-Have a level 2 Herb Garden with a follower assigned to it and have them only plant Frostweed.
-Invite a friend to your garrison, they will be able to pick all of the herbs.
-The herbs will respawn after a minute or two, and they will be able to pick them again.
-Rinse/repeat as needed, it seems that after the owner of the garden picks one of the herbs they will no longer respawn.
I was unable to pick anything except Frostweed, including ore, so I'm not sure why this one works. Enjoy!