Poisoner's Nook Plants don't seem to grow back
Solution 1:
The cell reset time in Skyrim is a whopping 30 days according to what inspection of the ESM is so far possible with existing Fallout 3 tools. So rest assured, the nook's plants will regrow eventually, it'll just take a long while.
For those who think this is way too long for streets to be cleared of bodies and plants to regrow, there is already a mod to set the reset time to 1, 2, 3, or 7 days: Skyrim Respawn.
Solution 2:
It is only 10 days, but entering the sanctuary before the time is up, will reset the time forcing another 10 day wait.