Surviving Frost vrs Freezing temps- please explain this

[GrannieAnnie]

I don't understand this:  how can my lettuce survive 20 F. (-5 C.) night temps protected only with an old window  (while the sides are open to the air) yet without the window above they'd be fros ted and I'm sure the plants would die? They've done fine this way since January when we removed th e cold frame covering them. They were well hardened in the coldframe but I did not expect them to flourish under a bare window

  I really thought lettuce was more tender than this.

[GrannieAnnie]

[euronerd]

I've read somewhere that certain plants have in their genetic make-up the ability to produce their own anti-freeze, probably glycol based. I start to research this every winter but then spring comes along and my attention is diverted  ;D

Geoff.

[Robert_Brenchley]

If the air's reasonably still (ie not blowing a gale) then the glass traps a bit of air round the lettuces. It's obviously nowhere near as efficient as a complete cold frame, but it doesn't take much to keep a bit of night frost off.

[bupster]

Some lettuces are also remarkably hardy anyway. I had a hedge of oriental salad greens that stood throughout winter.

[Tee Gee]

You have created a micro climate which is enough to raise the temperature around the plants a couple of degrees, plus frost comes down vertically so the glass again protected your plants.

The nearest thing I can think of in human terms is;

Say you are out on a windy cold frosty day and the wind is in your face, you are likely to find the 'wind chill' is quite intense.

Then you walk into the shelter of say a building and you are out of the wind, then you find the temperature seems to rise when all that has happened is ; you are not feeling the effect of the 'wind chill' because the shelter of the building has created a micro climate just as your piece of glass has.

Does that make sense ???

[bupster]

My neighbour raises very early broad beans with a similar theory - he puts canes around them and sort of wraps them with a wall of fleece, but doesn't cover them over. They don't get the wind chill, and aren't affected as much by frost.

[goodlife]

I think it is also the raising heat from the soil...during day it will warm up under class and release it in the night.....

[GrannieAnnie]

Thank you all.  :) Based on this we may try it with other varieties in the Fall.

[OllieC]

Lots of funny things happen to water between 0 degrees C and 4 degrees C. One example is that it starts to fall out of the air. You're stopping water from falling out of the air & landing on them - when this water comes out of the air without the protection of glass (or fleece), it then freezes on the plants, so they have ice sitting on them. You're stopping this.

That's what I reckon!