Like the rest of the Survivalist Challenges, this one is especially tedious. My method was to write out a check list of the 10 plants, then check them off as I collected them. Make sure to take advantage of Survivalist Maps to make the plants pop up on your radar and map. Use the stage coaches / camp fires to get around the world quickly.
I have already picked up 10 Violet snowdrops plus 1 Humming bird sage in Tall tree just can't found the last one, any ideas?
-Head out to Tall Trees, right around where the snow line begins. USE A SURVIVALIST MAP and look for the plant symbols to pop up on your radar! Most of the Hummingbird Sages are in that area (both in the snow and south of it). When you start seeing the Violet Snowdrops, turn back south.
Don't make a list like the first poster said. Instead, while the challenge is highlighted in the start menu press triangle or Y and it will list all the herbs and the amounts you have of each. I personally did not find this challenge hard, just keep buying survivalist maps every time you stop at a store.
Sorry for last post I was thinking of rank 9. For rank 10 several flowers are found just outside of Chuparosa. wooly blue curl, prickly pear, butterfly weed, and another but can't remember the name right now. There are two of each.
Also when you are in Mexico, if you use a survivalist map in Diez Corornas you can rarely find Desert or red sages, i can't remember which ones.
http://reddead.wikia.com/wiki/Survivalist_Challenges, you can use this to help tick off the plants you need in order and remember when using survival maps to press down on the D-pad to enlarge the minimap and see more plants in your radius