The prohibition against the grafting of diverse tree species is implied in the verse: "You shall not sow your field with diverse seeds."¹ 19:19. Therefore it is forbidden to graft a branch of one kind of tree upon another,²—Shulchan Aruch 295:1. such as the branch of an apple tree upon a citron tree,³ 295:6 (and Shach 295:3) permit the grafting of a fruitless tree onto a different kind of a fruitless tree. Chasam Sofer Responsa 288 says it is best to refrain from this as well. However, you may not graft a fruit tree with a fruitless tree. (Shulchan Aruch 295:3) or a branch of citron upon an apple tree. Even between similar species, like a branch of a [cultivated] apple tree upon a wild apple tree,⁴ Chapter 106 says these are different species because their taste is very different. However, the Mishkanos Yaakov Responsa 66 reinforces the prevailing custom that permits grafting an apple tree with a wild apple tree because they are one species and this does not depend on their taste. since they are two varieties of apples,⁵ Chapter 10 says it is considered like a fruitless tree and it is forbidden to graft it with an edible variety. Mishkanos Yaakov Responsa 66 disagrees, and considers them one species. they are forbidden to be grafted with one another.⁶ 295:6 says it is best to refrain from grafting any two trees that are not exactly the same kind because not everyone is knowledgeable if they are the same species or not. A Jew is forbidden to allow a non-Jew to graft two diverse kinds of trees for him.

footnotes: ¹ Leviticus ² Even outside of Eretz Yisroel ³ Ramah ⁴ Levushei Sarad ⁵ If one variety is not edible, Levushei Sarad ⁶ Ramah

It is forbidden to maintain a tree upon which another kind has been grafted, but it is permitted to eat its fruit. It is permitted to take a branch of a grafted tree and plant it elsewhere.

Sowing a vineyard with two kinds of seeds, or sowing a field with two kinds of seeds, is not prohibited outside of Eretz Yisroel⁷ 296:69) unless you have sown two kinds of grain or two kinds of vegetables together with the seeds of the vineyard.

footnotes: ⁷ This refers to both sowing and eating the produce. (See Shulchan Aruch