When it comes to fig trees and northern climates you have three options when it comes to storing your fig trees for the winter:
Grow fig tree in container and bring inside when winter comes
Grow Fig tree in ground when winter comes wrap the fig tree with burlap and other insulating materials.
Burial Method a.k.a Zombie Method
Grow Fig tree in ground when winter comes Bury the fig tree under ground (I call this the zombie method because you bury them in winter and they 'rise' in the spring)
Now I have 8 fig trees in containers and 4 trees that I keep outside. The container figs are pretty simple to store for the winter. After the first frost comes and burns the leaves off the fig tree they are then ready to be stored in either a garage or a dark basement.
For the other 4 fig trees I use the burial method. These outdoor figs are grown in the ground from April until November. Once the cold weather comes and burns the leaves off the trees I take a spade shovel and dig a foot from the base of the trunk all around the tree. Once I've severed enough roots and the root ball becomes free I lean the tree to one side and begin severing the bottom roots. I repeat this step on the opposite side until the tree is able to be lifted root ball and all from the location it has been growing. Don't worry fig trees can take heavy pruning of their root systems. As long as you don't damage the central core of the root ball the fig won't feel a thing.
Once you have the tree free you can perform some light pruning in order to make the tree more compact and make it ready for burying it underground. I like to bury the fig tree in my raised beds as the soil is easy to dig and work with. I dig down about a foot and a half or more depending on how large the fig tree is. You want to make sure that the trees root-ball and trunk will be covered by at least 12 inches of soil so the freezing temps stay at bay while it's buried.
Here are some photos of me digging up my zombie fig trees that I buried this past November. They are all healthy and the buds were greening even though they've been buried for 5 months. Figs are hardy and can take being buried as long as they are covered with enough soil.