After much experimentation the best quest ive found for these is part 6 of tangleroot. After completing it you can run it again and again and again....(just dont talk to Ungurz to update your quest line). Ive been running this and have found multiple sets of the 9 I need for the force ritual. Just take someone who can,
a) run fast, or
b) kill everything
I take my monk cause nothing hits her, she has evasion and she runs fast
Collectible Locations:
1. By the first set of spiders I take a right to the oozes at the end of the path. In parts 1-4 there is only 1 mushroom collectible here, but from part 5 onwards there are 2.
2. I follow the path around to where the bugbears spawn, veer around to the right, there is a spider and an adventurers pack on the gound, it can spawn in this pack too. (Lightning-split soarwood also drops here)
3. I generally run past everything for a while til i get to the big square room, there is a cabinet on the left side of the room where the Lightning-split soarwood can drop.
4. I run past the glass spiders that spawn in the hallway leading out of this room. Instead of taking a right to the end of the quest, I go left. There is a side hall with spiders and a mushroom.
5. I backtrack to the main hallway and keep running anticlockwise...the whisperdoom hallway has 1 mushroom.
6. I then head towards the shrine, in the first part of that hallway there is 1 to the left before the shrine.
7. I then go through the two traps (dragging my mobs behind me ) and there is 1 more mushroom by the chest at the top. I then jump off the edge, run to the end of the quest and go into the second part. I finish out from there without completing.
Now, I'm not saying that the fragrant drowshood will drop everytime at every location, but on average you should get 1 or 2 per run.