"..If the hackberry is deciduous then you should start the air layer at the end of winter, right before it wakes up" I don't know where you got this info from, but it is not a very good idea. And will most likely not work at all. 1) If you air layer before there is foliage, there is nothing to provide the sucrose to develop roots. 2) If you "leave thin strips", the wound will just heal over, and not develop roots. "...this way it will still have some of the cambian layer to supply the upper portion," The cambium does not supply the leaves with anything. The cambium is the pathway for sucrose made by photosynthesis in the leaves, to travel to the roots. The inner core is the pathway up from the roots that supplies water and nutrients to the leaves. When we sever the cambium, water and nutrients can still reach the leaves who, who in turn send the sucrose developed (energy) back to the roots for storage (down through the cambium). Once this energy reaches the severed section of cambium, it has nowhere to go and collects there. This initiates the formation of roots at that spot. If the cambium is not entirely severed, there is still a pathway for energy to travel to the roots. And the tree will begin to heal the severed section while still functioning normally. It is a one way loop -

I did some fig green-wood air-layering in the past. Only because the tree was either too small (lack of preferred old-wood) and/or it was a precious/rare tree. Sometimes, I even air-layered just the growing tip. I use either Gene's peat-moss seed-pellet method if the twig is thin, or rooter-pots if the twig is fat. In both cases, I left the rooting medium visible (plastic) so that I can watch the roots. Do not girdle - green wood snaps off very, very easily. Support the air layered branch and tie it to a stake. This prevents snapping by any wind and also prevents the rooter-pot from moving around relative to the twig (bad news for any developing tender roots). Be patient! I think green wood takes longer to root. After that, let the roots develop well before cutting. To give you an idea, old wood takes ~4 weeks to root, while green wood may take twice as long. Depending on the root/leaf ratio, a humidity dome may also help after cutting the twig. Pot in the shade. George (NJ).

I would suggest you leave the plant(s) as is do not remove BUT do keep the soil around the plant DAMP not wet. this will prevent the roots (but not the soil) from freesing

Early spring is the best time to start doing your air layering. You can also observe deciduous trees to see when their buds become green. That would be the best time to start.