Background

The Hemp Plant

Hemp Plant Types

Industrial hemp is made up of varieties of Cannabis sativa L. that contain less than 0.3% Δ9- Tetrahydrocannabinol (THC).

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 nondrug use, such as Cannabis sativa L. or narrow leaf hemp (i.e., hemp).

Typical Hemp Leaf

There are three species of the hemp plant: Cannabis sativa L., Cannabis indica and Cannabis ruderalis. Delta-9-tetrahydrocannabinol (THC), the active ingredient of marijuana, is the main difference between the different species.

Cannabis sativa L. a subspecies of Cannabis, is known as hemp. Varieties have a THC content of less than 0.3%. Hemp is a non-psychoactive form of cannabis. Cannabis sativa L. is generally tall and randomly branched, high in fibre and grain. It is low in THC. Many fibre and grain products and industrial uses have been made from Cannabis sativa L. (hemp).

Cannabis indica has poor fibre quality and is used to make drugs for recreation and medicine.  The plant is relatively short, conical and densely branched. Cannabis indica tend to have a higher Δ9-THC and a lower Cannabidiol (CBD) content than C. sativa L. Marijuana, dependent on the strain, can have THC concentrations of 18% to 38%.

C. sativa L. and C. indica varieties are sensitive to day length to induce flowering (photoperiod sensitive).

Cannabis ruderalis is not common in North America. 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.

Plant Description 

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, 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. 

Typical Hemp Roots

In compacted or wet soils, the taproot remains short and the plant produces more lateral, fibrous roots. A hemp root is well equipped to grow deep into the soil profile to pick up nutrients that have been left behind by previous crops or nutrients that have leached down in the soil profile. This attribute of the large root is also beneficial in finding water in dry years or in sandy soils.

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.

Opposite Leaf Arrangement

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.

 

Weekly Hemp Stalk Growth

 

Under ideal growing conditions hemp can germinate and emerge within three to five days. Details are found in the PDF, PowerPoint, and Movie below.

Growth Stages of Hemp

Growth Stages of Hemp

 

 

Plant Fibre Structure

The primary outer bast fibres in the phloem tissues surrounding the hollow woody core of hemp stalks is similar to bark on a tree. These long, strong fibres surround the hemp stalk and are low in cell binding lignin.

Processed Hurd and Bast Fibre

The bast fibres have properties similar to other fibre crops like flax. The bast fibres are known for their length, strength, durability, absorbency, anti-mildew and antimicrobial properties.

Hemp Hurd

The core, or woody part of the stem, contains the hurd which is high in cellulose. Hurd can be up to twice as absorbent as wood shavings, making it an excellent animal bedding and garden mulch. It can be easily blended with lime to create a strong yet lightweight concrete or plaster (hempcrete). It is biodegradable and possesses anti-mildew and antimicrobial properties.

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.

Hemp Male Inflorescence at beginning of pollen release

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.

Female flower. Seed development within bracts.

The flowers of female plants are arranged in racemes and can produce hundreds of seeds.

Female head with seed protruding from bracts. Near maturity.

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.

Monoecious Hemp Head

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.