This is a good topic that definitely needs more attention!
There are MANY ways to graft Japanese Maples, and many high-level Japanese Maples have been grafted in one way or another. Unfortunately, there is stigma around grafting because people immediately think of the unsightly grafts from garden centres.
There is a term in Japanese 'koromo-gae' which means 'changing the clothes', and it is used to refer to changing the foliage type on trees. Bjorn spoke about this recently on BonsaiU when I asked him about it. He mentioned that many Japanese Maples that began their lives as bonsai in the 1920's and 30's were not necessarily chosen back then with their foliage type in mind. Nowadays, many of those same trees have their foliage swapped for a better foliage variety. Bjorn went into considerable detail about how/when/why this is done. There is a lot more nuance to what he said than I can possibly repeat, so please understand that I am not quoting him in full here. In sum, Bjorn discussed 3 ways of doing this:
- Thread grafts through the primary branches
- Thread grafts through the trunk
- Ebihara-style peg grafting of branches
I would add that peg grafting does not necessarily need to occur with a single branch being grafted into the trunk. Sometimes it is appropriate to graft an entire trunk into the nebari (see attached).
Among the options Bjorn reviewed, I am personally using thread grafts passed through both the primary branches and the trunk to convert one of my vigorous Japanese Maples into a Deshojo (see attached).
I would also add that one other place in which thread grafting can occur is directly through the nebari. In the images 3 and 4 attached here, my teacher Yves used a vigorous Acer palmatum variety to quickly develop a nebari, and then thread-grafted the slower-growing deshojo strains into that nebari. These were just (multi-year) experiments, of course, and aesthetics was not a primary focus. I tried to put this idea into a sketch in image 5.
The latter is an approach that has fascinated me for years, and I currently have many projects underway where I am using vigorous Japanese Maple strains to create a large nebari using a variation on the 'Ebihara method' where I employ multiple trunks instead of just one. Once the nebrari is satisfactory, I intend to remove all of those trunks and replace them with very thin, elegant trunks. The goal is to reproduce the images of Koga and Musashi Ga Oka in a much shorter timeline. (I have investigated, and Ebihara seems not to have produced any kabudachi style maple bonsai, which drives me nuts, and I wish I could ask him why).
There is so much more to say about grafting Japanese Maples in ways that are not just appropriate but strategic and advantageous in bonsai, but I'll stop here for now