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As one might know, plant morphological characteristics refer to the physical attributes or features of a plant that are visible. These characteristics differ among various plant species. It is important to note that there may even be variations within the different strains and cultivars. As in the case of Cannabis sativa, however, here are some common morphological characteristics of hemp plants:
● Growth pattern
The growth habit of hemp can vary depending on factors such as genetics, environmental conditions, and cultivation practices. Hemp is an annual plant that typically grows upright with a sturdy, fibrous stalk. Hemp plants are typically tall, with some varieties reaching heights of 3 to 6 meters depending on the variety and growing conditions. However, the upright growth pattern is characteristic of the species and is often desired for optimal fibre production.
The life cycle of hemp is divided into four growth phases, from four to six months, germination and emergence, vegetative development, flowering and seed generation, and senescence. The vegetative phase is further separated into two stages: juvenile photosensitive and flowering. After planting, the hemp fibre growth phase consists of emergence, active growth, flowering, full flowering, and end of flowering.
Development stages of hemp
Prior to seed dispersal, seed hemp will go through an extra seed development and maturation period. Floral hemp (for cannabinoids) can be started with seeds. If planted from seeds, the plants will grow in phases similar to seed hemp. However, the female flowers of flowering hemp are supposed to bloom should not be pollinated.
Cultivation of industrial hemp for fibre and for grain in France
Seed germination starts with water absorption, leading to root emergence. Depending on environmental conditions, this process lasts between 3 to 7 days. The vegetative growth phase is marked by stem and leaf development, progressing slowly until the fifth pair of leaves. Afterward, rapid growth occurs, with the stem and internodes elongating quickly. During this stage, hemp requires the most water and nutrients.
The transition from the vegetative to the generative phase, when flower and seed formation begins, is indicated by a shift in leaf position. Soon after, the first flower buds (primordia) emerge, often at the leaf axils, and flowering progresses in stages. The time between flower formation and full bloom ranges from 4 to 6 weeks. In dioecious plants, male flowers appear about two weeks before female ones. Male flowering starts when the first pistillate flowers open to release pollen, beginning from the lower flowers of the branched inflorescence. Full male flowering is reached when at least 50% of anthers are open, marking an essential stage for determining the ideal fibre hemp harvest time. At this peak, known as technical maturity, harvesting is conducted to maximize fibre yield and quality.
Unlike male flowers, female flowers are more subtle in appearance. They first develop as green calyx flowers at the leaf nodes, followed by the emergence of stigmas. The female flower reaches its peak when 50% of the calyxes are fully formed. After successful fertilization, the stigmas turn red, signalling the start of fruit development. It takes another 4 to 6 weeks for the seeds to mature, with technical maturity for seed harvesting reached when more than 50% of the seeds harden. For dual-purpose hemp (both fibre and seed production), this stage is typically awaited before harvesting.
The third and final phase of senescence is marked by the drying of the foliage and stem, followed by the stem's deterioration.
● Root system
It consists of a spindle-shaped central or primary root, usually penetrating up to 40-50 cm, sometimes up to 1.5 - 2 m in depth and lateral branches, growing radially up to 30-45 cm. Up to 90% of the total root mass is located in layer 1 - 30 cm. The mass of the root system of female plants exceeds that of males by 2.0 - 2.5 times. The root system of hemp has a poor absorptive capacity.
Hemp root system
● Stem
One of the most distinctive characteristics of hemp plants is their strong and durable fibres, which are found primarily in the stem. These fibres have been used for thousands of years to make textiles, rope, paper, and other products. The stem of a hemp plant is typically tall, straight, and robust. It is often characterised by its fibrous nature, containing strong fibres that are used in several industrial applications.
The stems are upright, usually angular, furrowed and branched. As shown below in the figure, stem is divided in three parts, the outer part is generally referred to as “bark” or bast, the inner woody interior referred to as hurds or shives, and the core of stems of most varieties are hollow. Male hemp plants tend to be 10%-15% taller but are slimmer than female plants. The stems’ height can vary from 1 to 6 meters.
On the outside of the stem are the strong bast fibres, commonly known as hemp fibres (Figure 4). The so called primary bast fibres are suitable to produce textile fabrics; the secondary bast fibres are shorter therefore suitable for use in fibre reinforced composites and insulation, however, they are less appropriated for textiles. The high strength (and stiffness) result from the high content of cellulose (about 70 wt.%) and the relatively low angle between the main fibre axis and the orientation of the cellulose molecules in the fibre. (UNCTD, 2023). The outer portion of the hemp stem contains strong and long bast fibre, which provides the strength and quality attributed to the crop.
Cross section of a hemp stem
The stem is made up of a woody material that, after being separated from the bast fibres, is known as shives or hurds. It is a byproduct that can be used to make paper, animal bedding, and building materials. A glue layer exists between the bast fibres and the woody material that must be removed during processing.
Hemp’s weight distribution is roughly 25% bast fibres, 50% shives, and 25% other crop elements. Therefore, the stem is a predominant component of hemp, contributing significantly to the amount of biomass that can be harnessed. When planted at a high density, hemp has a long stem and few branches. As the plant matures, a sturdy, woody stem develops, typically measuring between 1 to 6 meters in height and 0.5 to 2.0 centimetres in diameter and does not have much foliage besides near the top of the plant.
