Okay, I got two answers, the philosophical one might not be as philosophical

-- The Norman answer;
Normanny, in order for the grass to be natural, it needs to have been watered by clouds. So, that means if you ever decided to visit some random seed, and pissed on it every day. It will grow unnaturally, it'll probably die anyways due to the piss.

--philosophical;

So, I've been googling, and there's this thing called "Natural laws" but it's related to humans, there's also another thing saying green is a natural color. So if we went by colors, technically hulk is natural, and so is the grass. This is kind of a half-assed answer, so I'm thinking of researching into this more, but not many people talk about what defines "natural" in philosophy, or its that I just studied math, so my mind was pretty fucked up while researching (nothing against math, though, they just dont fully explain the equations its annoying when they dpnt RAGHSHSH)

So there it is, grass is unnatureal if you grew it using piss