Didnt work :/
I dug out a stockpile for seeds only and set max barrel to 0. There are now some bags, and all of them contain seeds of rat weed, rock nuts, sweet pods, prickle berries, wild strawberries, and fisher berries, but not plump helmets, dimble cups or pig tail. And guess what I have in those farm plots... thats right, plump helmets, dimble cups and pig tails, and none of the plants of which there are seeds in the bags.
First, make sure seeds are not allowed in any other stockpiles except this one. If they're being stored in a general purpose food stockpile 7 Z-levels away, it's undermining your efforts here.
Second, make more bags. You should most certainly have bags full of plump helmet spawn at this point.
another 169 plump helmet spawn to be found.
Edit: Found them, they were sitting in a bag in my dining room. I have no idea how it got there. But nobody is hauling them to the stockpile.
A dwarf got hungry, grabbed a plump helmet, and took it to the dining room. Upon finishing the meal, this generated and left a plump helmet spawn on the dining room table.
At some point after that, a job got generated to put the plump helmet spawn in a bag. Therefore, someone took the bag of plump helmet spawn to the dining room, and put the seed into it, and left it there.
In order to get the bag moved back to the seed stockpile, another job has to be generated and carried out. This can only occur if there's an empty tile in the seed stockpile. And then the usual hauling rules apply (some dwarf has to accept the job, go get the bag, and haul it back to the stockpile). If the dwarf who accepts the job is very far away at the time, it could take a while. Or, if all your haulers are busy, it could take a while for the job to be accepted at all.