Yeah, you have to consider that when you're living in nature you actually need to identify the plants that are edible so you can't actually just eat everything you find even if you could eat it in theory.

So if you wanted to gather plant food how do you make sure that the food you find is edible?

It's quite simple. If you know a plant is edible you take the seeds and you carry them with you and broadcast them all over the place.

The interesting part about this is that this is also a form of selective breeding, since you're choosing the edible plants over the inedible plants.

Note that you do not tend to the plants or do any form of agriculture, you're still foraging from random plants you encounter.