Is one allowed to plant a fruit tree bought online it that tree produces fruit that came about from a hybrid (e.g. a grapefruit)? Also a fruit tree like a Cherry, which is not a hybrid, but is grafted on a different Cherry rootstock, is this permitted to plant?

ב"ה Shalom I consulted with a colleague from the Torah VeHa'aretz Institute T on this issue, who is more familiar with the issue of grafting. Basically he said that 98% of fruit trees are from some form of grafting, connecting a tree rich in fruit to a weaker tree. However, mostly they are from trees of the same species which from the Halachic standpoint, do not constitute the forbidden hybrid= Kilai'im". The same is the case with grapefruit which is grafted with some other citrus tree. According to the Chazon Ish (חזו"א, כלאיים סי' ג ס"ק ז) , citrus trees of different species are questionable if they are considered hybrid. In addition, even though the Shulchan Aruch (יורה דעה רצה:ב) says that a Jew may not allow a non-Jew to graft two different species, see Chazon Ish (חזו"א, כלאיים סי' א:א) who says that in a case in which we are not sure that it is a hybrid we perhaps can allow the hybridizing to be done by a non-Jew. ( See also שו"ת יביע אומר חלק ה - אורח חיים סימן יט, שו"ת משפטי עוזיאל כרך א - יורה דעה סימן כד, זרע הארץ הלכות כלאים ה"י (עמוד רנח) Also, according to the Chatam Sofer (שו"ת חתם סופר חלק ו - ליקוטים סימן כה) once the grafting is completed and is no longer noticeable, on may maintain the graft. Therefore, once the trees are already grafted, they may be planted. This of course, applies this year only outside of Eretz Yisrael, where one may plant. However, in Eretz Yisrael, we are in a Shemitta year and one may NOT plant. In addition, you may look at this very comprehensive and professional chart of permitted and forbidden tree grafting prepared by Machon Hatorav ve'Ha'aretz: https://en.toraland.org.il/beit-midrash/halachic-guides/mitzvot-of-the-land/kilei-ilan-grafting-scion-and-rootstock-combinations-for-trees-and-vegetables/ All the best