@BonsaiGuyNJ
Clearly your friend knows just enough about bonsai to be dangerous, not enough about bonsai to keep a tree alive. Pots from ''Bonsai Boy'' tend to be the cheaper lower production grade pots. These pots are not freeze-thaw resistant. The pot seems to be your friend's focus. Not the health of the tree.
Wisteria, Japanese Wisteria, Chinese Wisteria, American wisteria all are hardy through zone 6, a full zone colder than where you are at. Some are hardy through zone 4, which is -25 F to -30 F. Wisteria, if acclimated to winter over a 2 month period will tolerate their roots freezing. In my area, the Chicago area, building code requires water lines to be a minimum of 48 inches below ground. This means the ground does freeze to 4 foot depth, and all the tree roots in the top 4 feet of ground do freeze along with the soil. Wisteria have no problem at all with their roots freezing. Wisteria, and majority of hardy trees, shrubs & vines, all need a winter dormancy to set flower buds, and vegetative buds for new growth in spring. To get this dormancy the tree has to experience a certain number of hours at temperatures below 40 F. To stay healthy your wisteria needs about 2 to 3 months of temperatures below 40 F. If at any time in the winter the temperature stays above 40 F for more than a day or two, the tree begins to loose its winter hardiness and begins to wake up. It will try to grow. If this happens, you can not ''put it back to sleep''. So if you see buds sprout and push leaves, you need to put it in brightest window you have for the rest of the winter, and keep it indoors until danger of frost has passed, then out for the summer.
If this were my tree, I would find a plastic pot that was a bit larger than the bonsai pot that came with the wisteria. Check the bonsai pot, look at the bottom side, see if there are wires holding the tree into the pot. Cut those wires, then lift the wisteria out of the pot without breaking up the soil mass. Set tree in the plastic pot. Fill in with bonsai soil, or perlite, or vermiculite or whatever. Water the plastic pot and tree. Let drain, then set out in the storage shed. The empty bonsai pot can also go in the storage shed for the winter. No need to put in plastic bag. It is okay to freeze in this set up. Check at least once a week or more often to see if it needs water. Only water if the original bonsai soil near the trunk has gotten somewhat dry. In spring, lift the tree and its original soil & root mass out of the plastic pot and set back inside the bonsai pot. Tie it in with wire if necessary. Place outdoors in the sun where you were planning to grow it for the summer.
The bag can cause problems with fungus. I would not use the bag.
This moving a bonsai out of a nice pot for the winter is done more often than the pro's and most authors care to admit. Good quality bonsai pots are supposed to resist freeze thaw cycling. What is a good pot? Hard to say definitely, usually in the description it will be described as fired to cone 7 to cone 9, or it will have other terms in its description to indicate it is non-porous, high fired, and freeze resistant. These pots tend to be more expensive, usually over $35 and often into hundreds of dollars. Inexpensive pots rarely tolerate freeze thaw cycling. Occasionally good pots have minor flaws that cause them to be susceptible to freezing, and will break on you regardless the fact that you thought you had a resistant pot. So many growers, if they have a very fine pot, will have a second pot, plastic or otherwise inexpensive, that is roughly the same size and shape as the very fine pot, and will put their tree in the fine pot for the display season, then transplant it back to the less expensive pot for growing. This is done, and if care is taken to not disturb the root ball, it can be done as needed. This is not what some people refer to as ''slip potting'' but it is similar. You could do this with your wisteria. Thereby keeping it healthy and preserving the pot.
Key is, you need to focus on what the wisteria needs to be healthy, not the pot.
