What is the bestiary equivalent for plants?

"Bestiary" comes from the French "Bestiare" which itself comes from the Medieval Latin, Bestiarum Vocabulum where bestia (beasts) + -arum (genitive plural of) and voc[a] (to name/call/summon) + -bulum (noun suffix denoting vessel/instrument); And roughly translated "Instrument for Naming Beasts." I am not a teacher of the Latin language, so if you'd like to get more accurate, then see to them. But from my understanding that gets you close to the roots of the word "Bestiary" which might help you decide on what name you want to consider.

Now, the more commonly found labels for Latin Bestiaries would be the phrases liber bestiarum ("book of beasts") and liber animalium ("book of animals"). Just as there were books for beasts and animals, there were also treatises and books written for the trees, shrubs, and herbs, such as the one written by the 16th century Parisian, Robert Estienne (Latin: Robertus Stephanus), titled "Liber de Latinis et Græcis nominibus arborum, fruticum, herbarum, piscium, et avium […]" (or the "Book of Latin and Greek names for the trees, shrubs, herbs (grass, weeds), fish, and birds […]").

Now the key word to start working with is herbarum (which is the genitive plural of herba (herb, grass, weeds)). If you had a book on herbs, you would have [liber herbarum] or a collection of herb names, [herbarum vocabulum]. Unlike Bestiary, there isn't a French word to bridge from herbarum to something like Herbiary as there was with Bestiary (bestiarum > bestiaire > bestiary), but that hasn't stopped there being from being limited cases of historical precedence for referring to a/the collection of herbs and herbage as a "Herbiary," but the usage is very few and far between (at least searching Google Books).

You could refer to this Bestiary Equivalent as a "Herbiary," but you'd be doing so only with creative license. It otherwise isn't a truly recognized word in any dictionary, or as a commonly used word in the English language.

Now if you're trying to title your own book, you might try other name combinations using 'Herb' or 'Herbal,' such as "The Book of Herbs," "The Herbal Companion," "Herbal Anthology," et cetera.