These are a great idea, however, it is quite frustrating that they appear as progress requirements on plot signs very early but how are they crafted? They appear nowhere in the tech tree. Is there a hidden prerequisite? Players do need to have some sort of pointer or clue as to how they can achieve the goal. If the requirement is introduced but a player can see no route to reaching the goal then they will likely give up.
Don’t forget the game is still in early stages of development. 
The tutorial for this is still missing and yes, there are no hints for this either. 
I agree:
future version of the game should explain somewhere, that those can be found by exploring. 
Go explore your island a bit. 
Alpha 5 has added a few new plants, that you can “knock down”.
These will give you the plants you need.
So you do not craft the plants, but rather collect them in the world.
Once they are in your inventory, you can put them on the hotbar
and then select them to place them in any pot.
And yes, you have to build one of the plant pots, too.
Many thanks. Having different plants appear was lovely to see. I did not want to chop them down - silly me. 
I looked for an interaction hint, but could not interact. Oh well!
I was not concerned for myself because this is, as you say, very early days, but, I was concerned that some sort of hint was missing and something, needs to be provided to give players a clue. It’s a good idea that there should be some sort of logic route (for other things as well) rather than direct research. Quite how to achieve the balance I am not quite sure. (Maybe, one of the incomers could bring one of the potable plants with them and place it in a pot in the camp area. Just a thought. Or, how about a small circle of soil around the potable plants and a researchable shovel.)