...and, worst case scenario if you see little to no roots by early October you can just leave the layer on over the winter. I started an air layer on a crabapple late last year (maybe mid-July) - it didn't throw roots, but I just left it on over winter. This spring I re-wounded the callus (didn't remove the callus, just put some fresh cuts in it) and I noticed it beginning to produce roots within about 4-5 weeks.