On one of the loading screens in Oblivion, the message reads: 'Ingredients taken from plants will grow back in later months.' Does anyone know how long they take to 'respawn', if they actually do so, that is? I ask because most things in Oblivion respawn within three game days, but harvested plants definitely don't.

By harvested plants I mean an ingredient you take from a plant which is just lying around on Tamriel, not the already-harvested ingredients you can find in alchemists' shops and the like.

Except for Nirnroot, plants do respawn after 3 days. Keep in mind though, you have a random chance of actually getting an ingredient when you try to harvest. So you might be coming up with nothing just from chance.

If you're still having trouble, make sure you have avoided the 'cell' in question for the three day period. Pick an ingredient. Go away for 3 days. Come back and pick it again.

A little more reference material: http://www.uesp.net/wiki/Oblivion:Respawning

Off topic, I really think the Arboretum should have what they have in New Sheoth (can′t remember its name), lots of different plants to pick ingredients from