A herb garden represents an area of land where herbs grow. A herb garden can be of almost any size - it could be a tiny personal garden in an Urizen spire, meticulously tended and cared for, or it could be a vast forest in Wintermark or Varushka that a character scours in search of wild herbs.
It is possible to obtain a higher yield from a herb garden. You may spend 6 crowns to have the herb garden produce an additional 3 true vervain, 1 cerulean mazzarine, 1 Imperial roseweald, 1 bladeroot and 1 marrowort in that season.
A herb garden can be upgraded using Weirwood obtained from the Bourse. Each time a herb garden is upgraded, the owner chooses a single herb from the list. The herb garden produces 2 additional doses of that herb from then on.
To upgrade a herb garden requires Imperial wains of weirwood equal to the level the herb garden is being upgraded to. So the first upgrade of a starting level 1 herb garden costs 2 Imperial wains of weirwood, from 2 to 3 costs 3 Imperial wains, etc.