I don't mind there being a restriction on mountains or other areas; I just would like an advance warning before building and, as I said, consistency: there are other plants there, so the restriction seems arbitrary. If the area (and yes, this is a mountain--well, Volcano) is meant to be infertile, then shouldn't there be no wild plants for me to gather, either?
Plants can be farmed in the biomes in which they grow, and no crop, fruit or tree plant has the "mountain" biome enabled, only non-grain grasses.
Gatherable plants grow = plants can be farmed
no plants can be farmed = no gatherable plants grow
I think you just ended up missing the biome border. You cannot embark on a mountain-only map, and borders between biomes can be difficult to spot. Apparently you gathered plants in the non-mountain biome, then built a farm plot nearby but in the mountain biome.
Re: Suggestions -
i agree, when building an aboveground farm plot, it'd be good to have a "no farming possible here, why even try" warning.
In addition, changing mountains around so that flat places were likely to accumulate some soil and became able to grow farmable plants and trees would make for better mountains. Great heights, steep slopes and particularly arid or exposed terrain could reasonably grow only grass or be entirely bare, but not entire mountain ranges, invariably.