Uses of industrial hemp (Cannabis sativa subsp. sativa var. sativa.) have recently expanded beyond fibre and seed to include extraction of medicinally useful oil from flowers.  When growing industrial hemp as a source of cannabinoids like cannabidiol (CBD), the common practice is to plant only female plants.  This can be done by transplanting female plants to the field or by seeding feminized seed.  The process of establishing female only crops can be expensive, and the plants are still susceptible to being pollinated by neighbouring hemp crops that may contain males, or even from populations of wild hemp that grow throughout the province.

I’ve wondered how the timing of pollen shed differs in the industrial hemp cultivars that are permitted to be grown by Health Canada and, if differences did exist, could female only plantings be staggered to avoid peak pollination periods.  To address this, I conducted an observation trial over two growing seasons at the Simcoe Research Station.  Fourteen different industrial hemp cultivars, all from the Health Canada approved list, were seeded in June.  I monitored the plants to record when the first flowers appeared, and when the pollen shed began.  I also recorded plant height each week to get a sense of how the different cultivars grew. 

The trial included both common types of hemp.  Monoecious cultivars have both male and female flowers on the same plant, whereas dioecious cultivars have male and female flowers on separate plants.  Under some stressful growing conditions, particularly during periods of high temperatures and drought, dioecious female plants can develop male flowers which are capable of releasing pollen.  These plants are called hermaphrodites.

The conclusions of this study were that there is little difference between the cultivars in terms of days to first pollen shed, suggesting it may be difficult to stagger cannabis plantings to avoid pollen from neighbouring hemp crops and that, although the period of pollen shed was much shorter for dioecious hemp cultivars in 2020, there was no difference in 2021.  This suggests that growing conditions (2021 was much wetter than 2020), may play a role in how quickly the flowers of both monoecious and dioecious hemp varieties mature.  In some years, it may be that if dioecious cultivars are planted near outdoor cannabis fields, the exposure period of the cannabis to hemp pollen would be limited.

I also took weekly height measurements for the 14 cultivars in the observation trial. In general, all cultivars grew slowly for 2-3 weeks after emergence, but then grew rapidly for several weeks.  Monoecious cultivars like Altair, Anka and Ferimon, continued growing rapidly for a longer period than dioecious cultivars like CFX-1, Finola and Picolo, which results in the monoecious cultivars being taller by the end of the season.  On average, the dioecious cultivars reached their maximum height 2-3 weeks before the monoecious cultivars.  While taller plants may have more biomass, they are also more of a challenge to harvest, especially if the plants are being grown exclusively for flower production.

A second part of this study looked at the impact of pollination on cannabinoid yield.  It is commonly believed that pollination reduces the potency of cannabis flower oil.  My collaborators and I recently did a study to compare cannabinoid levels and yields in oil extracted from unpollinated and artificially pollinated industrial hemp flowers grown under identical growth chamber conditions.  Of the 13 cannabinoids analyzed using HPLC, the levels of 7, cannabichromene (CBC), cannabichromenic acid (CBCA), cannabidivarin (CBDV), cannabigerol (CBG), cannabicyclol (CBL), cannabinol (CBN) and cannabinolic acid (CBNA), were near or below the limit of quantification.  Total ∆9-tetrahydrocannabinol (THC), was present at concentrations below the legal limit of 0.3% (w/w).  The level of cannabidiol (CBD) in oil from pollinated flowers was the same as that from unpollinated flowers, but cannabidiolic acid (CBDA) and cannabidivarinic acid (CBDVA) levels were not.  This suggests that, although pollination changes the pool sizes of the precursors in the metabolic pathway leading to CBD production, cannabinoid production is not eliminated, and the level of CBD remains similar to that found in oil from unpollinated flowers.

https://onspecialtycrops.ca/wp-content/uploads/2022/03/Final-Report-Dec-2021.pdf