According to the interwebs they are the the ultimate survival food. https://plants.usda.gov/plantguide/pdf/cs_tyla.pdf says that one can make 0.75 kg of flour from each square meter of cattails. This flour has 266 calories per 100 grams or 2000 calories per 750g.
I’m no farmer, but making flour from starch should just evaporate the water, leaving the nutritious stuff behind. Therefore, each farmed tile of cattail should yield 7-8 rhizomes.
The games says that there are 34 calories in flour made from cattail starch (iirc), 13g per serving.
750g / 13g * 34 cal = about 2000 calories. So, just the number of rhizomes per plant should be adjusted.
I might have missed it, but to my knowledge no indication about the size of the fields in game exist, so we can’t be sure that they are 1 m² (even though I do think of them as if they were) or that they are planted/growing in the required density, so it might be wrong to adjust the amount of rhizomes to get from one tile…
Do the generated cattails differ from planted ones?
This is relevant to my interests, so I did some Googling. It seems the size of a tile in CDDA is basically “whatever” unless it has since been more closely defined. If each tile is 0.5m x 0.5m that would be consistent with some stuff (but not with other stuff). For example, a road lane is a bit under 4m (3.56m in the UK) and that is 8 tiles in CDDA.