I live just north of the 8a/7b zone line, technically in 7b. I have a 6ft wooden fence in my back yard, aged to a dark grey, and faces south in full sun. Do you think that the radiated heat off the fence creates a micro climate of 8a or even 8b? And if so, how far out from the fence would that extend?  

Gene, 

A microclimate in what way?  Will it give the plant some extra heat?  Yes.  It won't give much if any winter protection though as a fence would have no real thermal mass.  Guess I am just confused by the question because zones are based on low temperatures.  

Some here have done it with stone walls and wooden fences on 3 sides to take advantage of stored and reflected heat.  I accidentally did it here by placing a fig between a west facing house wall and east facing deck, plus it had protection from south and west winds.  The fig didn't go dormant when the more exposed ones did. 

Good luck!
Suzi

it might give wind protection.. but i don't think it will add much heat at all. brick seems to hold lot of heat in. 

Hi GeneDaniels,
IS the fence full wood or aired with holes in between the wood ?
If full wood, it will provide wind protection and temp rise . Go there on a sunny day and touch the wood . If it is hot, then it works .
But that said, one can understand that if you don't have sunny days, you don't have heat from the fence ...
The last two winters here have been the cloudiest I ever remember of .
I do use dark plastic trashcans for the same effect, and since there is no sunny days ... they only provide the wind protection . But, that helps a bit already IMO.

As for the distance, I wouldn't count on more than 30 to 50 centimeters ...

The fence gets quite warm on cold sunny days. But I didn't think about zones being based on low temps, thus overnight temps. So I guess it really will  not make much of a difference, just perhaps slightly warmer in the day time.

Hi GeneDaniels,
It will make a difference if the frost doesn't last for weeks.
If you get -50°C for a month - then it is of no help.
But if you get -15°C or -20°C at night only, then the fence will surely protect the tree and it will get -10°C or -15°C "only" which is of a big help for the tree .
At least, IMO, it is worth the effort .

You would get more benefit from a stone or concrete block wall. Wood is more of an insulator. You need something with more mass that can store the heat. The fence won't hurt, but probably won't add much.

Now, if it protects from a nasty wind that comes from the north, that will be a benefit. Cold winds, esp. dry ones, can dessicate a tree fairly quickly.