Even though I'm no expert in tree hybrids, I would say that what you want to do is more a matter of selection than hybridizing. You would need to get seeds from the hardiest of the japanese maples, sow them and see which one will survive in a colder zone. Then you could select these ones, use their seeds again and see if you would find some that would survive colder conditions yet. It is a long process, but it is one of the best way to improve a plant.

Another way to go would be crossing 2 or more of the hardiest of the japanese maple and proceed as above. In any case, it comes down to selecting the hardiest plant from a large number of their seeds and try to improve on that from selection to selection.

You could also try to cross a japanese maple with a hardier type of maple and see which seeds will do best in colder conditions, but of course, they wont be japanese maples anymore after crossing them with another species. And you would have to know if such a cross between 2 different species of maples is possible in the first place and second if it would yields viable seeds. Hope this helps a bit!

I wonder If I graft the japanese maple onto a regular hardy maple in my climate?

I'm hoping to find someone that has seeds for a dwarf maple variety so i can try this.

>>I wonder If I graft the japanese maple onto a regular hardy maple in my climate?Grafting

I'll use the example of roses here. Just like japanese maples, most roses are not hardy to zones 4 and lower. The are zone 5 and up. Hybrid tea for instance are not hardy but they are grafted onto hardy roots. Unfortuntely, this doesn't make them hardy in zone 4 and down, we have to protect them heavily so they will come back the year after and even then you run the risk of loosing them. Not because the root system is in danger but because the grafted part is. But even if you loose the grafted part during the winter, you still may have roses that will emerge from the root stock, below the grafting point. Of course, they wont be the same hybrid tea that you bought since that part died, but a hardy rose from the plant on which it was grafted. I would say the same rules would apply to japanese maples grafted onto hardy maple roots.

You probably know the Explorer Serie roses. These roses are hybrids but are not grafted, they use their own roots to grow from and they are hardy to zone 3 and some to zone 2. I have 5 of them doing very well and they never needed any kind of winter protection and I never even thought about giving them any either (zone 4b QC, Canada). Why is this? The crosses were made between hardy species and/or cultivar from the the start and the breeding was done not to impart hardiness in the first place but to develop other caracteristics such as colors, trailing habits, short plants, diseases resistantce, etc. They probably used less hardy roses in their breeding program to bring in some new genes unvailable otherwise.

Hybridizing

If you cross two zone 5 plants like two acer palmatum, you'll end up with a bunch of other zone 5 plants. But if you cross a zone 2 plant with a zone 4 plant, you will probably end up with 80% of the plants hardy to zone 3, 10% hardy to zone 4 and 10% hardy to zone 2. Then if you take one of these new zone 3 hardy plant, roses for example and cross it back with a zone 2 rose, you may end up with a 50% zone 3 and 50% zone 2 stock. And if you cross back again one of these new zone 3 rose with a zone 2 rose, you may end up with a 75% zone 2 - 25% zone 3 ratio.

These same rules would also apply to maples making crosses between 2 different species, one of them being the japanese maple for the first cross. The difference is here: in the example of roses I took, the desirable plant and the one used in every cross was a zone 2 and the goal was to bring a zone 4 plant to zone 2. With every cross I did I added some zone 2 hardiness to the gene pool. With the japanese maple, we would be working against the odds, the desirable plant and the one that should be used in every cross is a zone 5 plant and that would bring a zone 3 plant to a zone 5 only. And if I added more and more zone 3 maple, I would be loosing more and more of the japanese gene pool. Either way leads you away from the wanted result. And we haven't even said a word about what could be recessive of dominant in these crosses, which is also a very important factor to keep in mind.

This can be observed in the new clematis hybrids that are reaching the market these years. Old clematis were hardy in zone 4, but crossing them to warmer growing one has yielded many new very desirable hybrids, but only hardy in zone 5, 6 or even 7. We have gained in beauty but lost in hardiness.

In a word

I don't think grafting would be successfull in the way you would want it to be. And breeding wouldn't work quite the way you would want either from what I understand from what you said. These are two of the reasons why I said that selection was a much better approach in your case. It is not an easier or shorter one way to do, only a safer one. Hope this helps!

The above information is correct as far as it goes. There are 120 species of maple trees ,I do not know which is the most cold tollerant check out the sugar maple and the Norway maple or whatever maples grow best in your area .

YOU MIGHT WANT TO do a F1 then a F2 cross to get your best result . I know there is an apple that is hardy to 75 below zero. I know one of its ancesters is called "rescue" .
If all the desired traits are dominant then you will be culling out the bad traits for many generations . If your bad traits are dominant then the f2 will be 9 ,3 ,3,1 .

This would mean that out of each 25 generation F2s you would get : 9 ,weak , Norway type , 3 hardy ,Norway type ,
3weak , Japanese type, and one hardy, Japanese type . That means throw 24 away keep one.
As far as grafting goes the hardy root stock shall probably survive but if you graft nonhardy shoots on it they will die if they are planted outdoors .However if you top-graft your F1 and maybe your F2 shoots to it and grow it in a green-house you can use this to get to the F2 generation .

The best comparison would be the pecans (Southern ) and hickory (winter hardy Northern ) . If you grafted the pecan onto the hickory trunks they would not last the winter outside .Some of the F2 hybrids might make it if grafted onto the hickory stock . The "Hicans " (the hybrid pecan X hickory ) do exist in some nursery catalogs .