Timing of Pollen Shed for Industrial Hemp Grown in Southern Ontario

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 at the Simcoe Research Station this past summer.  Fourteen different industrial hemp cultivars, all from the Health Canada approved list, were seeded on June 8^th.  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 data in table 1 shows that dioecious hemp cultivars begin to shed pollen sooner than the monoecious cultivars, which indicates the monecious cultivars have a longer vegetative growth period and overall longer growing season.  The average start to pollen shed for the earliest flowering cultivars was almost 10 days sooner for the dioecious Picolo vs. the monoecious cultivar Joey.  On average, Picolo was the earliest flowering cultivar at 22.8 days and Ferimon was the latest at 44 days.  The data suggests there may be some flexibility in choosing an industrial hemp cultivar that flowers later than other common cultivars, and thus is less likely to be pollinated neighbouring hemp crops.

Table 1.  Number of days from seeding to when pollen shed (opening of male flower) begins for different industrial hemp cultivars.  Each cultivar was represented by multiple plants and data is presented for the first and the last plant to display male flowers for each cultivar.

Weekly height measurements for the observed cultivars are shown in figure 1. In general, all cultivars grow slowly for 2-3 weeks after emergence, but then grow rapidly for several weeks.  The monoecious cultivars continue growing rapidly for a longer period than the dioecious cultivars which results in them 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.

Figurere 1.  The weekly change in plant height and the maximum height reached by 14 cultivars of industrial hemp.