What temperature is "frost free"?

[Ian in Cumbria]

Hello All

Some of you may remember me posting as a greenhouse newbie a short while ago. (I don't mean I'm not a newbie any more! I certainly am). I know that scientifically "frost free" means above 0C but how much above 0C should I aim at? I have over-wintering pelargoniums (both established plants and rooted cuttings), some potatoes which are just showing, carrots, lettuce and spring onion which I'm hoping for an early crop from (not a winter crop) and some sweet peas which I have just pinched out. I've got a fan heater with thermostat and have lined the whole greenhouse with bubble wrap. It gets really good light all day and the auto vents are still operating when the sun shines, keeping the temperature down to around 20C max. I am happy to just keep the plants "ticking over" and wait for longer days to get them going again. I'm also looking to minimise my heating cost. So what's the balance? 2C? or can I let it go down to 0C?

Regards

Ian

[oldherbaceous]

Evening Ian, not sure if others would agree but, if you have things already growing, i personally wouldn't let it drop below 2C, i would also have fleece ready for very frosty nights.

You will soon know if it's getting to cold in the greenhouse, as the pelargonium leaves will turn white.

[Geoff]

The biggest problem in trying to set a minimum temperature is the quality of the thermostat. If you set it let's say at 5dC that doesn't mean that is the minimum you will get. As it cools down it will switch on at 5, 4, 3 or whatever degrees then as it warms up it will switch off at 5, 6, 7 or whatever, and it is not always consistent. You have to find by trial and error what the setting on the device is that gives you the minimum temperature you select. Aiming for 2 is probably a bit risky, especially at first, I would go for nearer 5 then perhaps lower it a little when you are more confident about the performance.
A problem with basic thermostats built into heaters is that the gap between the switch on and switch off can be quite high, this wastes energy as your greenhouse spends more time hotter than you want it. Also the thermostat is where the heater is rather than where the plants are. The way round this is to use a good quality rodstat with a stated accuracy that can be placed in the most appropriate position. Regrettably they are expensive and I am not sure if they payback but they do last, I've had mine many years and it has seen off several heaters.

[oldherbaceous]

Morning Geoff, what very good infomation you have provided there.

[Ian in Cumbria]

Thanks for the comprehensive replies. I'll have a look at a rodstat. Meanwhile I'll continue to try to "calibrate" my heater. Last night it went down to -2C outside but the greenhouse temperature went between 2C and 4C. When I set the max/min it was just about 3C so I think the heater must have kicked in at least once. The thermometer in in amongst the pots so should give a reasonable indication of what the plants are seeing. There's no doubt at all that near the heater will be more variable, particularly in front of it (it is a fan heater) so I've kept this area clear. Thanks again, the replies have given me more confidence that I'm in the right area.

Regards

Ian

[Ian in Cumbria]

Geoff

I see what you mean about cost of rod stats. I think I'll persevere with what I've got for now. Mind you a stat that works both to switch a heater on in winter and a fan in summer does sound a good idea.

Regards

Ian

[Monika]

Ian, I started by using a fan heater (and still use it when it is unseasonably cold in spring when the greenhouse is full of seedlings and young plants) but, to keep the greenhouse just from freezing, we bought an oil-filled electric heater with a thermo-switch. The heater stands below the most vulnerable plants and the heat just drifts up between the slats. The thermo-switch lies about three feet to the side and is turned to 5 degrees C, so the most vulnerable plants probably get about 8 degrees, the hardier ones about 5 degrees and spanning out from there, the plants near the glass probably would not survive a hard frost if they are not hardy.

Having said all that, this would only apply from about February onward because I no longer try to keep tender or half hardy plants over the whole of winter. Having realised that our winter electricity bill always shot up alarmingly when the greenhouse was only slightly heated, I calculated that it is considerably cheaper to buy seedlings of, say, pelargoniums or buy one plants and take several cuttings, than to keep last year's plants!

