flax is too long/heavy compared to other stuff. straw is also too heavy
one flax seed gives you an average of 8 stems and due to length, you need chests or wardrobes to store them. that means that we need a chest for every 15 tiles or a wardrobe for every 30.
each chest can hold enough stems for 2 hanks of linen (which reduces that weight to 2 stone, which is a bit insane), but it gets better!
to make one rope, you need 5 hanks of linen (=5 chests)
to make one linen cloth, you would need 25 chests for flax storage.
i dont think this is balanced when compared to the rest of the storage system.
short term suggestion: make flax lighter and shorter. i would suggest .5 or 1 stone instead of two, and shorten it to be able to put it into crates.
long term suggestion: implement a 100 farming ability to make a container called "bale".
material could be 2 rope, size should be big enough to store flax for 5 linen cloth and of course it just holds flax or straw. also, everybody should be able to loot them.
alternatively (or additionaly), barns could function as secure storage houses for flax (and straw)
edit: fixed mistake of assuming 10 stems = 1 fiber, when in fact 10 stems = 10 fibers.