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Minimum temperature for a frost-free greenhouse

As the nights are getting frosty and I am having to heat the greenhouse to protect the tender plants: what is the minimum temperature you let a frost-free greenhouse drop to?

The RHS in its encyclopaedia gives 2 degrees Celsius (strikes me as too low). Monty has mentioned 5 degrees celsius in the past...

For context: dahlia tubers are stored in compost in wine crates, cannas are potted, ensete is dry-stored. I am just glad I haven't splashed out on a plumeria and having to look after that, as well...

Posts

  • Flo7Flo7 Posts: 33

    Thanks for the reply!

    I'm new to the greenhouse world, and have to admit that I have underestimated the importance of insulation. I only have a paraffin heater, so the scope for raising the temperature is not vast. Bubble wrap is ordered, though, and I am going to use the coming days - which are forecast to be milder - to get the greenhouse ready for the coldest months. 

  • PosyPosy Posts: 3,601

    Most people seem to suggest 4 to 5 degrees. The key plants are your half hardies, like Canna. They don't forgive mistakes. If you have electricity in the greenhouse it is worth getting a little heater that is more controllable and less wet.

  • PalaisglidePalaisglide Posts: 3,414

    Flo7 had trouble with parafin heaters years ago and bought a small electric heater with a frost guard thermostat. You need to trim the wick on parafin heaters or as I found out one morning you wake to a sooted up greenhouse. If you have staging put the plants under it and cover over with bubble wrap, the floor temperature will be a degree or so warmer than higher up. Make a tent within the greenhouse not touching the glass, lay fleece over the plants and then drape bubble wrap over that. You cannot put enough heat in to a greenhouse with normal heaters to stop a severe frost, I live in the N E of England, we have had hard frosts all weekend, the glass is frosted but inside under cover above freezing. Some plants can take a few degrees of frost and wrapping pots will also work. Remove the covers during warmer days, let the plants breath, keep watering to a minimum though do not let them dry out. Heating depends on what you can afford, electric fan is cost efficient if you have a cable in, gas and parafin best used sparingly.

    Frank

  • Flo7Flo7 Posts: 33

    This is all really helpful, thank you to all of you! The minimum temperature so far was 3 degrees, and I don't want to let it drop much further than that - but I guess that I won't be able to maintain that once the frosts hit really hard. I only have paraffin, unfortunately, no electricity in the garden... Bubble wrap will arrive tomorrow, so I am a bit more fortified, at least. The idea of the tent is good, I will try that!

    How do you best ventilate with paraffin if it stays frosty during the day? Open the louvre vents slightly? Or one big air exchange (I reckon that will let the temperature drop too much, though)?

  • PalaisglidePalaisglide Posts: 3,414

    Flo7 crack the top vent a little the fumes rise but so does the heat. Condensation is the main problem and it can cause problems with the plants. It is difficult to organise if you work and would be out all day, if it looks cold enough to last all day then leave the GH closed up otherwise some air flow is best. No easy answer and I did at times lose plants. I did keep some in the garage it was warmer but does not get the best light so had to be turned daily.

    Frank

  • Flo7Flo7 Posts: 33

    I am finding that my echinops seedlings are not liking it, they are developing some brown and limp leaves. I am ventilating as much as I can, but some seem to struggle. I am going round and wipe the condensation off to minimise the effect, but not to too much avail, yet...

    Last edited: 05 December 2016 21:28:55

  • 1 deg C.

    Everyone likes butterflies. Nobody likes caterpillars.
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