I can’t find any very complete information about grafting chestnuts.
I have a 9-10 year old Chinese chestnut seedling that was coppiced by nature every winter when it froze back to the ground. Every year I’d prune off all the extra trunks, then it would freeze back again the next winter. I decided it needed all those trunks and quit pruning a few years ago. It hasn’t died back since. It’s about six foot tall with around eight main trunks. I had planted another tree for a pollinator, but it didn’t make it. I need a pollinator as soon as possible. Last night I ordered four varieties of 12” scions from Burntridge to graft onto different trunks of this tree. Is this a good idea? What grafts should I try and when?
Maybe I should try pruning it to one trunk now that it has older wood that’s less likely to die back? But I kind of like it the way it is.
I've got a whole bunch of seedlings and have been trying to graft my "known quantity' cultivars atop them. So far, only one has taken and it's been planted out. I'll be doing another round in the spring (it's autumn here). I haven't seen much info on grafting them apart from cautions about incompatibility and rejections, but did note that the nursery trees I got were obviously grafted. My plan is to make a bunch of dual trees where I let the rootstock branch once beneath the graft union, and then grow them on in order to get them out around the village in reserves and on berms. The idea is that each tree will be more likely to self pollinate if it's got two varieties. We'll see.
Chestnuts are notoriously picky about grafting - if the scions you've ordered aren't of Chinese ancestry, they are very unlikely to take.
Rule of thumb is that a european cultivar will graft to a european seedling with about a 2/3rds success rate (for professionals at least) in industry they typically even graft the cultivar on a seedling of that specific cultivar to maximize success rates.
Same goes for chinese if you use both chinese, or japanese etc.
If that cultivar fails on that tree, try another (same species) cultivar and you'll probably get another 2/3rds successful grafts. Then repeat until all your trees are eventually grafted.
As far as I know, the lines blur a bit with hybrids, but I would want at least one to match. For instance a euroXjapanese would probably work on a euro seedling, but a chinese probably wouldn't.
If some of your scions aren't at least partially Chinese, it may not be worth it to try to graft them onto it. But it may work who knows! If I were you I would look for some euro seedlings for those scions if you have any.
As far as timing, what I've seen is people leave the scions in the fridge until mid spring, after the tree has leafed out, and then graft with the dorman scion on the growing seedling. But I don't know if this is more or less successful than grafting while dormant. Probably is a delicate balance between keeping the scions fresh and grafting at the correct time.
As far as pruning that multi-stemmed tree, I would try to graft it with a different variety each stem, and then in two years when you're pretty sure the graft has taken, cut out all but one of the stems which the graft fails on. Then you'll get perhaps 3 or 4 known cultivars on one tree, plus you get to see what the quality of your seedling is! The tree is likely to be healthier in the long term if it has fewer stems rather than a whole lot. Doesn't mean it has to be one stem though!