"..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 -

Murky, My figs are towering giants right now. I bet they are 12 or more feet tall. If you had asked me that same question in 2003 I would have jumped at it, as these root cups look like they cannot be improved upon. But I am just not airlayering anything these days. Back in 2003 I was using yogurt cups that I had first melted a hole in the center bottom, then cut with scissors down the side to that hole. Next I would fit it around the fig stem, and then duct tape the whole thing solid. After that I would fill the cup with some potting soil and keep it moist for a month or two. They rooted very nicely, best done in early June. The reason I even went to this extreme is because a friend from California had given me some potted figs that had severe nematode infestations, so this method was incorporated to give the clones disease free roots. Then junk the mother plants. Any more, my figs are so branch-prolific that I just take the more limber of the lower branches and weight them down with rocks of bricks to the ground, making superb fig-babies for whoever wants them. Right now I have plenty of Desert King and Negronne, if you know anyone that needs that sort.

The main root mass is still supplying water & nutrients to the top via the xylem, and that burden shifts to the roots produced by layering at the point in time when the layer is separated; so, some care should be taken to ensure things don't go awry, but by and large you should be fine for at least a week or two, doing nothing more than keeping the layering medium moist. Al

Well it probably will fail but you've got nothing to lose by treating as a cutting.As it's not dormant you probably won't get away with leaving outside in a cold frame 'til next year(as in hardwood cuttings)so I'd dust the calloused area with rooting powder,put it back in the pot,water and cover with a plastic bag and leave on a window sill out of direct sun.Keep an eye out for fungus though,remove the bag daily to let it breathe.It's a long shot but not sure what else you could try.Btw,can sugar maples be propagated by cuttings? or is that why you layered? Edit..Found this,apparently timing's everything with cuttings though I wouldn't expect you to have a set up like this. http://www.rngr.net/publications/tpn/19-1/pdf.2005-05-20.7826838949/at_download/file