Pheno-hunting: the criteria that define a good phenotype

Terpenes are the volatile aromatic compounds of hemp: myrcene for herbal notes, limonene for citrus, linalool for lavender, beta-caryophyllene for pepper. A breeder assesses a candidate's terpene profile first by nose, by crushing a leaf during vegetative growth, then by chromatographic analysis of the dried product. Two plants from the same cross can show very different dominant terpenes, which explains the aromatic diversity observed between closely related varieties.

In France, the THC content of the finished product must remain at or below 0.30%, in line with the framework confirmed by the Conseil d'État decision of 29 December 2022. A phenotype of interest for CBD hemp production therefore shows a high CBD/THC ratio and, above all, one that is stable from one harvest to the next, rather than a one-off CBD peak. This stability is verified over several growing cycles before a variety is considered reliable for commercial production.

Beyond aroma and cannabinoid profile, a selected phenotype must resist the main threats to hemp outdoors or under glass: powdery mildew, grey mould (Botrytis) at the end of flowering, and water stress. A plant that requires repeated plant protection treatments to survive is of limited interest for large-scale production, even if its aromatic profile is remarkable. Yield at harvest, expressed as the weight of dried flower per plant, completes this agronomic assessment.

A plant that is too tall, or whose branches are too thin, complicates harvesting and drying. Breeders also assess the overall architecture: short internodes, a compact structure, dense inflorescences but not so tight as to raise the risk of internal mould. These criteria, less spectacular than aroma, weigh heavily in the final choice of a phenotype intended for repeated production.

In France, a variety may only be grown commercially if it appears in the common European catalogue or in the official French catalogue. The selection work described here therefore takes place upstream, at the breeders, before any registration. Our article on the stages of pheno-hunting sets out the selection process itself, from germination to cloning the selected plant.

Can the terpene profile vary depending on growing conditions?

Yes, to a certain extent. Genetics set the aromatic potential, but light, temperature and the timing of harvest influence how terpenes are actually expressed in a given plant.

Why does the stability of the CBD/THC ratio matter more than a one-off peak?

Because commercial production must guarantee constant compliance with the legal threshold of 0.30% THC. A phenotype whose ratio varies sharply from one harvest to the next presents a risk of non-compliance.

Does disease resistance take priority over aroma?

Neither takes priority in absolute terms: the breeder is looking for a compromise. A phenotype that is exceptional aromatically but very vulnerable to disease is rarely selected for repeated production.

How long does a full assessment of a phenotype take?

Several growing cycles are generally needed to confirm the stability of the criteria observed, notably the cannabinoid ratio, before a phenotype is considered reliable.