I am looking for the bestiary equivalent for plants. But I don't want to use 'compendium' or 'herbal'. Is there such a word? If not would it be 'okay' to use 'Herbiary' or should I more go to the something like 'The herbal compendium' then?
"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.
I've never heard of any book called an herbarum: not even Latin or Old English. I found the word "plantum" used. "Flora" would pertain to the plant life in a particular area (more localized.) Botany and medicine were so intertwined for centuries that almost any book on herbs or plants also had to do with medicine.
For centuries, even today, because some plants are very difficult to differentiate (even with full scientific descriptions); botanists, herbalists, gatherers, growers, and medical men have kept what are called "herbariums." These are collections of the actual plants, ideally showing all the parts as it grows during the year, pressed and attached to paper with as full a description as possible. This would include the area in which it was found, manner in which it grew, companion plants, colors, really anything and everything that would help identify it. I have done several plants. I can tell you that no photo, video, drawing, or rendering can come close to the security one feels in identifying a plant, armed with an herbarium.
I see now that herbarum is from the French. I still cannot believe that I have not heard this word, working with herbs for many years. Probably people don't want it mixed up with "herbarium." I shall forever treasure my few specimens.
You could Shakespeare "Floriare" depending on how scientific you wanted to go... lol good luck!