The main prohibition regarding tree grafting is the actual act of grafting – i.e., inserting the branch (scion) of one tree into the wood (rootstock) of another tree. This prohibition is not explicit in the Torah; the gemara[1] derives it from the textual proximity of the prohibition of crossbreeding animals and that of crossbreeding in the field (kilayim).[2] This derivation is presumed to be of a Torah-level.[3] Because it is derived from the prohibition of crossbreeding animals, which is not a land-based prohibition, it applies even outside of Eretz Yisrael.[4]
It is forbidden not only to plant kilayim, but even to allow it to remain in one’s field.[5] This extends to grafting trees as well.[6] However, although one may not leave such grafted trees intact, one may eat or benefit from the fruit that have already grown there.[7] An exception to this rule is when vineyards (grapes) are involved. In addition, the offspring of crossbred animals are invalid to be brought as korbanot.[8]
It is unclear whether leaving the grafted tree intact is a Torah-level prohibition or a Rabbinic one.[9] This question might depend on whether one is passive or active in the matter of preserving or improving the tree.[10] In any case, it appears that the opinion that this is only a Rabbinic prohibition is the stronger one.[11]
The Chatam Sofer[12] was bothered by the practice in his time (outside Israel) of observant Jews buying orchards containing grafted trees. In justification of the practice, he suggested that since the source for the prohibition and its extension to chutz la’aretz is the prohibition of crossbreeding animals, the prohibition in chutz la’aretz applies only when one is active in preserving the grafted species. Thus, claims the Chatam Sofer, the practice he witnessed, although not ideal, can be justified, especially in a case in which a non-Jew has a financial stake in the orchard, by means of which he can prevent the Jewish partner from uprooting the grafted trees. The Chazon Ish[13] rejects the Chatam Sofer’s nuanced leniency. Thus, at first glance, it is quite problematic to keep and cultivate a grafted fruit tree, even though its fruit are permitted.
In practice, however, you probably have no problem. Grafting trees is forbidden only when the scion is of a different species than the rootstock.[14] It is completely permissible to graft a nectarine branch onto another nectarine tree. In addition, nectarines are simply a variety of peaches; thus, it is also permitted to graft a nectarine branch onto a peach tree or vice versa.[15] Unlike what many think, nectarine branches are not (or at least not usually) grafted onto plum trees. Thus, even if your tree was grafted, it was probably grafted in a permissible manner.
Since we do not want “to go out on a limb” botanically, let us work with your assumption that it is possible that your tree was grafted in a forbidden manner. (In most cases, someone with horticultural experience can tell you if it was grafted at all.) The Chazon Ish[16] writes that if one is unsure whether a grafted scion and the rootstock are (considered) of the same species, it is permitted to keep the tree and cultivate it. This conclusion is based on a halachic type of “divide and conquer.” On the level of Torah law, there is a s’feik s’feika (double doubt): Perhaps there is never a Torah prohibition to maintain an already grafted tree, and even if there is, perhaps the case before us is not one of forbidden grafting. Therefore, regarding Torah law, we permit keeping the tree based on the double doubt. On the level of Rabbinic law, although the Rabbis forbade maintaining an improperly grafted tree, one may be lenient regarding the single doubt of whether they are different species. If this is true, when one knows that the tree was grafted and the question is only whether the scion is considered the same species as the tree, one certainly may be lenient when it is not clear whether any grafting even took place to begin with. In addition, in chutz la’aretz, there is slightly more room for leniency.[17]