Overnight Temperature- How low do you go?

I do recognize that our growing climates are vastly different and

I see that many "over there" have greenhouses but lows are lows no matter where you live. Numbers don't lie. So when is it that most of you with greenhouses decide that it is okay not heat at night?

I'm trying to decide when I'm going to tempt the fates and move plants from the heated (kept to a minimum of 50f) to the unheated ones. Will the plants stand the shock of going from 50f down to 40f? Or if it goes even lower but not all the way down to freezing?

I've had my small greenhouses for quite some time now. One heated and two not. As of Tuesday, the weather guessers are saying the overnight lows won't go below 39f but you know how they lie sometimes. And every year I agonize over this decision. I spend so long growing my seedlings that I dread doing something stupid and killing them!

Has anyone got a tried and true personal rule for making this choice?

Thank you ​

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This is difficult because it pretty much depends on what you are growing, I have just one setting on my thermostat these days, and its about 4-5C (38-40F), and that is for overwintering tender, but established tender shrubs.
But I have a propagator that I use that plants can stay in for a week or two after germination, that is set at 75F.

Apart from that I can put any tender seedlings in my conservatory that rarely goes below 50F in mid winter and around now not below 60F.

But now I just use the greenhouse as its unlikely to go low enough over night for the heater to come on anyway.

 

@pete thank you for your reply.

I am mostly worried about tissue damage to the main bodies of these plants. Mostly what I would like to move into the unheated greenhouses are peppers because it is getting more and more difficult to get far enough into the heated one to fill the water receptacles.
Next year might not be such an issue because my hero, @rustyroots , came up with the collapsible staging idea.

The other reason I wanted to move the pepper plants into the unheated greenhouses is that their final pots are awaiting them there. There is not enough space for these pots in the heated greenhouse and I'm sure the peppers are getting pretty root bound. I'll water the empty pots today and that will give the wet soil enough time to heat up to ambient temperature no matter how chilly that is. The stuff coming from the hose is extremely cold right now so daytime ambient should be warmer.

I'm assuming that ideally then, 40f would be as low as I should go. I do know that it is also the same marker for when peppers will drop their blossoms. At this point blossoms don't matter to me. They'll make more.

 

Peppers, I would put them in my unheated greenhouse here, in fact I have seedlings in the part with the heater, but I dont think the heater is coming on anymore.
I go with the idea that at this time of year in order to get a cold night it has to be clear, so it will probably follow a sunny day and the greenhouse will have warmed everything up as the days are now longer than the nights.

I never water from a hose in the greenhouse, the water is far too cold at this time of the year, so if I dont have enough at greenhouse temperature I go indoors and mix some from the hot tap.

But having said that, I have water buts which I fill from the hose, and they are in full sun, so water from them is mostly always warmer than from a hose.

 

Thank you @pete. Yes, I've been using a temperature mixed watering can full in the greenhouse so far even though very few are directly water as I have most drawing from a capillary feed. I was trying to cheat and soak all of the pots pre: potting up, by using the hose as there are around 25 of them. Naughty Lori!

 

Always a problem at this time of year when your greehouse is busting with fast growing young plants and nowhere to go because the nights are still too cold.

Could you just put a small electric heater in your unheated greenhouse and set it on Frost so it uses the least electricity but should save the plants if it drops that low ?

Like @pete we have our main heater set at 4c and it keeps most things ok, but no idea about peppers as we do not grow them.

We use a couple of 40ltr trugs full of water and out of the way under the bench, they do seem partially effective as a heat sink, keeping things warmer at night, plus its warmer than out of the tap for watering.

 

@ricky101 electricity is deadly expensive over here so using even a small electrical heater would create a massive bill. My heater in the one greenhouse is natural gas from the main house supply. It was propane but I changed it out. I was told I'd blow myself up but it's been about 5 years now and I'm still here.

I have rain barrels but like the bird bath, the water freezes overnight or seems to get just as cold as the outside. They hold 50 gallons so maybe that is the problem. They never get warm enough during the day to have enough heat to retain through the night .

What I'm trying to figure out is the difference between when it is safe, "under glass" even though there is chance of frost out in the open. Since it seems a lot of you can use your greenhouses for most of the year, I thought maybe you had the magic number for temperatures sorted.

I realize that the temperature in the greenhouse pretty much drops at night in lock step with the outside but does the enclosure significantly reduce heavy frost from killing the plants.

