The Biological Species Concept is what Mandrake was referring to (where 2 species can't have fertile offspring together)- however, this is most useful when studying animals, not plants.
The Morphological Species Concept is more useful when studying plants, due to both cryptic species and hybrid-swarm species (more plant taxonomists use the morphological species concept).
Multiple plant species can interbreed and can have fertile offspring, no doubt. Closely related species would likely be more successful than plants from different families (Psychotria is Genus level, so it can be done).