Planting herbs would make it so easy to make flux though and make steel no longer the large commodity it is. So either herbs are not farmable or that it should take a long time or there to be a limit on the quality of the herbs.
I dont think so... i
I highly doubt you will mass produce flux this way. These "gardens" will be use to moustly produce high quality herbs for alchemical use(potions).
Not to mention that these gardens(alchemical ones not decorationals) should be hard to maintain, these are wild herbs after all. So i doubt people would be able create so called "flux fields". Of course small procent could be able to create more high quality flux for example better quality steel.
Not to mention time required to grow these herbs and amount of work it would need.