Sorry... air layer, not air root!
Sounds like it may be a better idea to find a sapling and then bend it. Ok, that works too.
“Air root” would be a better term for it than “air layer.” I understand that the process involves the different layers of the tree’s live tissue, but you’re not prompting the tree to grow more layers. You’re prompting it to grow more roots.
Regardless, as others have said, it is very difficult to convince an oak to grow roots above the root collar. I’m not convinced it’s even possible, at least on white oaks. I tried for a couple years to layer a burr oak, scraping the bridges of tissue off after the first year and replacing the sphagnum moss. After two full years, the tree produced a massive, ugly callus, but no roots. It was far more interested in bridging the gap than sending out any new roots.
That said, I’m finding pin oaks like the one pictured above respond extremely well to bonsai techniques, and I highly recommend everyone give the species a try. They have relatively tight internodes, smoothly heal over chops, ramify quickly, and tolerate extreme root pruning. I haven’t gotten my pin oak’s leaves to reduce much yet, but I’ve only had it in a pot for two years. I’m confident they will reduce, but even if they won’t, wild trees’ leaves are usually small enough to work well for bigger bonsai.
Edit: On a second look, that
might not be a pin oak, since the leaves look a bit big for it, but the conifer-like growth habit and deep leaf sinuses lead me to think it could be a pin oak.
. Street word is Oaks hard to air layer and bigger is harder verging on improbable/impossible.
