The sex life of a cannabis plant begins and ends with a seed. The genetic potential contained in this modest package is the result of millions of years of evolution, and thousands of years of human desire. Sexual reproduction shapes every aspect of cannabis cultivation and germplasm development.

Cannabis seeds vary in appearance based on their provenance.  Mature cannabis seeds are typically 2-5 mm in diameter, or about the size of a match head. 

Seed Dormancy

The same features which enable seed durability can present challenges to achieving high germination percentages for cultivation purposes.  Position zero in the lifecycle of a cannabis annual is seed dormancy.

Seed dormancy describes the development stage where the seed serves as a fortress of genetic information and will resist germination by design. This is an evolved condition to ensure that seeds of this annual herb do not germinate in the fall, and instead overwinter for more favorable growing conditions. 

Evolution has favored a diversity of reproductive strategies for various species. About 94% of all plant species are monoecious with both male and female reproductive parts occurring on each individual plant. In contrast, cannabis plants are typically dioecious with male and female reproductive organs occurring on separate plants.   

Self Pollination

Monoecious plants self-pollinate to varying extents. Self-pollination is the most intensive form of inbreeding as it involves the re-combination of genetic information from the same plant to produce seeds.

This reproductive strategy enables the preservation of traits, or plant characteristics, that have evolved to suit prevalent conditions. The extent to which monoecious plants self-pollinate is enabled by physical flower characteristics, genetic self sterility, timing of pollen release and other factors.

Outcrossing

Many monoecious plants also outcross to some extent and pollen from one plant can cross-fertilize a different plant of the same species and create seeds.

Frequent outcrossing enables plants to quickly adapt to changing climate conditions due to the phenotypic variability that is introduced through gene recombination between different individuals at each generation.

Dioecy

Dioecy in cannabis is determined by the inheritance of sex chromosomes resulting in populations that typically consist of about 50% male, and 50% female individuals. This distinction is unusual in the plant kingdom, but common for mammals like us.

Male plants are known as staminate plants and develop stamens and anthers for pollen production and distribution.  Female plants are referred to as pistillate plants and form pistils, or reproductive organs which enable seed development at maturity. 

A small percentage of cannabis plants will express both male and female reproductive parts.  These individuals are known as intersex plants.

Biosynthesis

Gynoecious populations consisting exclusively of pistillate, or female plants, are desirable for cannabis flowers, and cannabinoid production.

Cannabinoids are bioactive compounds and are highly valued by humans for their psychological and medical attributes.

In cannabis, these structures produce a variety of bioactive compounds including terpenoids, flavonoids, and esters, in addition to cannabinoids. These consortia of secondary metabolites are highly valued for their aesthetic and psychoactive properties.

There are also compelling reasons to maintain and improve varieties through sexual reproduction.  Achenes provide stable storage of genetic information, and sexual recombination offers opportunities for breeding incredible cannabis.

For cultivation of autoflower varieties, seed production is required, as cloning is not feasible due to the determinate nature of these populations.

Sexing Cannabis Plants

When crops are grown from seed for cannabinoid production it is important to cull staminate individuals as quickly as possible to avoid fertilization of pistillate plants.

As a cannabis plant approaches developmental maturity, pre-flowers will form next to the stipule at the base of each node, allowing for visual sorting by sex.  A staminate plant will form anthers which resemble small balls with short stalks, while pistillate plants will display distinctive paired stigmas or white hairs which emerge from each pistil.

Visual identification is straightforward, but with some strains the pre-flowers may not be evident until after flowering has been induced.  This is inconvenient timing for plant culls, and can result in gaps in flowering canopies, and accidental seeding if growers are not attentive.

Genetic tests are available which can accurately determine the sex of individual plants soon after emergence from the seed.  Typically, a cotyledon or seed leaf is removed from a seedling and submitted to a lab for genetic testing. Within large scale production systems, genetic testing is typically less expensive than maintaining plants until their sex can be visually determined.

Once flowering has been induced, a series of reproductive developments enable procreation and seed production.  These developments are the result of interaction between genetics and environment as directed by hormone regulation.

Hormone Regulation

Hormones are chemical signals that are used to communicate environmental conditions, and genetic intent within organisms. 

Without the guidance provided by hormone signaling, plants would consist of masses of undifferentiated cells.  Hormones interact with each other, and with other plant signaling systems in ways that remain imperfectly understood. The primary hormones involved with sexual differentiation in plants are ethylene, and gibberellin.

Following induction, staminate plants will undergo a period of rapid etiolation, or elongation as stamens begin to form in clusters before opening to release pollen grains by the millions.  These pollen grains are 20-30 microns in size and anemophilic or wind loving. 

