I've just become the owner of a greenhouse that was surplus to someone else's requirements and my intention is to overwinter my cacti in it. I'm in the north of England. I know there are certain species that prefer not to get too cold, Melocactus, for instance, so they'll be going into the spare bedroom, with a south facing window and heat of around 60F, in general.
On checking care instructions it tends to be the case that most cactus prefer not to get colder than say 50F and this includes quite a few species.
I don't have a large collection at the moment (the new greenhouse will allow me to expand) and I have them all outside in a frame that I put together. My intention is to move the frame into the new greenhouse and add other countermeasures against the cold.
So my question is, which species definitely DO NOT like to be cold, as against those that just prefer it but will cope quite well down to a few degrees above freezing. My plants include Echinopsis, Echinocereus, Thelocactus, Stenocactus, Neoporteria, and a bunch of Mamms and Gymnos.
Any feedback would be appreciated. I've attached a photo of the frame for reference. Thanks.
Overwintering in the UK
Overwintering in the UK
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"Once in a while you can get shown the light in the strangest of places if you look at it right"
Location: The sunny North of England!
Location: The sunny North of England!
Re: Overwintering in the UK
But maybe a few species, basically all cacti are happy with anything just above freezing for the long term and occasionally a bit of freezing every now and then at night. The main exceptions are Melocactus (although I believe they don't really need to be very warm too), Discocatus, Uebelmannia's and some other Brazilian species.
On the whole most cacti really don't mind temperatures up to -5C and many can also take an occasionally -10C if they are really dry and dehydrated. All my plants in a 30 square meters have to cope with a bit of frost in late autumn and early spring. I bring most inside in early winter, but they have to manage whatever comes by regarding temperatures at night before that time. But my plants are grown hard, and don't get any water after about now. So by the time it starts to really get cold, they have been dry for two to three months already. As the autumn provides a lot of sunlight still, and the greenhouse temperature can still get high, they have shrinked a lot in size. This is helpful for frost survival.
African succulents are much more tender than many cacti. So keep a good eye on the night temperatures for those succulents if you have them and better keep those at 0C the lowest. But on the whole, most cacti are very robust!
For safety of your plants and for your own reassurances and practicality, I recommend to keep them out until the nights start to become structurally cold and night frosts will become frequent (late November for me, usually). Then bring them in. Take them out as soon as the night frosts are frequently above freezing again (somewhere between midth and late March, at least for me).
On the whole most cacti really don't mind temperatures up to -5C and many can also take an occasionally -10C if they are really dry and dehydrated. All my plants in a 30 square meters have to cope with a bit of frost in late autumn and early spring. I bring most inside in early winter, but they have to manage whatever comes by regarding temperatures at night before that time. But my plants are grown hard, and don't get any water after about now. So by the time it starts to really get cold, they have been dry for two to three months already. As the autumn provides a lot of sunlight still, and the greenhouse temperature can still get high, they have shrinked a lot in size. This is helpful for frost survival.
African succulents are much more tender than many cacti. So keep a good eye on the night temperatures for those succulents if you have them and better keep those at 0C the lowest. But on the whole, most cacti are very robust!
For safety of your plants and for your own reassurances and practicality, I recommend to keep them out until the nights start to become structurally cold and night frosts will become frequent (late November for me, usually). Then bring them in. Take them out as soon as the night frosts are frequently above freezing again (somewhere between midth and late March, at least for me).
Re: Overwintering in the UK
You have to remember the smaller the greenhouse structure the quicker it heats up in the sun and the quicker it cools down at night. The greater internal volume the structure has the slower will be the temperature changes. Yours is small so if unheated I would think it would not remain more that a few degrees above outside temperature at night.
I can get many cacti through the winter in my unheated greenhouse if I cover them with newspaper on frosty nights. However if it is sunny during the day a lot of that heat is carried through the night in larger greenhouses. Many cacti will stand below 50F overnight and some greenhouses go down to 40F. The advantage of course is a smaller structure like yours needs less artificial heating and costs less to run.
My greenhouse goes below freezing most years with the plants dried out and covered over, but some may mark and the tender ones are brought into the house over winter. Also they will stand dry cold better than damp cold and the UK is a damp cold country in winter with a "freeze thaw, freeze thaw" type climate our plants are not keen on.
See:-
https://cactusnursery.co.uk/temp.htm
https://forum.bcss.org.uk/viewtopic.php?t=167445
http://www.succulentaddict.co.uk/overwi ... in-the-uk/
I can get many cacti through the winter in my unheated greenhouse if I cover them with newspaper on frosty nights. However if it is sunny during the day a lot of that heat is carried through the night in larger greenhouses. Many cacti will stand below 50F overnight and some greenhouses go down to 40F. The advantage of course is a smaller structure like yours needs less artificial heating and costs less to run.
