When I tried to turn in the quest A Return To Your Roots, all Avrusa would say was, "Just go away please." I searched online and found people suggesting dropping all the nirnroot then picking it back up again and other similar things. They also said it's important to make sure you have all 30 of the plants. Dropping didn't help and I had over 30 of them. But I did get it sorted!
In fact, I had 31 of the plants. I gave 1 to my follower and voilà! She was willing to talk to me.
TL;DR: make sure you have the exact number of plants needed, no more, no less.