Do plants grow back?
Solution 1:
Night Blossoms seem to be the only plant that grows back. I've been to Crazy Joe's (I keep wanting to say Crazy Dave) swamp a few times and the Night Blossoms have reset. Comet Bloom and Desert Spores must be found, and I have seen no evidence that they regrow.
I don't know if the Night Blossom's regrowth is due to a timer (real world or in game) or tied to story completion. However, it doesn't really matter as Night Blossoms are the only plant you can purchase (in Subway Town) in the game.
Solution 2:
Comet Bloom and Night Blossom appear to randomly respawn when you reload the area. I found you can regularly find Comet Bloom outside the Ghosts' hotel near where you start and Night Blossom in Crazy Joe's swamp.
Both are used in recipes that last 60-90 seconds, so I guess they should be replenishable.
Desert Spoors are only used in that one that permanently increases your health, so don't respawn.
Solution 3:
Some plants do not grow back. This is most likely due to the fact that they can be used to
engineer an item that permanently increases your maximum health
later in the game.