My greenhouse goes below freezing most years with the plants dried out and covered over, but some may mark and the tender ones are brought into the house over winter. Also they will stand dry cold better than damp cold and the UK is a damp cold country in winter with a "freeze thaw, freeze thaw" type climate our plants are not keen on.
See:-
https://cactusnursery.co.uk/temp.htm
https://forum.bcss.org.uk/viewtopic.php?t=167445
http://www.succulentaddict.co.uk/overwi ... in-the-uk/
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Re: Overwintering in the UK
Unfortunately you need to be quite specific - many genera contain some species which will be perfectly happy in typical UK winter temperatures if completely dry, and others which won't!
Generalisations such as "Mammillarias are OK with -5deg C" or "Mammillarias need +5dec C" don't reflect that some are more cold tolerant than others.
For example many Echinocereus are frost tolerant, but I expect there are some from Baja California for example that will be unhappy or worse if not kept warmer.
Generalisations such as "Mammillarias are OK with -5deg C" or "Mammillarias need +5dec C" don't reflect that some are more cold tolerant than others.
For example many Echinocereus are frost tolerant, but I expect there are some from Baja California for example that will be unhappy or worse if not kept warmer.
Re: Overwintering in the UK
Do you intend to cover the front of that installation? Because if it stays open rains in coming months will keep moist many of those pots and their hardiness will be weaker for coldest time. If you have space bring inside more plants or at least more valuable ones. Not sure what are the minimums in your region but if temps fall below -5 C you can see damages on many of those Parodia, Astrophytums, maybe Mammillaria, Ferocactus and this depends on the species.
Other thing: even if the species is considered more or less hardy if it's grown in a heated GH and not trained for outdoors from early age it can easily perish in winter from cold.
Other thing: even if the species is considered more or less hardy if it's grown in a heated GH and not trained for outdoors from early age it can easily perish in winter from cold.
If your cacti mess in your job just forget about the job.
°C = (°F - 32)/1.8
°C = (°F - 32)/1.8
Re: Overwintering in the UK
Hi 7george, a slight misunderstanding it seems to me. The picture is a frame I put together to house my plants during the summer. It has a perspex screen that goes on each evening for a bit of overnight protection.
I'm intending to move the whole thing into my newly acquired 8' x 6' greenhouse. I imagine I'll leave the front uncovered to allow air to circulate and put some extra protection on the sides and back.
I don't think I'll be able to heat the greenhouse because of the difficulty in running a cable (it will be about 50m from the house) so I will follow DaveW's advice and protect with newspaper and keep an eye open for frost warnings and act accordingly.
I'm intending to move the whole thing into my newly acquired 8' x 6' greenhouse. I imagine I'll leave the front uncovered to allow air to circulate and put some extra protection on the sides and back.
I don't think I'll be able to heat the greenhouse because of the difficulty in running a cable (it will be about 50m from the house) so I will follow DaveW's advice and protect with newspaper and keep an eye open for frost warnings and act accordingly.
"Once in a while you can get shown the light in the strangest of places if you look at it right"
Location: The sunny North of England!
Location: The sunny North of England!
Re: Overwintering in the UK
OK, I see it now. If the frost is occasional and with the careful approach it may work fine. I'm generally not very concerned about non-gentle and non-tropical plants if the cold is up to -3 C especially if I have several plants some of which are taken indoors. In any case you will provide more curate data about those cacti hardiness maybe next spring. 
If your cacti mess in your job just forget about the job.
°C = (°F - 32)/1.8
°C = (°F - 32)/1.8
Re: Overwintering in the UK
Kill or cure! 
Thanks, everyone, for your inpur. Much appreciated.
Thanks, everyone, for your inpur. Much appreciated.
"Once in a while you can get shown the light in the strangest of places if you look at it right"
Location: The sunny North of England!
Location: The sunny North of England!
Re: Overwintering in the UK
We used to use paraffin heating when I started and paraffin was "sold on draft", you took your can along to the hardware shop for a refill. Now it is sold in tiny containers in the UK like paint thinners and is about as expensive so too dear to use in anything other than a very small greenhouse.
I looked it up and UK paraffin is now almost = £8 a UK gallon and the last I bought years ago when I stopped using it was the equivalent of 12.5p a gallon! Most now heat their greenhouses in the UK with either electric or gas.
I looked it up and UK paraffin is now almost = £8 a UK gallon and the last I bought years ago when I stopped using it was the equivalent of 12.5p a gallon! Most now heat their greenhouses in the UK with either electric or gas.