees

The plants should not be exposed to temperatures less than 57 degrees or growth will stop

Setting the minimum temperature in your greenhouse depends on what you are growing, whether you want the plants to continue to grow and whether you are willing to pay the heating bill. My DH is not a gardener, has a heart attack every time I get the propane tanks filled and has to be watched to make sure he doesn't lower the thermostat. Our nighttime temperatures can get down to 23ºF. The daytime temperatures can be in the mid - 80s. Temperature differences between day and night can be over 40º. As a compromise, I set the thermostat at 45ºF. At that temperature most plants will slow down, some will slow way down. My bananas slow down but don't go dormant. Last year, my DH lowered the thermostat down to 40º. At that temperature, the citrus trees refused to bloom, some Brugmansias and my pink banana died. The other banana trees survived, but sulked until I discovered the problem. Many of my Brugmansias have about 2 flushes while overwintering in the greenhouse.

There is a difference between the temp at which growth stops and the temp at which the plant is immediately killed. Growth may stop for bananas at 57F, for citrus at 54F. It is at this temperature that the roots will no longer take up nutrients and water...some people have called it "absolute zero." This isn't an immediate problem for the plant. The temp swings are the problem, and they are particularly a problem for container plants.

Lets say it's 3pm and you have a nice warm GH and the soil temp is 80F and the GH is 84F. The leaves might even be 90-100F, but the roots are taking up water, the transpiration stream is working, and the plant, roots and leaves, are in balance. The GH cools off at night, perhaps rapidly, the "demand" from the top of the plant is reduced, but the soil is still warm from the day, so the roots are working. Then by morning the soil is 55F, the GH air is 55F. Everything is still in balance, since the top of the plant doesn't require much water and the roots aren't working well to provide it anyway. Then 2 hours later, you have a problem. The GH has warmed to 75F, the leaf temp may be 80F, but the soil temp has only come up to 57F. The top of the plant demands water, but the roots aren't warm enough to provide it. The leaves wilt because of transpirational losses which cannot be replenished by the roots. In citrus, this is "Winter Leaf Drop," but I see it happening with other plants to some extent as well. Plants are particularly prone to overwatering in this period as well.

The plant can grow during the winter, but it depends on all of the factors discussed above...day length, GH temps, root zone temps, humidity, and other plant-specific factors.

HTH

SB

Thanks
I think that I now understand what large fluctuations in temps will do to my plants. I had thought that I would also grow tomatoes but with heating cost these days it would be expensive to maintain 65 degrees even where I live.
Thanks again Oldude
.

stressbaby..... thanks for sharing this information. I too wondered the same thing.
Just to go one step further ... What would happen if one just wanted to maintain the plants rather than expect growth. Lets say as an example the citrus or even the others you have seen, IF "Winter Leaf Drop," occured would the plant maintain itself as sort of semi dormant until spring arrived to give consistant warmer night temps or hubby left his finger off the thermostat:)? I have a vision of the citrus plant having bare branches as the leaves have fallen. Would it just be semi dormant and if about February if the temp was consistantly up would it put out new leaves and bloom in the spring? Is this asking to much or does it happen?

Bea

Bea, I'll ask for others to chime in, that is a good question. In the case of citrus winter leaf drop, the plant is sort of self-pruning. You generally won't lose all the leaves, just enough to restore the balance between the top and bottom of the plant. I think that the leaves that remain are photosynthetically active, but at a reduced rate given the day lengths and temps. In the cases involving citrus where I have had significant WLD, the plant has usually (but not always) bounced back in the spring.

SB

I don't have a lot of citrus-an orange and a Myers lemon, but last winter they both dropped their leaves-I just figured they were going semi-dormant and in the spring they flushed out new leaves and are fine. In fact the orange has about 12 oranges on it right now. Thinking back-I could have let them dry out a tad bit too much and they dropped their leaves because of that. Not uncommon for me!

Bea
I can‘t speak for greenhouse citrus but my in ground citrus will have leaf drop when the temps get below freezing. Last year we had two freezes with temperatures at 23-25 degrees and I saw a lot of leaf drop on the Hamlin oranges. In fact they fit your description of bare branches. The Satsumas had some leaf drop but not nearly as severe as the Hamlin’s. This spring they bloomed and flourished with very good crops on all of the citrus.
Citrus can take colder weather later in the winter than they could in early winter ,before they acclimate to colder temps. If kept at an artificially higher temp in the greenhouse and then suddenly exposed to cold temps then I would think that the damage would be much more severe.
Oldude

Thanks everyone!
I have a Myers lemon and a lime tree in the greenhouse and it looks like they will be ok. Now I just have to find out which is the best tasting dwarfish orange tree:) I thought they only grew in Florida but now I am going to give them a try.

Bea

Bea
I have two owari satsumas “carizo root stock” in 25 gallon containers. You can control the size very easy in containers since this will naturally dwarf the tree and citrus respond well to pruning. The trees are in their second year and I just harvested 65 fruit from the two. If you want dwarf citrus then ask for these root stocks. Semi-dwarf is regular trifoliate and ultra-dwarf is flying dragon rootstocks. The Carizo rootstock is not as cold hardy but produces higher growth rates.
Oldude

My overwintering citrus have never suffered from winter leaf drop. They grow at a slower rate, are in bloom sometime in February and set fruit while still in the greenhouse. Stressbaby, you have brought up something I hadn't really considered because it has never happened. The plants are in large pots so the temperature shouldn't fluctuate as much. Now, I'm curious. Are there any soil thermometers that transmit readings to a base unit? Reasonably priced. Maybe a temperature sensor placed in a baggie and buried would work.

I think that citrus are not the easiest northern GH plant...they can be temperamental. Betty, I wonder whether you may see less WLD with larger containers or in more favorable climate (ie, less total container/root time at or below 54F).

I haven't seen remote soil thermometers, but I am sure they are out there.

Mommum, look at Four Winds or Brite Leaf if you are interested in good mail-order citrus. Stan Mackenzie has some good cold hardy citrus too, I think.

Thanks Stressbaby!

I am heading there now

Bea