מדרש על ערלה 1:2

2) "when you come … and you plant": to exclude (from arlah) what was planted by the gentiles before they entered the land. — But perhaps I exclude what was planted by gentiles after they came to the land! It is, therefore, written "every tree" — whence they ruled: What our ancestors found to have been planted before they came to the land is exempt (from arlah). What they planted, even before they had conquered the land, is subject (to arlah). "and you plant": to exclude what grows of itself. "and you plant": to exclude (from arlah) what is engrafted and what is sunk (in the ground to grow as an independent plant) — whence they ruled: The connection of grape vines and the engrafting on an engrafted part, even though it is sunk into the ground, is permitted (vis-à-vis arlah). R. Meir says: If (he sunk into the ground) a healthy (self-sufficient) engrafting, it is permitted; if, an unhealthy engrafting, it is forbidden. This tells me (as being subject to arlah) only of his planting a nut and an almond. Whence do I derive the same for planting a young shoot (thereof)? From "tree" (including a shoot thereof). "food tree": and not a barren tree. "food tree": to exclude what is planted for fencing, beams, and fuel. R. Yossi says: Even if he said: The inner is for food and the outer for fencing, the inner is subject (to arlah), and the outer not.