Certified GKH Gardening Expert
Generally, trees of the same genus can be grafted together. An example would be different types of apples on the same tree, or most citrus can be grafted onto other citrus. In other cases where there are more cultivars within a genus, you have a little more freedom. Cherries, apricots, peaches, and plums are all in the genus Prunus, so they can be grafted together. You just can't cross species. Apples won't graft to pears or anything like that. As long as you stay within the same species you can come up with many cool trees.