Orlah, Chapter 1, Mishnah 2. The halachah is that every fruit tree is subject to orlah, even if it came up on its own and even if it was planted by a non-Jew; there is no requirement that it specifically be planted by a Jew. That said, the mitzvah of orlah took effect only when the Jewish people entered the Land of Israel.
Entering the Land as the Start of the Obligation:
Therefore the Mishnah states: "Mishebau avoseinu la'aretz" - at the time our forefathers entered the Land of Israel in the days of Yehoshua bin Nun, "mah shematz'u natua - patur". Any tree that was already planted at that time is exempt, and the laws of orlah do not apply to it, as the verse says, "When you come into the Land" - meaning after the entry into the Land.
By contrast, "mah shenat'u, af al pi shelo kivshu - chayav". Even if they had not yet conquered the Land of Israel - and this is essentially a historical matter - the obligation applies, since the entry into the Land itself is the criterion, and there is no need for it to have actually been conquered.
Planting for the Public:
From here the Mishnah moves from the historical matter to one with practical implications: "Hanotea larabim - chayav". One who plants a tree for the public and not for his own personal fruit has, as it were, renounced ownership of the fruit and made it ownerless, and nevertheless he is obligated in orlah, even though the planting is not for his personal use. "V'Rabbi Yehudah poter" - Rabbi Yehudah disagrees and holds that a planting that is not for personal use is exempt from orlah, and he derives this from a verse.
Cases That Are Obligated in Orlah Even Though One Might Have Thought Otherwise:
"Hanotea birshus harabim" - one who plants in a place that is not his own, in the public domain.
"V'hanochri shenata" - a non-Jew who planted a tree.
"V'hagazlan shenata" - one who stole land and planted in it.
"V'hanotea bisfinah" - one who plants inside a boat.
Planting in a Boat:
In the case of a boat there are several possibilities, and first we must define what a hole in a boat is: a hole that is stopped up with clods of earth to keep the boat from sinking.
An earthenware boat: since the roots would naturally penetrate through the earthenware and could draw nourishment from the riverbed, it is obligated even if there is no hole in the boat.
A wooden boat: it must have a hole that is stopped up with clods of earth.
It is worth noting that there is a dispute about this: some say that it makes no difference whether the boat is of wood or of earthenware - either way it need not be perforated.
"V'ha'oleh me'eilav" - a tree that came up and grew on its own. As mentioned above, all of these are obligated in orlah, meaning that the obligation of orlah applies to them.
In summary: in this Mishnah we learned that the obligation of orlah began with our forefathers' entry into the Land - what they found already planted is exempt, and what they planted is obligated even before the conquest; that one who plants for the public is obligated according to the Sages while Rabbi Yehudah exempts him; and that one who plants in the public domain, a non-Jew, a thief, one who plants in a boat, and a tree that came up on its own - all of these are obligated in orlah.