Cultivated plants may be described from various viewpoints: botanical, agronomic, agricultural and more. These viewpoints often result into different specific formal representations (i.e., ontologies). Linking concepts describing these different viewpoints is difficult and demands domain expertise. Still it is necessary as it supports the agricultural planning processes. In our case, there exists no standard knowledge pattern to represent alignments between thesauri describing cultivated plants in agriculture and organism taxonomies or classifications; in addition, basic ontology mapping properties (e.g., from SKOS) are not sufficient. We have conceived the Crop Planning and Production Process Ontology (C3PO) to describe agricultural knowledge for diversified crop production. In this paper, we describe the ontological representation of the Plant module of C3PO, which addresses the aforementioned linking needs. It integrates crop usage information about cultivated plants from the French Crop Usage thesaurus and botanicalclassification and nomenclature-information from the TaxRef taxonomy. This Plant module is valued in two systems both developed by Elzearda french SME: (i) a web application to support farmers in crop planting activity; and (ii) a web portal, La Serre des Savoirs (under development), which will share general agricultural information about crops.. The C3PO ontology and its knowledge graph are publicly available at https://gitlab.com/serre-des-savoirs/c3po.