In Oblivion, Nirnroots were a collectible - there was a finite number of them*, and they did not respawn.
I've been noticing, however, that when I pick Nirnroots in Skyrim, the roots of the plant remain behind. This got me wondering...
Do Nirnroots regrow in Skyrim?
What about the Crimson Nirnroots of Blackreach?
*discounting the respawning food-nirnroot in Frostcrag Spire
22 Answers
Nirnroot does indeed grow back and can also be bought from Alchemy stores. It is a lot less rare than in Oblivion. With regards to the Crimson Nirnroots, I am not sure, because I haven't come across them yet. However, I believe they would re-spawn too. (All alchemy Ingredients I have seen so far do.)
EDIT:
I just wanted to confirm that crimson nirnroots do indeed regrow after 15 in game days, as seen here.
A quick warning: do not pick to many crimson Nirnroots before starting the quest, as it has been reported that this will cause negative progress through the quest.
6I can confirm they grow back after 15 days also. I'm using the PS3 version and picked about 20 of them before doing the related quest. It does give you a negative number if you try dropping them and picking them back up. So, I was forced to wait 15 days to pick a total of 50 of them.