I'm sure I sound thick but there's got to be a difference. Somehow

 

Maybe our cold is different to yours @CanadianLori , not meaning that as funny.
But you get a more intense cold and I remember you saying that frost stayed in the ground for a while after winter.
That kind of thing just doesn't happen here.
What are your day temps like at the moment, I know that in a month or two yours will be above ours but are you still cold during the day there.

 

I don't having any heating at all in mine Lori, hubs makes an inner frame and bubble wraps it , that good enough for our over wintering plants, we also have one large and one small cold frame too and they are fine for the remaining over wintering plants. I always have fleece to hand too which I can throw over any of the more vulnerable ones if temps get too low, but I rarely use it. We are way down on the south coast.

 

@pete we've had a unusually cold spring. We were only getting up to 37 and sometimes as high as 41. Now the switch is flipped- today it was at least 55f out there. I had to take my jacket off and work in my shirtsleeves (non plaid or red!) and I still felt warm. Total heaven! Our next few days will be even warmer. Just the nights I'm worried about.

I did fill up the empty pots and left them in the greenhouses to warm up and the ones that will be going in the heated greenhouse - once I get space in there - are also watered and left out in the sun. We're expecting rain tomorrow so they'll get a good rinse too.

I have raised beds for outside stuff because it takes a long time for the ground to thaw. And I may be good with an axe but I sure don't want to have to use a pick axe just to start gardening.

@Upsydaisy at what temperature do you fleece your greenhouse plants please?

 

To be honest Lori I am overprotective of my seedlings, mind you I have got tiny seedlings in the greenhouse now that shouldn't really be out there yet, it's really still too cold for them at night. It's down to 2C tonight

The teeny tiny 'shouldn't be out there' ones will be fleeced until they are bigger, no matter what the temps will be, I will just vary the number of fleece layers. The rest get fleeced at this time of year only when there is a big difference between day and night temps ....this week we've had day temps of 25C but night temps down to 2C
I feel ( rightly or wrongly) that such contrasts will be too much for the young plants to handle. So at the moment they have all got one layer of fleece over them.

My mature plants that are over wintered in the greenhouse are kept in the bubble wrapped frame unit until March and only get fleeced when temps drop to freezing or below.
All the bubble wrap has now been removed.

 

Very interesting thread. Forgive me please if my comments are a bit mixed up. Firstly night temperatures do tend to worry many gardeners. Perhaps a quick recap of what happens to plants during a typical day in it's life. Much of the plant's activity is governed by the sun and UV rays. So wherever you are, noon is usually the hottest time of day. So plants are employing probably their ultimate abilities. As the sun lowers and night falls. The plants cells and vascular systems are slowing down and to a certain point closing.

As a result plants are now less susceptible to heat changes. The dangers to night temperatures is very low temperature and moisture. This as we know brings frost. Frost will cause the cellular structure to abnormally expand and eventually split. This as we are all aware is fatal for plants.

So if your greenhouse is generally frost free and draught free during the night, all well and good.

I have used various forms of heating. Hot water piped (gas). Electric, fan heating. Paraffin oil heating, propane gas heating. Sooner or later the cost rise and often become too much. I have known many growers, who have in time become household names in the commercial plant world, using 'night lites' Those squat little candle lites. Placed here and there on the GH stagging. A 5-6inch clay pot upturned with a nite lite burning away below. It always worked.

Now thanks for the piccies. Much is written about greenhouse insulation and shading. The design of greenhouses. Most are all glass and metal frames. John, my friend. Wowee yo have a lot of ground there and glass everywhere. If I may, my friend. With glass going from the apex to the ground. A suggestion. Make use of internal insulation also knock up some kind of screening to protect at least four feet of the outside glass area. All in all. With respect to fellow gardeners. Perhaps it's time to review our practices.

Friends. Please continue to enjoy your gardening and to share the ups and downs with us all.

 

So, when it comes down to it, 4 or 5C (40f or so) is the magic number. Frost cannot land indoors at that temperature or ? I have solar fans running 24/7 in the heated and insulated one and every 5 minutes in the others. They are a combination of overhead DC and little "desk top" style fans also powered by solar or batteries fed from solar.

I just wanted a definitive temperature tolerance level for plants in an unheated greenhouse. Does frost happen in there at temps just above freezing or no?

 

I would not put peppers or tomatoes in greenhouse if it drops below 10c , I have had them in the past down to 4c and they survived but it sets them back weeks imo.
Regds Sandy