One of the central tenets of Tolkien's mythology is that a sentient (he would say "rational") creature must have a soul. Elves have souls, Men and Hobbits have souls, Dwarves have souls, rabbits do not.
So in order for a tree to "go Entish", as Treebeard says, it must acquire a soul from somewhere.
As far as Huorns go, we're never given an explicit confirmation of their origin; but there are two guesses.
Ents Going Treeish
This is Merry's theory; from The Two Towers:
Treebeard won't say much about them, but I think they are Ents that have become almost like trees, at least to look at.
The Two Towers Book III Chapter 9: "Flotsam and Jetsam"
If this is true, then we have to ask where Ents come from. The best information I've been able to find on that subject is from a note at the end of Letter 247, where Tolkien writes (emphasis mine):
No one knew whence they (Ents) came or first appeared. The High Elves said that the Valar did not mention them in the 'Music'. But some (Galadriel) were [of the] opinion that when Yavanna discovered the mercy of Eru to Aulë in the matter of the Dwarves, she besought Eru (through Manwë) asking him to give life to things made of living things not stone, and that the Ents were either souls sent to inhabit trees, or else that slowly took the likeness of trees owing to their inborn love of trees.
The Letters of J.R.R. Tolkien 247: To Colonel Worskett. September 1963
This letter presents two possibilities, but neither seems open to the idea of newly-planted trees becoming sentient:
- Ents are trees with souls, in which case the only way a newly-planted tree can become sentient is by acquiring a soul; this is probably impossible, unless Eru really, really happens to like that tree
- Ents are tree-shaped spirits, in which case they're not actually trees at all, and are unconnected to them; they just become more like trees over time, until eventually they go to sleep forever
In which case, you can't plant a tree and expect it to wake up and start talking to you. That's just not how it works.
Then again, that's only one guess.
Trees Going Entish
Treebeard admits this as a possibility:
Some of us are still true Ents, and lively enough in our fashion, but many are growing sleepy, going tree-ish, as you might say. Most of the trees are just trees, of course; but many are half awake. Some are quite wide awake, and a few are, well, ah, well getting Entish. That is going on all the time.
The Two Towers Book III Chapter 4: "Treebeard"
As I said before, the only way for a creature to become rational is to acquire a soul; but only Eru can make souls (or provide a mechanism for their creation, such as the begetting of children). So it has to come from somewhere.
The most plausible explanation is that these trees are being inhabited by pre-existing spirits, possibly of a lower order than the Valar or Maiar. In The Silmarilion, Manwë says:
When the Children awake, then the thought of Yavanna will awake also, and it will summon spirits from afar, and they will go among the [fauna] and the [flora], and some will dwell therein, and be held in reverence, and their just anger shall be feared.
The Silmarillion III Quenta Silmarillion Chapter 2: "Of Aulë and Yavanna"
Presumably, then, these "trees getting Entish" are trees that have attracted the attention of some spirit floating around, and they decide to inhabit it and take it for a spin.
Which is correct?
I've presented two theories, but unfortunately they're equally possible and Tolkien himself never comes down on which one; he wrote precious little about the Huorns.
It is worth noting, however, that Treebeard's knowledge is not infallible; Tolkien writes in the draft Letter 153:
Treebeard is a character in my story, not me; and though he has a great memory and some earthy wisdom, he is not one of the Wise, and there is quite a lot he does not know or understand.
The Letters of J.R.R. Tolkien Letter 153: To Peter Hastings (draft). September 1954
So we really don't know.