Cannabis is predominantly dioecious with staminate "male" and pistillate "female" flowers occurring on separate plants. However many monoecious varieties have also been described, in which individual plants bear both male and female flowers. (Although monoecious plants are often referred to as "hermaphrodites", true hermaphrodites – which are less common in Cannabis – bear staminate and pistillate structures together on individual flowers, whereas monoecious plants bear male and female flowers at different locations on the same plant.)
Marijuana, hemp and cannabis are common names for plants of the genus Cannabis. The term "hemp" is used for cannabis plants that are grown for non-drug use. The presence (or absence) of Delta-9-tetrahydrocannabinol (THC), the psychotropic ingredient, is the main difference between industrial hemp and medicinal cannabis.
The term marijuana is not used by professionals due to its association with the demonization of hemp in the 1930s by the US military industrial complex.
Cannabis sativa L. and cannabis indica are “short-day plants”. In other words they flower when the night lengths exceed their critical photoperiod they require a continuous period of darkness before floral development can begin. Natural night time light, such as moonlight or lightning, is not of sufficient brightness or duration to interrupt flowering.
In general, short-day (i.e.long-night) plants flower as days grow shorter (and nights grow longer) after 21 June in the northern hemisphere, which is during summer or autumn. The length of the dark period required to induce flowering differs among species and varieties of a species.
Photoperiodism affects flowering by inducing the shoot to produce floral buds instead of leaves and lateral buds.
Cannabis ruderalis is “autoflowering” and will flower when the plant reaches maturity.
There are three main sub species of the cannabis plant: Cannabis sativa L., Cannabis ruderalis and Cannabis indica.
Cannabis sativa L. has been selectively bred to create varieties used in the cultivation of industrial hemp for industrial application and nutrition. Hemp is a less psychoactive form of cannabis. Cannabis sativa L. is generally tall and randomly branched, high in fibre and grain (seed). Many fibre and grain products and industrial uses are made from Cannabis sativa L. Cannabis sativa L. can also be used to produce THC by using specific varieties and growing conditions.
Cannabis ruderalis will produce flowers based on its age rather than the light cycle (photoperiod). This kind of flowering is also known as auto flowering. Cannabis ruderalis was developed for Russian climate, latitude and elevation and was traditionally used for a multitude of industrial and nutritional applications. Cannabis ruderalis can also be used for the production of THC by using specific varieties and growing conditions. Cannabis ruderalis has often been crossed with the sativa L. and indica in order to produce an “auto-flowering” plant.
Cannabis indica has poor fibre quality and is used to develop drugs for recreation and medicine. The plant is relatively short, conical and densely branched. Cannabis indica tends to have a higher Δ9-THC and a lower Cannabidiol (CBD) content than C. sativa L. Medical cannabis, dependent on the strain, can have THC concentrations of up to 38%.
The hemp plant is a stout, aromatic, erect annual herb. The slender cane-like stalks are hollow except at the tip and base. The leaves are compound with palmate shape, and the flowers are small and greenish yellow. Seed-producing flowers form elongate, spike-like clusters growing on the pistillate, or female, plants. Pollen-producing flowers form many-branched clusters on staminate, or male, plants.
Lanceolate: narrowly ovate and tapering to a point.
Palmate: when all lobes originate from a central point (lobes are incomplete divisions in the leaves).
Scabrous: rough to the touch.
Hemp Plant Types
Industrial hemp is made up of varieties of Cannabis sativa L. and Cannabis ruderalis that contain less than 0.2% Δ9- Tetrahydrocannabinol (THC).
Hemp is an annual broadleaf plant with a taproot. Hemp generally requires 110 days for its growth and should receive around 10-12 inches (25.4-30.5 cm) of rainfall throughout the growing season. Soil moisture will affect the ability of the root to penetrate deep into the soil profile although hemp can demonstrate adaptations to a variety of soil moisture conditions. In some soils the taproot may penetrate 6-12 inches (15-30 cm) deep. PH should be between 6 and 7.5.
