Low minimum winter GH temperatures

[fanaticactus]

Now that we're having an early blast of colder than normal air (frost & freeze warnings tonight), I'm hurrying to locate my individual cacti in the best places for a possible very early, very cold winter. Although I know we will one or two more isolated days of 70-degree weather (21-22C), I want to be sure most of them are already placed in the most optimal locations for the winter for the dormancy (in hopes of spring & summer flowers next year) without freezing either bodies or roots beyond salvage. I'm getting a new propane heater for the GH with a BTU rating of 30,000 to heat a 8x10 space with a peaked roof of 10 feet. Last winter I found I cannot rely on my valiant electric heater that couldn't even keep the temperature in the mid-20s (-4C).

When I read that a certain cactus is, for example, "hardy to -5C", I'm thinking that it's for in-ground established plants only, not potted plants in a GH. Is that correct? What would be the minimum temperature to maintain inside a GH that would help the cacti to go dormant safely and to improve the chances of a good floral display in the next growing season? Would 40F (5C) be a safe bet? Higher? Even down to 35F (2C)? I am aware that some cacti (e.g., Astrophytum, Disocactus, Epis) need a higher temperature of around 50F to winter over safely; I'll research those myself. Should I take the extra precaution this year of layering newspaper over them to help insulate a bit? Do the pots themselves need some insulation to protect the roots through the (relatively) thin sides of the pot? I am near the Canadian border and the sun does not get very high all winter, hence very little natural warmth even during the best of days.

[adetheproducer]

My greenhouse went down to about -10°c a couple of times last year before I got nervouse and brough them indoors to windowsills the only one that suffered was my matucana madisoniorum which I nearly killed. I do have a few more sensitive ones now so they will be coming in soon. I also have made insulation for the green house out of bubble wrap that worked last year with my old plastic covered frame g/h. I have done a better job this year and have small wooden frame with the dual layer polycabonate thats like thin double glazing so should already be warmer than in was in last years freeze.
Im bringing in my madisoniorum, uebelmannia and maybe a couple of gymnos and maybe some of my smaller plants which a still a bit young amd not yet hardened to the cold.

[iann]

You can't pick one temperature. Many cacti will be perfectly happy at 20F in a pot, some quite a lot colder. Others will be stone dead before they hit freezing. Being in the ground could be good, but any dampness is a risk. Under really cold snow is OK, but not many will endure freezing damp, ice, etc.

I let my greenhouse go down to more or less whatever temperature it likes, with the hardiest plants in it unprotected. Slightly less hardy ones go in an insulated frame which can be heated if necessary to maintain about 25F. Anything not frost hardy comes indoors.

[DaveW]

As Ian says rather than heat the whole greenhouse to higher temperatures some construct an enclosed frame heated with soil warming cable or something similar for the few that require higher winter heat. Simply building a mini greenhouse within the greenhouse and any warmth that escapes in winter helps heat the rest of the greenhouse. You can build them any size to suit your needs and as high as needed depending on how tall your tender plants are. Really just a heated "cold frame" within the greenhouse.

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