Wikipedia defines a herbarium is a collection of preserved plant specimens. These specimens may be whole plants or plant parts: these will usually be in a dried form, mounted on a sheet, but depending upon the material may also be kept in alcohol or other preservative.
The term can also refer to the building where the specimens are stored, or the scientific institute that not only stores but researches these specimens.
A waymark for this category should be a repository for pressed, dried and mounted plant specimens used in scientific research. The herbarium must be an herbarium acknowledged by the International Association for Plant Taxonomy (IAPT). For more information on the IAPT see the following: (Link) (Link).