We're talking about medicinal herbs, folks, so don't get it twisted.
Medicinal herbs in this novel are associated with independent, seemingly dangerous women. It's Anys and Mem Gowdie who are Eyam's resident herb experts before their untimely deaths, and it's Anna and Elinor who pick up where they leave off. As we know, not everyone thinks that this is a good idea. As Anna says: "I knew how easy it is for widow to be turned witch in the common mind, and the first cause generally is that she meddles somehow in medicinals" (2.2.50).
In this society, a skilled woman is something to be feared. Sigh. Those are some swell gender roles, huh?
Herbs thus represent a world that exists outside of certain societal constraints. They represent how the women we meet in the novel—Anna, Elinor, Anys, and Mem—must overstep the boundaries placed in front of them by their society in order to create their own sense of community and strength.