I had a mystery tree that I think misidentified as a hawthorn for a long time, and because of that I tried grafting Asians pears to it, which worked quite well except they sucker like crazy, suckers from the ground and waterspouts from everywhere…
But I came to really like these big bushes, they have Jasmin scented flowers sometimes pure white sometimes pink, usually white fringed with pink and tuning mostly pink before falling off.
The fruits are about 1/4 of an inch to maybe half an inch, absolute maximum, colored anywhere from yellow to red, usually orange and one tone, shinny, and no steaks, they have apple like but white and softer seeds.
The leaves are massively different, from small marquis shaped almost smooth leaves to massively lobbed with a Christmas tree shape. Different leaves can be found on the same branches.
The leaves usually turn yellow in the fall with red veins, but sometimes orange or red, my profile picture is what color Korean Giant leaves turn to when grafted on Toringo Apple, Scarlett red while they turn yellow on my tree on betulafolia.
The bark is smooth on young shoots up to about 2” and then becomes flaked like a black cherry tree or perhaps like oak.
The young shoots have some fine spines or thorns and fruit spurs grow like thorns, although generally blunt, they can be sharp. The larger branches, anything over ~1/2”, are completely thornless.
They tend to grow like bushes but in forests they sometimes can be found growing as a single trunk straight tree I’ve seen up to 25 feet tall and maybe 6” in diameter, certainly I think they’re capable of getting at least a little larger. In the open they mostly grow no more than 10 feet tall and spread quickly from the roots, putting out new trunks at 30 feet away or even more.
As a rootstock, I wouldn’t really recommend it. Grafting on it overall is not great; no apple grafts survived, no European pear grafts survived, but my Korean Giant and Raja grafts did great, unless we talk about the suckers over taking them. I don’t think I’ll use it as a rootstock any more unless I run out of better rootstocks for Asian pears, or maybe I want to grow an Asian pear on its own roots and bury the graft union very very deep.
But I’ve come to really love this tree-bush. And what I most appreciate it for is its value as a thornless, flowering hedge, it grows quickly, spreads quickly and even kinda trains to spread in straight lines pretty well, and its long thin runner roots don’t interfere with mowing. The fruit it’s great after really ripening or after a frost, but it’s hard enough to collect that it’s not worth much, but it’s pretty after the leaves fall.
If anyone can try to help me confirm my identification as Malus Sieboldii I would appreciate it….if only we had a crab apple expert on this forum, who could I ask? @clarkinks do you know any crabapple experts? 
(And my apologies, I’m not 100% sure this first picture is the same tree, I’m 100% sure on the other photos that they’re all the same species, aside from the Asian pear grafts you see on them of course)
This is literally the first tree that I seriously tried to graft and it worked! I love that they took on my first real attempt, after practicing with chestnuts on beech trees too early in the year, and knowing that would would almost certainly not work.
