The Living World

ICBN stands for the International Code of Botanical Nomenclature.
It is the set of internationally agreed rules and principles by which plants are given scientific names, so that one plant has only one correct name all over the world.
It is independent of the zoological code (ICZN, International Code of Zoological Nomenclature) and of the bacteriological code.
The code applies equally to the names of all taxonomic groups treated as plants, whether or not these groups were originally so treated.
Under these rules each name has two components, the generic name and the specific epithet (binomial nomenclature given by Linnaeus).
So ICBN is the International Code of Botanical Nomenclature.

According to the biological species concept (Ernst Mayr), a species is a group of individuals that can interbreed freely among themselves in nature and produce fertile offspring, and that are reproductively isolated from other such groups.
So the conclusive test for two plants to belong to the same species is that they can reproduce freely with each other and form seeds that give fertile progeny.
Similarity of appearance or of secondary metabolites is not conclusive, because different species may look alike.
Different species can have the same number of chromosomes, so chromosome number is not conclusive either.
A high percentage of similar genes is also found between related but distinct species.
So the answer is that they can reproduce freely with each other and form seeds.