Originally posted by jonnin:

wait a second.
seed onions don't pick the bulb, they sprout a pretty flower and you just pick the tall plant and it spits out seeds. If it has a bulb, its a food onion, and that should poke above the ground, turn brownish, and then you can pick it. If its white, its not ripe.

when the seed onion matures, some plants sink into the ground by themselves, I didn't even interact with them.