Pollen Dispersal

Pistillate plants also undergo a rapid internodal elongation or etiolation following induction that can result in a doubling of biomass within a few weeks. 

Plant elongation and leaf production decline as pistils conglomerate to form complex flowers. This development coincides with the peak of floral expansion and reproductive viability.  When reproduction has been frustrated by pollen exclusion, pistillate inflorescences will continue to swell and accrue trichomes for several weeks before their pollen receptivity wanes.

Pollen Pollution

As cannabis production increases, unwanted pollination events have become an increasing liability for cannabis cultivators. It is important that sensimilla (without seed) cannabis growers consider this issue when designing air filtration systems for grow facilities.

Triploid cannabis germplasms have been developed which resist pollination by diploid pollen pollution.  All growers should understand that pollen pollution is a growing issue and take every possible measure to avoid pollinating their neighbors’ crops.

While dioecy is the norm for cannabis cultivars, a diversity of sexual expression is evident within various populations.  At some point, far back in prehistory, Cannabis was monecious, and later developed dioecy as a reproductive feature in response to evolutionary pressures.  Some commercial hemp populations have been selected for monoecy to optimize production of seed.  Some ruderal, or wild type landrace varieties have been reported to consist of mostly staminate plants.

Intersex Plants

Although the sex of Cannabis plants is inherited through sex chromosomes, the formation of male and female flowers is managed through hormone regulation. 

Intersex plants can be more difficult to detect than staminate plants as the timing and physiology of male flower formation is atypical.

In cannabis populations propagated from seed, a minimum of two to three plants per thousand can be expected to express intersex tendencies. These plants should be culled, and seeds resulting from unintentional pollination should be discarded to prevent the inheritance of intersex tendencies in subsequent generations.

Seed based propagation is generally more cost efficient than vegetative propagation. 

For cultivation of autoflower varieties, seed production is required, as vegetative propagation is not feasible due to the determinate nature of these varieties.

Silver Thiosulphate Solution

Creation of feminized seeds is achieved by treating the pistillate pollen donor plant with an ethylene blocker. Silver thiosulphate solution (STS) is commonly used to block ethylene signaling that would normally direct formation of pistillate reproductive organs. This results in female plants forming pollen producing anthers in addition to pistils.

The anthers of these XX plants produce XX pollen which is distinct from the XY pollen produced by staminate plants under natural reproductive conditions.  Seeds which result from pollination by reversed pistillate plants will be all female as only X chromosomes can be inherited from either parent.  Intersex plants can occur in feminized populations as well as in dioecious populations.

Intersex Avoidance

The extent to which male flowers are produced on pistillate plants following STS treatment varies between cultivars.  Some pistillate plants will produce profuse male flowers and distribute pollen widely, other plants will form very few male flowers within the floral mass, with very little pollen escaping naturally.

It is important to avoid using parent plants that have genetic intersex tendencies for seed production. The extent to which a pistillate plant produces pollen in response to ethylene blocking is likely related to that plant’s inherent tendency to express intersex traits when exposed to environmental stressors.

Breeders should consider selecting pollen from plants that produce the fewest male flowers following STS treatments.  This requires pollen harvesting and conservation strategies. To collect pollen from these donors, the flowers must be dried rapidly in a desiccant chamber and then mechanically tumbled in a pollen extractor.

Selective Pollination

Once pollen has been harvested, it can be desiccated, and stored in the refrigerator where it will remain viable for several months. 

Pollen is often mixed with flour or talc to serve as a bulking agent and make distribution of the microscopic grains more efficient. Female flowers can be manually pollinated using a makeup brush or paintbrush.  In some cases, the pollinated flowers can be enveloped in a paper or polyethylene bag for a few days to prevent pollen contamination of nearby plants.

In addition to creating feminized seed for flower production, the maintenance of gynoecious cannabis lines through induced monoecious phenotypes offers additional tools for germplasm improvement. Both parent plants in a cross can be selected based on their floral characteristics, rather than by selecting males in each generation based on other criteria.

Selfing

Cannabis plants can also be self-pollinated (selfed) by pollinating a cultivar with its pollen donor clone. Self-pollination is the most intensive form of inbreeding and is useful in creating true breeding cannabis lines within realistic timeframes.

True breeding lines can then be hybridized to restore heterosis (hybrid vigour) while retaining a high degree of phenotypic consistency in the F1 hybrid seed line for flower production.

The complexity of sexual expression in cannabis populations presents opportunities for cultivation and germplasm development.  Our ability to leverage the sexual frustration of pistillate cannabis plants increases the synthesis of valuable bioactive compounds, and our function as human pollinators has enabled the cannabis plant to extend its range globally. The reproductive features of this crop guide its cultivation and ensure the continued co-evolution of our species.