I've had many of these beautiful trees, but can't keep them alive.
Two years ago I came across a two gallon can size Koshobai in a nursery for a landscape shrub. It was about 2 feet tall. I cut it back, wired a little and put it in a deep bonsai container immediately. Well, it has survived two winters here in Rochester, NY in my cold greenhouse, kept about 50-60F. This year its in my garage kept about 27F. I may chicken out and put it in the cold greenhouse later on.
Yes, the larger Koshobai in Japan are usually grafted on to
Ilex serrata with heavy trunks. Mr. Takeyama has a couple of the best ones in his garden. I just looked through over 40,000 images on my computer to find a photo, but could locate it. But, attached is a larger size Koshobai bonsai from Mr. Takeyama's garden which I photographed about a week ago
. This one does not look like it was grafted on the thick trunk.
Small plants are usually propagated by grafting in Japan.