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.)
That sentence made me think about the future of the game
and how this could improve the villagers happiness 
Right now we just look at the plot-sign of the home and it tells us what to build.
But we never see anything improve, apart from the happiness indicator when the day ends.
I believe this decorative item could be introduced by a villager,
directly coming to us with an request (approaching us, wanting to talk)
as soon as we upgraded the first house to the level right before the plants. 
So the next day (after the house upgrade), one villager wants to talk to us,
telling us that some plants would make him happy (and his home look more alive).
And by telling that to us, we will just then unlock the next requirement on the plot-sign.
So I think the plants should only appear on the plot-sign, after the villager talked to us.
The villager could say something like:
“I would love to have some plants in my home. Could you gather some for me?”
… this way we know what he wants and the player gets asked to look around the area
to collect the plants (and later place them in the home, to unlock the next upgrade bonus).
I guess this is something for @Daniel to think about and maybe add to his long list of things to do.
Nice ideas for sometime in the future. It will be interesting to see what direction Daniel has in mind to take this.
Darn, right after your message there was a log right where I wanted to plant a tree. Instant plant.
Indeed it needs to be explained!
Thanks for the discussion, I took a note 