You want to argue for legalization of marijuana? Fine, argue for the tax potential.
Or argue that it never made sense to ban it in the first place, because it was never anywhere near as dangerous as the government made it out to be.
Or take a philosophical perspective to liberty and how severe ill effect should the be before we limit that.
I think that's exactly what "you shouldn't need a license to grow a plant" is. How is "you shouldn't need a license to do XXXXX" anything other than a "philosophical perspective to liberty"?
But the "It's a plant" and "You can't criminalize a plant, man" are just stupid.
Why? I think the burden's on you to give at least one reason why criminalizing a plant makes sense.
If you are saying that everything natural should be legal just because it is natural, you are arguing for cannibalism, murder, incest and numerous other things that do occur in nature but we prefer to keep illegal.
Too bad for you he's not saying that.
When arguing whether substance X should be legal or illegal is really quite irrelevant from whether it is created by growing plants or synthetizing it in a laboratory (aside from the "difficult to control" thing, which is whole another argument)
Maybe your problem is that you're thinking of it as a substance instead of a plant.