From: Rabbi Moshe Sofer
To: Rabbi Aryeh Leib
Jews in Hungary who purchased vineyards containing grafted almond and peach trees asked whether maintaining such kilayei ilan is permitted. The Chasam Sofer finds retroactive justification based on two grounds: Tosfos holds that passive maintenance of kilayim is permitted even according to R' Akiva, and according to the Rambam the prohibition of grafting in chutz la'aretz derives solely from a hekesh to kilayei behemah, meaning only active involvement—not passive maintenance—is forbidden. For future practice he recommends giving a non-Jewish partner a share in the fruit to avoid the prohibition entirely.
Source: Shu"t Chasam Sofer YD §288