I chickened out...

[Mike]

I think there are a lot of factors that influence the effect of a minimum temperature on plants.
Soil moisture and air humidity are well known, but duration of exposure less often considered.
Just as we could withstand the temperarure of a walk-in freezer for a few minutes, but not a few hours, a brief exposure to an extremely low temperature will be less damaging than a long one. And for plants the soil may act as a temporary reservoir of some warmth.
So, I'll always try to ensure that the greenhouse temperature lifts a bit in the day after an extremely cold night. That's not often a problem when low temperatures coincide with clear skies - a little winter sun can work wonders - but recently here we've had both cold and cloudy conditions. So sometimes I turn the heater thermostat up a few degrees in the day, then back down at night to save money.
Sadly however the electricity went off altogether a few nights back and I recorded a minimum of -5.9C in the greenhouse. Some plants already look very sad indeed. Mike

[ralphrmartin]

Sorry to hear that Mike. I had an incident some years back when the (oil-fired) heater cut out on the coldest night of the year. It turned out it had been set up for the wrong grade of oil and overheated, and a safety cut-out tripped. Extreme conditions are ones that stress equipment most. Still, it was a one off problem, when I reset it and following nights were not so cold, and fortunately, nearly all the plants survived. Good luck with yours...

[edds]

I think there are a lot of factors that influence the effect of a minimum temperature on plants.
Soil moisture and air humidity are well known, but duration of exposure less often considered.
Just as we could withstand the temperature of a walk-in freezer for a few minutes, but not a few hours, a brief exposure to an extremely low temperature will be less damaging than a long one. And for plants the soil may act as a temporary reservoir of some warmth.

I agree. I think the forecast of prolonged minus temperatures and the awful placement of my current greenhouse triggered me into bringing all my cacti inside as I was sure things weren't going to creep over freezing for the whole period of time.

I'm still in a protracted planning for a new greenhouse and am thinking about what features I can add at the design stage to maximize solar gain and heat retention during these short winter days to try and ensure that it needs as little additional heating as possible to remain frost free and also, even if it does dip below, it soon rises above freezing again once daylight arrives.

I'm definitely adding two very large water reservoirs under the benches to store rainwater for watering and act as a heat sink. I'm planning to insulate under the floor slab and on the outside of the walls so that that mass of concrete acts as a thermal mass too.

I'm debating what to make the greenhouse out of for best visual appearance v thermal performance (multi-layer polycarb versus a modified 'traditional' greenhouse) and whether to paint all the concrete surface below bench height black for maximum solar gain. I think I might need some mats or something I can throw on the black floors in summer though!

[EdMcKenna]

I thought until now my tealights and terracotta pot in enclosures was working well. The downside of going to work in the dark and returning home in the dark means I only got a good look at everything yesterday after just under two weeks of sub zero temps.
I thought that I had kept the enclosure temperatures above zero but a minus 4 had been recorded. My two Aronium Schwartzkopf plants look wilted, I'm hoping the root ball and crowns are alive the rest, mainly cacti looked ok but I brought a couple of trays into the house as my confidence is shaken. I have noticed a few plants with rust and mould, mainly opuntias and some Fero's which produce nectar.
I calculated roughly 1.8 litres of water vapour produced over 10 days, (6 tealights per day for 10 days).The sub-zero temperature means I can't ventilate as I would like, a catch 22 situation. I went out today with the shower vac and tried to dry the bubble wrap, FYI I filled it 4 times
I'm just waiting now for the BBC promised 11 degrees to ventilate well.

[iann]

Supposed to be 14C and 90% humidity tomorrow morning, not sure how much condensation that's going to shift even with a nice breeze. Barely a glimpse of the sun though all week.

[iann]

I've been inspecting my plants now the weather is warmer. No obvious damage to any of the cacti. The ones exposed to the lowest temperatures are all very hardy. I thought maybe some of the ones in the hotbox might have suffered at -5C that first night, but they look OK so far. There might still be casualties; in previous years cacti that I've frozen have looked fine until spring.

The Lithops aren't so good. Some of them are almost certainly dead. Strangely not all the obvious suspects, but species that I consider to be the most hardy. More than one L. hookeri that have been through many winters look very sick. In some pots, some seedlings have gone soft and lost colour while others look fine. Maybe 5% of all the Lithops left outside could be dead. It will be interesting if there is any obvious pattern of less hardy species, but so far it looks very random. I might just have to keep them all above -5C and make sure the fleece is on.

[Aiko]

I also noticed some dead Lithops that have survived a few winters in the frost, like the -10 to -15C from February almost two years ago.
They are turning to mush as we speak.

The only casualties for me, it seems. All cacti and Aloinopsis and such are fine (visually).

This week the temperature went down to -9C for two nights and between -5 and -8C for bout five nights.

[Christian]

Interesting to read about all these experiences... Newspaper and fleece covering seems to have made some difference here - though I brought some plants into a frost-free outhouse just to be on the safe side. The Conophytum kept covered in the cold greenhouse look fine so far, but I seem to have lost Aloinopsis acuta from MSG seed which I assumed to be hardy.

[iann]

I find the warty-leaved Aloinopsis, which is most of them, are not as hardy as you might expect. A. schooneesii on the other hand is very cold-hardy; mine have been out in the greenhouse unprotected for many winters.

[habanerocat]

Sinking the greenhouse a couple of feet below ground, that is below the frost layer, is a technique used in several colder countries to keep the greenhouse cool in summer and frost free in winter. It certainly is worth looking into for anybody looking at a new build. Additional expense of course trying to keep it dry, but certainly would save on heating.