Do plants respawn after a certain amount of time?

Solution 1:

Plants do in fact respawn. Just outside of the starting village (to the left of the path Dennis leads you down to point out the first radio tower) there's a green plant in a small field; I have harvested it several times. I don't know what triggers respawning, or if the respawn interval is somehow fixed.

Solution 2:

Through my experiences, fast traveling and reloading the game allowed my to loot the plants again. Granted this was after a short period of time ( questing /exploring / etc) - nothing more than 5-10 minutes.

If you have a few fast travel locations available, I recommend going from camp to camp harvesting. By the time you come back around to the first camp you started at, the plants should have re-spawned.

Solution 3:

If you fast travel to "Hubris Farm" you will find 4 green plants close by. You can then fast travel to another location called "Hubert Shore" then straight back to "Hubris Farm" and the green plants will be there for the picking. Do this as many times as you want to load your rucksack.