Female plants have a longer vegetation period, which results in the formation of thicker fibre cells (lignification) and a higher proportion of secondary fibre bundles compared to male plants. However, male plants tend to have better fibre quality, as they contain a greater proportion of fine and long primary fibres. Generally, the longer the plant stem and the thinner its diameter, the higher the fibre content. For optimal industrial fibre processing, the ideal stem dimensions are 2 to 2.5 meters in height and a diameter of 6 to 10 mm.
Hemp stem
● Leaves
Hemp leaves are palmately compound, meaning they have multiple leaflets radiating from a central point, resembling the shape of a hand. These leaves are typically serrated along the margins and possess a rich green colour. The green, palmate leaves are comprised of seven lobes. A single plant can form between 13 and 15 pairs of leaves (TFZ, 2020). The size and shape of the leaflets can vary significantly depending on the crop’s genetic origins. Compared to other Cannabis species, hemp plants often have a narrower leaf shape, characterised by long, palmately complex leaves with serrated leaflets. Both the upper and lower surfaces of hemp leaves are covered with tiny glands called resinous trichomes, which produce a sticky substance. Additionally, other types of small structures known as epidermal trichomes can sometimes be found on the leaves (UNCTD, 2023).
Hemp field
● Flowers
Hemp plants undergo flowering in the third stage of their life cycle, during which they produce small greenish-yellow flowers that grow in dense clusters. Hemp naturally is a dioecious species, meaning it has distinct male and female plants. However, some commercial varieties are monoecious, meaning a single plant contains both male and female flowers. Male plants are crucial for seed production, as pollination is required for seeds to develop.
The male inflorescence is a loose cluster of flowers at the end of a central stem, known as a rachis. Male flowers contain five greenish-yellow sepals that enclose five densely packed stamens, which unroll to expose pollen-producing anthers. While hemp is primarily wind-pollinated, bees can also contribute to pollination. Male plants typically die shortly after pollination.
Female flowers emerge near the apex of the stalk and form clusters. Each female flower consists of two pistils and a seed pod, with the pistils protected by a tiny sheath called a bract. These flowers contain the plant’s reproductive organs and are typically rich in cannabinoids such as CBD (cannabidiol) and THC (tetrahydrocannabinol). The bracts have the highest concentration of trichome glands, which produce cannabidiols like CBD and THC. CBD hemp is harvested from the buds of female flowers.
● Seeds
Hemp plants yield small, spherical seeds that range in colour from dark brown to black. These seeds are highly nutritious and can be processed to produce hemp seed oil and other culinary products. Commonly referred to as seeds or "little nuts" or “nutlet”, hemp fruits are botanically classified as achenes—indehiscent fruits where the embryo (seed) occupies the entire volume. Each fruit contains a single seed covered by a thin pericarp or outer wall. The seeds are smooth and elliptical, measuring between 0.3 to 0.6 centimetres in length. The two cotyledons, which make up the bulk of the seed's mass, are rich in oil and protein. Typically, achenes weigh between 2.8 and 70 grams, with the most common range being 14 to 21 grams. Monoecious varieties tend to produce smaller seeds compared to dioecious ones. Hemp seeds do not develop uniformly and tend to shatter upon maturity, resulting in seed loss and the emergence of volunteer plants in future growing seasons (Gu & Fulk, 2021).
Hemp seeds
Hemp seeds are essentially small, one-seeded nuts with diameters ranging from 2 to 3 mm. When threshed, they split easily, allowing the seed to be separated. These seeds are oval and slightly flattened, with smooth surfaces that are gray-green or brown, often exhibiting mosaic patterns.
The mass of 1000 seeds generally is between 17 and 25 grams. Certain varieties from regions such as Afghanistan, Chile, China, Japan, and Korea produce particularly large seeds, with 15 to 25 seeds per gram, whereas wild plants in Kashmir yield the smallest seeds, with up to 1000 seeds per gram. Generally, darker seeds are heavier and better developed, while lighter seeds are often immature and sterile.
The seed consists of the embryo and a thin endosperm. The embryo contains the structures for the development of cotyledons and the rootlet. It is therefore rich in oil-containing reserve substances. The endosperm serves as an energy source for the seedling and is rich in starch. In terms of germination, the hemp seed loses its ability to germinate relatively quickly. If this is still 95 % in the year of harvest, it is reduced to 80 % in the following year and after two years the hemp seed is no longer able to germinate. The storage conditions have a considerable influence on the germination capacity. For example, storage at low temperatures of 1 to 2 °C and low relative humidity can prevent the rapid degradation of germination capacity.
These are some of the most important morphological traits of hemp plants, however it is important to note that they can vary between kinds and strains of hemp, as well as in response to growing conditions and farming procedures. However, understanding morphological characteristics is relevant to quickly identify plants, but also because is crucial to comprehend some classification systems, as UNODC (1956) which divided domesticated Cannabis into three different groups:
1. Fibre hemp, long, unbranched plants, with poor seed production
2. Oil seed hemp, short, early maturing plants, with rich seed production
3. Psychoactive/flower producing hemp, short, strongly branched plants, with small dark green leaves
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