I understand where you're coming from and on a larger scale such as with weather systems that is how it works. You're forgetting that each day the sun provides a HUGE amount of energy to the Earth - even in deep winter - the fleece of course acts both ways, it keeps the cold in too, but the sun wins out and the soil around the fleece warms - a little, and the fleece helps to keep it there for a while - every little helps.
Billericay - Essex
Knowledge is knowing that a tomato is a fruit. Wisdom is not putting it in a fruit salad.
Actually the ground (soil) holds its heat for a long time, much longer than the air does, and so it insulates plant roots - that's why plants in pots are more likely to be damaged by hard frost than plants in the ground. It works like the bricks in a storage heater. If it didn't, I guess ground source heat pumps wouldn't work...
However, I think you're right that the benefit of fleece is minimal. If you want to protect plants which aren't reliably hardy over winter, a thick dry mulch is much more effective, if they're growing in the ground. For protecting young plants in spring from the odd frosty night, dry newspaper works as well as fleece - provided you remove it in the morning, because light can't penetrate it.
Since 2019 I've lived in east Clare, in the west of Ireland.
I think it’s a useful discussion. It’s always good to explore and understand more. Learning. It’s not obvious why fleece works. It would be interesting to discover what kind of heat differential is created. I wonder that about little zip up plastic greenhouses. At -2oC do they really offer greater heat or just shelter? Has anyone measured the differences inside and outside?
Wind and draught is often over looked as a potentially drying and killing force. Draughts will change a house environment and the biosphere of plants.
As a side note, plants do generate their own heat, though generally not much. Skunk cabbage generates enough if its own heat to melt snow surrounding it.
Not really an answer to your question, but heat is generated by a secondary process of respiration called thermogenesis, which some plants are capable of (like the skunk cabbage Fire mentioned).
Re: the fleece, I expect that a big part of its usefulness is as an insulator not of the plants per se but of the air around them, which will generate a microclimate warmed slightly by the ground and sheltered from freezing and dessicating winds. As people have already said, the ground does lose heat much more slowly than air and water, which is why the sea temperature varies much more than the ground temperature, for example. The more consistent temperature and more stable microclimate will make things less stressful for the plant over winter.
I wonder that about little zip up plastic greenhouses. At -2oC do they really offer greater heat or just shelter? Has anyone measured the differences inside and outside?
I don't know about those specifically but pollytunnels are often colder than outside on clear winter nights. The still protect plants as they break the wind but purely temperature wise you need to add some form of heat sink to keep it above ambient.
All I know is I put fleece on all but one of my window boxes this winter, and all but one window boxes came through and are growing great. The uncovered one which was in the most sheltered spot did not make it past the first frosty day. Don’t really care about the science, just the result🤪
Marne la vallée, basically just outside Paris 🇫🇷, but definitely Scottish at heart.
Wind chill factor doesn’t really apply to plants as it is about perceived rather than actual temperature.
What is important with plants is whether the moisture within the cells of the plant freezes and in doing so expands enough to burst the cell walls.
Actually I think there is a 'wind chill' effect on plants although, as you say, it's not directly comparable to what we perceive. There is a connection though - plants transpire and we sweat and both are affected by a cold, dry wind. I have a cold, very windy garden and there are plants, like cherry laurel, that are very hardy but they die here. It isn't the cold on its own (it's not THAT cold here), but the combination of cold and wind - they appear to burn - the leaves crisp up and dry out.
Fleece acts as a wind break as well as preventing cold radiant losses from the earth to the sky, so keeping the soil surface a little warmer
Gardening on the edge of Exmoor, in Devon
“It's still magic even if you know how it's done.”
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You're forgetting that each day the sun provides a HUGE amount of energy to the Earth - even in deep winter - the fleece of course acts both ways, it keeps the cold in too, but the sun wins out and the soil around the fleece warms - a little, and the fleece helps to keep it there for a while - every little helps.
Billericay - Essex
Knowledge is knowing that a tomato is a fruit.
Wisdom is not putting it in a fruit salad.
However, I think you're right that the benefit of fleece is minimal. If you want to protect plants which aren't reliably hardy over winter, a thick dry mulch is much more effective, if they're growing in the ground. For protecting young plants in spring from the odd frosty night, dry newspaper works as well as fleece - provided you remove it in the morning, because light can't penetrate it.
https://www.google.com/amp/s/laidbackgardener.blog/2018/02/08/heat-seeking-bees/%3famp=1
Re: the fleece, I expect that a big part of its usefulness is as an insulator not of the plants per se but of the air around them, which will generate a microclimate warmed slightly by the ground and sheltered from freezing and dessicating winds. As people have already said, the ground does lose heat much more slowly than air and water, which is why the sea temperature varies much more than the ground temperature, for example. The more consistent temperature and more stable microclimate will make things less stressful for the plant over winter.
I don't know about those specifically but pollytunnels are often colder than outside on clear winter nights. The still protect plants as they break the wind but purely temperature wise you need to add some form of heat sink to keep it above ambient.
Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.
Fleece acts as a wind break as well as preventing cold radiant losses from the earth to the sky, so keeping the soil surface a little warmer
“It's still magic even if you know how it's done.”