I've been thinking about a herbalist as a companion character. Not a mage -- nothing to do with Hermetic Magic and Herbam -- but the mundane medicinal (and other) uses of plants and plant extracts.

Is this covered anywhere in the RaW? I thought maybe in City and Guilds, but no ...

There is a Mythic Herbalism major virtue in HoH: Societas. It is in the Ex Miscellaina chapter p 125.

The virtue allows you to creat potions to heal , to poison , and to bgive physical bonuses to the imbibers. So your herbalist could create a potion that allows the imbiber to hold their breath for an exceptionally long time but could not create a potion to turn the imbiber into a fish.

It looks like what you are searching for.

In 4th edition, there was the Exceptional/Supernatural Talent Herbalism (and one for Alchemy). I believe it gave an Ability to add into Potion Lab Total, and some bonus for various types (poisons, healing etc.). It was an add on to Hermetic Magic, IIRC it could not do much - or was very vague rule wise - by it self.
Hedge Magic sourcebook had a Folk Magician with Greater Herbalism, where various potions could be brewed, with no affiliation to Hermetic Magic. It had Minor Potions, that took short time, and Major Potions for seasonal activities. The same boni as above were granted to various potion types. That book was a fine inspiration source for Hedgies and Supernatural Companions. (The book also featured a Natural Magician/Guild Alchemist, wh could do some comparable things with Greater Alchemy).

I believe the Mythic Herbalism from Ex Misc. can do much of the same. The Ability would propably be used in stead of Medicine (which is the Academic knowledge of a healer) to treat illness and serious injuries. This could make a good Mythic Companion, with many good supprt Virtues (Way of the Land, Purifying Touch, Immunity etc.)

Others have mentioned HoH: Societates, but have only pointed out magical herbalism. However, there is a description of an Ability called Profession: Apothecary, which covers the mundane craft of medicinal plants. The description of the Ability in HoH:S states that more will be said on this process in the forthcoming Art & Academe - not really useful to you at the moment, although it seems that your question will be answered!