Cattail pollen makes a fair flour. You can also use the fuzz for insulation or starting fires. The pith - at least in dead, dried stems - makes a good fire starter. There are also multiple parts with medicinal uses.

I like sliced bamboo shoots. I plan to put some in a batch of brown rice and diced beef - with a couple of fresh-peeled cloves of garlic - this evening. I like alfalfa sprouts, but unfortunately local groceries stopped carrying them a few years ago.

Yes, they need soil. In the story the humans are on a planet where a huge natural disaster ruined the ecosystem a few centuries back. Native vegetation is recovering, but slowly. There's soil, it needs work in most places, but they are prepared for this.