Both Golden Currant and Burdock Root can be found north-west of Annesburg either side of 'ANOK' in Roanoke Ridge on the map. Specific areas where these spawn are Fort Brennand and along the river around the Old Tomb.
Yarrow can be found in good amounts near Rhodes, north of Bolger Glade.
Crafted with an existing throwing knife and Oleander Sage.
The pamphlet to make it can be found under the floorboards in an abandoned shack, North of Saint Denis cemetery, near the train and roadways.
Poison Arrow.
Crafted with an arrow, a Flight Feather and one Oleander Sage.
The pamphlet to make it can be found in a lockbox located outside the Mysterious Hill Home. Alternatively it is the reward for completing the bison hunting companion activity with Charles in chapter 2.
Rank 9
Pick one of each species of herb.
Reward: 150 Health XP and $20
There is no in-game checklist for this. It is highly recommended to make one and keep track manually. Anything found prior to unlocking this rank does not count so the compendium is not a reliable source of tracking.
If too many orchids are picked up before the fourth part of Duchesses and other Animals is completed, some species will not spawn (collecting one of each doesn't cause any problem).
Obtain the Legend of the East satchel as soon as possible, as this allows you to carry up to 99 plants of any type at the same time.
Collect any and all Burdock Root, Yarrow and Currant you find while doing the early challenges, as this makes Challenge 7 much faster to complete. The same applies to mushrooms, Sage and Indian Tobacco, although the challenges that require these plants aren't nearly as time consuming.