The Woody Plant Ontology lists all variables used for woody plant observations. Terms are collected from various sources (past and ongoing projects at national and international levels). Reference Plant Ontologies are being developed in the context of the Planteome project and are being integrated with the Crop Ontology, which provides harmonized breeders’ trait names, measurement methods, scales and standard variables for many crops. The Woody Plant Ontology is one of these reference plant ontologies integrated within the Crop Ontology.