Use a,b,c,d to cycle through the seasons and set Plump Helmet for each one. Then when that season arrives they'll plant and harvest them all season around.
Wrong type of seed
But then, by the looks of it, Katsuun made the same mistake before you. Weird...
sorry for not completely explaing about what i was talking about,and i was talking about world gen
Have you modified the raws in your game in any way, or the world gen settings? The vast majority of changes to things in the raws will have a huge effect on all kinds of things within the game, which includes fucking up world gen seeds...and obviously changing from the default world gen settings will mean that world gen simply will not produce the same results for you as it will for other people.
One from the wiki that should definitely work for you (because the world gen info is provided for you) is
http://www.dwarffortresswiki.net/index.php/Pregenerated_worlds#The_Portentous_Realms - open up your DF folder, go into data, then init and open world_gen.txt. On the wiki you should see a section under The Portentous Realms that says "Game Object Data [show]". Click that and lots of text will appear inside a box. Highlight everything in that box from the line [WORLD_GEN] to the bottom and copy it. Paste it into the bottom of your world_gen.txt text file (there's no need to overwrite anything, even though you may notice that there's already a world gen titled "LARGE"). Save the text file. Load up Dwarf Fortress. Select Design New World With Parameters. On the right-hand side you'll see a list of the world gen settings that is taken directly from that world_gen.txt file you saved a moment ago. Assuming what I've mentioned is the only change you've made to this file, the options should be: Large, Medium, Small, Smaller, Pocket and Large. Select "Large" at the bottom, and press enter to gen the world.
Many seeds will require everything in your DF folder to be default, out-of-the-zip-file crap (or the person who posted the seed was too fucking incompetent to realise that any changes they made to files would mean that their wonderful seed wouldn't produce the same results for anyone else because they didn't bother mentioning all of their changes)
Just to highlight for you how much can change even by making one TINY change to a single part of a file...I had a map where I found a great location for myself. It was a 5x4 area, with a cave river, magma pipe and HFS. It had obsidian, it had flux (marble)...the only downside was that the only soil layer was peat, so I had no sand. So, I modded peat to be [SOIL_SAND]. Re-generated the world (I'm not a fan of reclaiming sites) and found the location again. This time I had red sand instead of peat (I was lucky it was sand at all), instead of obsidian and andesite I had rhyolite and diorite, and instead of marble I had conglomerate. Simply by editing peat to be considered as sand changed every single rock/soil layer across the world map that'd be generated using the exact same seed. Similarly, messing with any options in the world gen design parameters can alter the number of site rejections on a seed and result in getting a completely different world.
make sure at least one of your dwarves has planting AND gathering jobs enabled(otherwise, your Plump Helmets will wither and you'll have wasted the seeds)
Not that this is a thread for farming advice, but that's a load of bullshit if you simply choose to have all dwarves do harvesting (and ideally with that setup your farmers should NOT have food hauling activated so they can plant fresh seeds while the plant is being taken to a food stockpile. If your farmers have food hauling activated, they'll spend more time harvesting and hauling than they'll spend planting, which is a waste of time and resources)