Horticultural fleece - how does it work?

I've read somewhere on here that apparently horticultural fleece only works down to -5 °C.

So I have a couple of questions:

1) If a plant is hardy down to, say, -10 °C, will the fleece add another 5 °C to this and thus make it hardy to -15 °C or would it have no effect at all seeing as it allegedly only works to -5 °C?

2) If I doubled up the horticultural fleece on the same plant, would it then be hardy down to -20 °C?

Additionally, if I move some plants into a cold greenhouse to overwinter, should I also wrap them in fleece to add further insulation or would they be okay without (specifically my Escallonia)? The greenhouse is in a sheltered shady corner behind the garage.

Many thanks

 

Hi Merleworld. We have debated this before and I have never seen a definitive answer.

My own thoughts are that its not insulation like the duvet you put on your bed. Its far too thin and that only works if you have a heat source inside. Without a heat source the inside will reach the same temperature as the outside eventually.

I believe it works by making the air inside still. You always feel warmer on a cold day when you get out of the wind. A cold wind is more damaging than cold still air. And thats why you get a small amount of extra protection. And yes I believe that it is always extra so a plant rated at -10C may survive down to -15C with a fleece.

However doubling it doesn't help much. It will make the air a bit more still, but once the air is totally still, like in a cold frame, you can't improve any further. But it certainly does no harm to have it doubled.

There is another mechanism that could be playing a part. On a clear night its always much colder than on a cloudy night. There is a good reason for this. Everything, including houses - people - the ground and plants, loses heat by radiation. On a clear night this radiated heat goes out into deep space. But on a cloudy night the clouds catch some of it and radiate it back. So it is possible that a fleece acts rather like covering your plant in a cloud.

 

Good explanation Peter I always wondered ,as well , how a 0.5mm thick weave had any thermal insulation properties for plant cover.
So would protecting plants with horticultural fleece or bubble wrap in an unheated greenhouse or potting shed have any advantages ?

 

Hi, it may also be that it slows down the rate of temperature change, a lot of the frost damage occurs when the sun rises and the plants warm up quickly, so a little shade and less moisture on the leaves can make a lot of difference.

 

I was always told that fleece doesn't so much keep the cold air from getting in - as obviously cold air, being heavier, sinks - but more a case of stopping the warmer air around the plants, from rising. My Grandad used newspaper which did exactly the same type of job, although it was a lot more 'hassle' to put down and of course, being heavier, could crush very delicate plants.

 

Worth remembering that fleece will help with the overnight drop in temperature but not much help in prolonged subzero temps.

 

Harry - if a fleece works by making the air more still, then I suspect that a fleece in a greenhouse won't help much - unless its a very drafty greenhouse. Though even a thin fleece will slow down the movement of air and hence the rate of change in temperature - and this might be just enough to give some protection. However bubble wrap does have insulation properties. It traps a layer of air in the bubbles, which unlike the fleece can't move. And of course air is an excellent insulator. With things like duvets, glass fibre insulation in the roof and double glazing, its the still air that does the insulating, the rest like the fibres and panes of glass are simply there to prevent the air moving.

So I think Blacksmith and Fidgetsmum are also right. By slowing down the movement of air it slows down any temperature change. However in long cold spells this effect is ultimately lost as the fabric is porous and the air will eventually pass through it, as Lazydog pointed out.

But there is no question that fleece and cold frames do work - but only to a limited extent.

 

Merle, there are two types of fleece, horticultural fleece, which is white. And frost fleece, which is blue/green. Horti` fleece will protect agains mild frost, frost fleece, because it is slightly thicker and heavier, will protect against even lowere temps. For greenhose protection line the inside with large bubble insulation. This will help make the greenhouse frost free. Left up, it will also act as shading in the summer.