This is a question I would like answered, as I don’t believe I’ve paid attention if there were new bush spawns or young tree spawns ever while playing.
No, they do not respawn naturally. Underbrush refreshes every season though.
True. As Trigon said, they don’t (re-/)spawn. I’d like to add that foraged underbrush resets themself when a new season begins, but only if they are not in the reality bubble (at least for 10342 and all versions before, as far as I know) during the season change. I think the same is true for fern, but I’m not sure.
Both types don’t regrow (at the moment) once destroyed, however.
Although… Queen triffids can (and will) generate new plants (even trees) around them, so I guess that’s one way to get new plant matter.
There are also artifacts that do the Triffid Queen trick, but you have to get pretty lucky to find one.
Oh really, now. . . . I didn’t know that. Is that effect still current?
Yep, it’s AEA_GROWTH in artifact.cpp.