Herbalism has been important throughout all cultures of the world and history. The knowledge of use of plants as medicine both has been blessing and curse to the people who share their knowledge. Many were revered for it, and many were killed for it (being accused of witchery, sorcery, devil worship).
Völva were known herbalists. Here is some good information about what plants they probably used on a normal basis. Herbalism would not only be practiced by Völva, but likley common knowledge, as many parts of plants would be used for food, and dying of cloth.
One interesting thing I’ve read in several sources, and while a bit odd to us, very clever, was broth. If a healer could not determine if a wound was fatal (internal injury/bleeding), they would make a very strong broth of onions (and possibly herbs). Essentially, something you could not mistake the smell of. Then they would give it to their patient. After drinking, the wounds would be smelled. If the smell of the broth was detected, it was known that the wound was fatal.