Supposedly tender plants like lemon verbena and French tarragon survive fine in our unheated greenhouse over winter with a bit of bubble plastic over them, when needed.

[PLUMPUDDING]

I haven't used any heat in the greenhouse yet and even though the air temperature in there has dropped to -2 and -3C on three or four occasions recently potatoes under fleece and lots of salad things, pak choi and chinese kale, and parsley all look OK. Because really cold weather is forecast I've also put a few sheets of newspaper over the fleece tonight, which I'll take off in the morning.

Don't copy me, as I'm just seeing just how much they can stand, particularly the potatoes without turning black! I've earthed them up a bit and am hoping there is enough warmth in the soil to protect them, but I won't be surprised if they are black in the morning.

[Ian in Cumbria]

Hello All

Thanks for all the replies. Lots of sense there. My experience over the last few days have borne out a lot of it. The only thermostat I have is the one built into the fan heater. The main problem is that when the fan switches on, it gets hot faster than the air in the greenhouse and turns off when the air temperature has only just risen a little. Having said that, with outside overnight temperatures down to about -4C, the range in the greenhouse overnight has been from +1C to +5C. This is with the thermostat turned well down. Nothing in there seems to be suffering, in fact the winter lettuce is growing quite well. It's been sunny during the day so temperatures then around 20C with vents opening. Not today though - snowing just now.

Regards

Ian

[Johnboy]

Hi Ian,
If I may be permitted to add a further comment. Temperature and frost are two different things where greenhouses or polytunnels are concerned.
If I may explain: Many plants will survive a low temperature below freezing just so long as there is no frost contact. Frost contact is what will actually do your plants the damage not necessarily the temperature itself.
You will note Plumpudding's comments about minus temperatures and her plants not suffering in the Greenhouse well that kind of bares-out what I am trying to convey. I do hope this has helped you and not confused you.
JB.

[Ian in Cumbria]

Hello JB

I've thought about your reply and am not sure what you mean by "frost contact" unless you mean condensation freezing on the foliage or the soil freezing around the roots. I thought the damage to foliage would be caused by the liquid in the plant cells freezing and damaging the cells. I can understand that the liquid in the cells might have a freezing point less than 0C because of dissolved compounds. Is this what you mean? Or am I on the wrong track completely?

Regards

Ian

[Johnboy]

Hi Ian,
An exceedingly good point to raise.
Let me try and explain what I mean.
With foliage that is in direct contact with frost eg, in an open situation, leaf tissue will be damage in a tender plant.
In a greenhouse or tunnel you do not get this direct contact although the temperature approaching the outside temperature it will normally slightly higher but in tunnels it can be actually lower than outside. (You will have to take my word on that as it is too difficult for me to put into words.)
Frost contact will destroy leaf tissue but a similar temperature without frost contact may not. So frost free to me is no physical contact between frost and the plant. You must of course be aware of the temperature range of the plant you are trying to grow.
I hope this has not further confused you.
JB.

[Ian in Cumbria]

Hi JB

I think I'm with you now. I've bought a plug-in thermostat which now gets over the problem with the thermostat in the heater. Over the past few days the overnight temperature in the GH has varied between 1C and 3C which is much better than trying to rely on the thermostat in the heater. This is with the plug-in thermostat set to 3C. As I said at the beginning, I'm not looking to provide enough heat for things to grow much (I can't afford that and I suspect I'd struggle with light as well), I'm just wanting to keep things alive so they get a good start next year. Thanks for the help, I'll no doubt be back with more questions in due course.

Thanks again

Ian

[PLUMPUDDING]

Just thought I'd mention that my potato tops are now soggy and black. They survived -3C OK under their covers, but -12C was a bit too much! I've not looked if they've produced any new potatoes yet. Ever the optimist I thought they might just send some new shoots up if there is anything still in tact underground and it got a bit warmer, but it doesn't sound very likely.