It's stated that the damage is increased against slowed enemies. Does it include the root effect too? What about stun and immobilize?
But it does work with other peoples slow, like the wizards blizz?
If you have the choice between
Cull the weak and Archery - both give 15% more damage (given you are using a bow)
So it's obvious that you will always take Archery before Cull the weak. I think having a condition: (here) enemy must be slowed, should result in a little higher bonus. Like 20%.