In compacted or wet soils, the taproot remains short and the plant produces more lateral, fibrous roots.
Hemp plants are warmth-loving (thermophilic) and sun-loving (heliotropic). Bio-mass and seed production will be reduced if plants do not receive enough sun and warmth throughout the growing season.
Hemp leaves are compound palmate with serrated leaflets. The lower leaf pairs usually occur in an opposite leaf arrangement on the stem.
Depending on variety and growing conditions, the first pair of true leaves usually have a single leaflet. The number gradually increases up to a maximum of about thirteen leaflets per leaf (usually seven or nine). At the top of a flowering plant, the number of leaflets diminishes to a single leaflet per leaf.
Depending on weather and growing conditions, hemp may be slow to establish and grow in the first few weeks of the growing season. Later, during hemp’s elongation phase, it is capable of very rapid growth under ideal growing conditions. Hemp can grow up to 7 to 10 cm a day during this vegetative growth period which usually occurs in July and early August in Canada.
Plant Flowering and Growth Stages
Hemp can be either of the dioecious type or monoecious type. In dioecious type the plant population is made up of pure female heads and up to 50% male heads. In monoecious type, the plant population has both male and female plant parts on the same head.
Industrial hemp is normally dioecious which means a plant will have either all male or all female flowers on it. The plants rely on the wind to complete pollination (anemophilous).
Prior to flowering, the sex of the plant is indistinguishable except for some general trends in growth habit. For example, in less crowded growing conditions, the female plants tend to be shorter with more branching than the male plants. The male plants will often break off easier when pulled due to less fibre at that growth stage.
When inflorescence development begins, male flower primordium hangs from long multi-branched loose clusters, formed of small individual flower buds along an axis up to 30 centimeters long.
The female primordium is identified by the enlargement of a tapered, curved, tubular bract (floral sheath). Female flowers are more tightly clustered and have two long white, yellowish or pinkish stigmas protruding from each bract. Each seed forms inside a bract.
The flowers of female plants are arranged in racemes and can produce hundreds of seeds.
Female flowering and seed set are indeterminate. The seeds continue to develop and mature over an extended period of time. There will be both ripe and immature seeds on the same plants at time of grain harvest. When about 50% of the seed is exposed in each bract, it is ready to harvest.
Monoecious plants have both male and female parts on the same branch or raceme.
An ideal monoecious plant will have a few male flowers arranged in whorls at the base of the flower head, while the female flowers are formed at the top. After shedding pollen, the male “flower” dries up and disappears. Monoecious plants are 30% to 40% self-pollinated.
A pure male plant will die soon after pollen shed. Female plants continue to grow and develop seeds.
Genetic factors dispose a plant to become male or female whereas environmental factors including the light cycle can alter the sexual expression. The light-dark cycle determines when hemp begins to flower (males appear).
A hemp plant in the vegetative growth phase requires more than 12–13 hours of light per day to stay vegetative. Flowering usually occurs when darkness equals at least 12 hours per day. The flowering cycle can last for weeks, depending on the variety and environmental conditions.
Regardless of seeding date, hemp will generally start to flower about the same time every year (photoperiod). Planting hemp early in the spring can produce a taller more robust plant. Late seeding of hemp can potentially produce shorter plants, but both seeding dates will result in flowering about the same time every year.
As the growing season progresses, the bottom leaves die (atrophy) due to the lack of sunlight that can reach the bottom of the crop. Male plants die soon after shedding pollen.
The EU approved seed list
In order to have a license approved by the Home Office, seed used in the production of industrial hemp in the UK have to be from the EU approved list. Selective breeding (not genetic modification) has been used to produce varieties that when tested contain less than 0.2% THC at the flowering stage. There are moves afoot to increase the level to 0.3% THC but the only difference this will make is to increase the number of approved varieties on the EU list.