What it does:

You've hit this bug, even if you don't know it. Every time you went to a place after the cell was supposed to reset, and wondered "hey why haven't those plants respawned yet?" it's this bug. When you go to Solitude and you see those Nightshade there and then you go grab a cup of coffee and come back and restart the game and they're gone, it's this bug. FloraRespawnFix.esp fixes this bug.

See, since the 1.3 patch there has been a bug in the Skyrim engine that it can't 'remember' that plants, mushrooms, nests, and some salmon have respawned, after you reload a save. So you might go to a place again, see that everything is as it should be (i.e. has respawned), then if you save and reload... poof! it's gone. You have to wait for the cell to reset again. You might not notice it unless you look for it, but I've seen it explained as "oh plants take 30 days to respawn" (they don't, they would normally respawn when the cell they're in resets - like anything else), or that certain plants just don't respawn (they all should), or whatever. Nope, bug. Most players have just gotten used to it.

Well, they don't have to get used to it anymore. This mod can't directly fix Skyrim.exe, but it can alter how these flora and fauna are processed by the executable, such that they are no longer affected by the bug.

http://skyrim.nexusmods.com/mods/31268