Burdock Root is a plant in Red Dead Redemption 2 that can be used in crafting, you can sell them to make money, and you will also need to collect it for some Trophies. Burdock Root can be used to craft Special Bitters. Burdock Root is quite common in the game, once you know where to look. It should be noted, Burdock Root grows in the same places in Red Dead Online, so if you need the plant for a challenge in the multiplayer portion of the game, the below will still help you.
You can find Burdock Root along the banks of most rivers, but a great place to search for it is on the banks of the Dakota River to the southwest of Valentine. The plant seems to grow along the above area quite a lot, so it will be straightforward to collect here. If you are a long way from Valentine, you should be able to find it on the banks of just about any river; you might need to search for a while.
It is a good idea to grab all the plants in the game as you come across them because you never know when you might need them, and make sure you don’t sell your entire supply because you might suddenly find yourself in need of a plant you just sold.