Was a bit annoying to get this. Doing the spell quest for completeness at 75, I one-nuked everything. Got the petals really fast, in about 5 mins. Then I cleared the entirety of the tunnels and was twiddling my thumbs when I decided to go to the southwestern area and clear there, just to see. And I can confirm it does drop there, and much easier spawns to kill, since there are big clumps of them just standing there, an easy nukefest.
Long story short: go to southwest open area for this drop.