Dwarf Fortress

Title really says it all. I am a new player. Every time I try starting a fortress and I get to an underlying cave.

I start a farming plot. Set them to plant plump helmets only for all seasons. I make sure they (my dwarves) don't use the spawn as cooking material. And still they seem to be harvesting them without getting any new or extra seeds back in return.

What gives? Am I forgetting something here?