Apothecarists used to grow much of their own herbs and traditional remedies.
If they couldn’t or didn’t have access to a particular herbal medicine, they’d of course go to market or a trader.
I would like to see the ability for Apothecarists to assign a single (or greater) square(s) to grow their own herbs rather than having to collect them/it (willow) from the wild.
As a side note, the greatest concentration of Salicin is actually found in the older, mature willow tree bark; but you could just use more of a lower concentration, so it’s still possible to use various subspecies, including bushes to achieve the same result