Entwives

[The Might]

I think there might be something to Aganzir's idea.
It may well be possible that Ents took the shape of the tree that they guarded as for example Quickbeam who is said to have guarded Rowan trees also resembled them.

Due to their many animal-like characteristics I would speculate that the first spirits that became shepherds of the trees belonged to animal kingdom, looked plant-like (probably to blend in with the other trees) and could assume the form of the trees they guarded.
As such I believe breeding would be pretty much like in most other animals.

An interesting quote for this is:

... 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. ~ Letter #247

Considering the infromation we get from the books I personally have to go with the second theory.

For more informatin I recommend the quite nicely done essay on Ents by Ardamir.

Before reading the essay I was myself quite sure that there were no more Entwives, though after doing so the idea of some Entwives having managed to escape Sauron's attack seems to me if not likely or probable, then at least possible.