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
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.)
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.