The distinction is tenuous for sure. But it has to do with whether the present location of the organism is the direct result of human intervention. Things like feral horses (in North America) and most invasive weeds are present indirectly because of past human activity, but occupy their present locations due to a predominance of factors other than direct human intervention. They are considered wild.
Throw a human-made enclosure around some of those horses, and they again become captive, as their current locations are now controlled primarily by human intervention. Release them from that enclosure, and other factors regain primary control and they are again wild (though still introduced non-natives).
For an individual planted tree, its location will always be the direct result of human intervention. The locations of its offspring, on the other hand, will generally be an indirect result, much like any other introduced population, maybe just fewer generations removed from their origin.
There are gray areas for sure. Like remnant alfalfa plants within the boundaries of a formerly cultivated but long abandoned field. There is no shortage of debate about such gray areas in existing topics elsewhere in the forum, though, so no need to rehash here. 