That's the big question everyone is asking cyclonic. They need to get a handle on how many thistles there are first. Its a brilliant idea given the savings that can be made in production etc. You could literally grow your own money. Of course that is also a cause for concern, people might take advantage by growing some. They will overcome this by marking legal thistle tender with dye. Anyone found trying to use illegal thistles will be receive the death penalty. Executions in town centres will provide entertainment for the public. I might move up there.
I hope they don't go imperial. 12 seeds to the thistle, 20 thistles to the weed. It's all too complicated.