I see Wagon Wheel Orchard is offering a large number of varieties for custom grafting. They will also graft from scion you snen to them per their website.
Looks like they only send you the freshly grafted rootstock for you to grow out, minimum order of 10 trees, and you are limited to 2 rootsock choices.
Throwing this out for anyone who is interested. I will custom graft trees for you as well. Only thing is I will grow them out for a year for you prior to shipping (can send them immediately if you prefer), rootstock of your choice, and no minimum order. If anybody is interested drop me a PM.
I suspect the late dropping, 2 inch rootstock crab that I have is columbia. I am going to try and graft it to dolgo rootstock. It is not really edible and is very hard.
One reference says reanetka produces a great late hanging crab of somewhat varying size.
So your looking more at the Columbia as the scion source and/or to just the grow the Columbia root stock into the actual tree? Not to use the the Coumbia as the rootstock only?
I am going to attempt to graft some colombia onto the dolgo. There must be some variation in the colombia-maybe seed grown.
If I could get some colombia at a reasonable price, I would just plant it as is.
I took a picture of it this afternoon-some scion and still a few apples.