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Due to lack of space indoors, some of our plants and mini-trees here in Switzerland will have to survive the winter on the balcony. My girlfriend is set on wrapping the trees in bubble wrap, covering the top-soil with some fleecy material, and buying an isolating tent (advertised as a 'cold greenhouse'). I understand that all these methods would be viable ways to keep heat trapped for a little bit longer. But do plants produce enough heat for such methods to actually work? I have my doubts.

I would imagine that a cold greenhouse, without internal heat supply, will within a day also drop to the average outside temperature, which these days seems to be about 4° C around here. My main question is: how do the cold protection measures really work, as I can't imagine they actually keep a plant warm?

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  • What varieties of plants do you want to over-winter? – Jurp Nov 26 '20 at 15:59
  • The cold greenhouse would be for a number of succulents (including a sizeable aloe vera), while the other measures (e.g., wrapping in bubble-wrap, putting something on the top-soil) would be for the evergreen mini-trees (say, 1.5 m high) for which I don't know the name. – Erik Nov 26 '20 at 16:29
  • Where in Switzerland are you? I see that minimum temperatures there range from -29 C to -9 C. No aloe can live outside at -20 C, so your location matters for how we answer your question. – Jurp Nov 26 '20 at 17:01
  • In the Zürich region, so it doesn't get that bad! – Erik Nov 26 '20 at 18:25
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A balcony with pots is going to be very difficult to protect from cold. I think the primary aim is to delay cold penetration. So ,when the day temperature is + 10 C , and the night is - 5 C , your protection may hopefully keep the soil temperature above 0 C during one night until the next day. ( The specific temperatures are arbitrary depending on what the plants are). If the day is warm ,you may want to remove the insulation to let the soil warm as much as possible.

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According to the US Dept of Agriculture, Aloe vera does not do well in temperatures below +7 C, so given that you're in an area with minimum low temperatures from -10 to -12 deg C, the Aloe vera will die over winter unless it's in a heated greenhouse. If you do try to over-winter it in the cool greenhouse, I recommend that you take some "pups" off of the mother plant, pot them up, and keep them inside for the winter. This will give you insurance in case/when the mother plant dies due to the cold.

As for the rest of the succulents, unless they're used to the cold temperatures that you'll be experiencing, they stand a good chance of dying as well.

Assuming that the evergreens are not tropical species (like Norfolk Island Pine), then they will probably be fine outside on the balcony. The big thing you'll have to combat is frost heave due to the warming and cooling of the pots, especially during sunny days. It would be best if you can:

  1. Allow the pots to freeze solid. This may be tricky, given your climate
  2. If frozen, mulch the heck out of them with straw. The intent is to keep the pots frozen through the winter so that they do not experience frost-heaving, which has the potential to break the roots and kill the plants.
  3. Remove the mulch in spring.

If you can't get the pots to freeze solid, then you'll have to come up with a way to prevent them from warming too much, especially on sunny days. Perhaps a shade screen? Burlap/muslim covering of the plants and pots? I don't know that the bubble wrap by itself is a good idea, because it is clear and will allow sunshine to heat the plants/soil. Perhaps use the bubble wrap underneath the burlap/muslim wraps? In any case, water the evergreens well now and when the weather warms in spring. Try to keep the soil as moist as possible, as any winter warmth may cause the roots to use the water in the pots, and if there isn't enough water there the evergreens will brown.

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