Hemp is an annual herbaceous plant of the species Cannabis sativa, meaning “useful hemp.” It is a high yield commercial fibre crop which flourishes in areas with temperate climates, such as Canada. Hemp grows successfully at a density of at least 150 plants per square meter, and reaches a height of two to five meters in a three month growing season. Every part of the plant can be used commercially. The stalk of the hemp plant is harvested for its fibres. The fibre length and the content of cellulose and lignin are important quality parameters for raw material used in the cordage, textile, paper and fibreboard industries.
Bast Fibres
Hemp has traditionally been grown for its valuable and versatile high quality (primary bast) fibres. The production of these fibres has traditionally been a very labour intensive process. After harvesting, the hemp stalks are soaked with water to initiate a process of retting (the decompositional separation of the bark-like bast fibres from the inner woody core, the hurd). After the retting process, the plants are dried and then the fibre must be separated from the hurds, shaken out, and cleaned. Recently, alternative fibre separation processes have been developed, using technologies such as ultrasound, enzymes and steam explosion, which are much less labour intensive. Once separated, the bast fibres are ready for spinning and weaving into textiles, or for pulping into high quality pulp. Because of their high tensile strength, bast fibres are ideal for such specialised paper products as: tea bags, industrial filters, currency paper, or cigarette paper.
Bast fibres come in two varieties:
- Primary bast fibres which are long and low in lignin. These fibres are the most valuable part of the stalk, and are generally considered to be among the strongest natural fibres known to mankind.
- Secondary bast fibres which are medium length and higher in lignin are less valuable and become more prevalent when the hemp plants are grown less densely (therefore less competition for light), and thus grow shorter, fatter stalks.
Hemp Hurds
The hurds are the short, fibred, inner woody-core of the hemp plant, which comprises 70-80% of the stalk. They are composed of libriform fibres which are short and high in lignin.
The hurds are essentially the by-product of the process of extracting bast fibre from the hemp stalks, and were traditionally considered waste. Though the fibres are shorter, the lignin content of hurds is similar to wood, so there are opportunities for using the hurds for tissue or newsprint pulp.
Hurds can also be used to produce a wide range of products including rayon, biomass fuel, cellophane, food additives, and industrial fabrication materials. The most exciting use is in construction, where the hurds are mixed with lime to form a building material which can be moulded into both interior and exterior walls.
Hemp Seeds
Hemp seeds are also a potentially valuable commodity. The seeds have exceptional nutritional value, and are second only to Soya beans as a source of complete vegetable protein. Hemp seeds contain all 8 essential amino acids in the correct proportions humans require, and also contain 30-35% oil by weight.
Hemp seed oil is approximately 80% polyunsaturated essential fatty acids (EFA’s). Furthermore, the proportion of these oils in hemp seeds most closely match the ratios which have been determined to be most beneficial to human nutrition.
However, although the oil is very healthy, this high percentage of polyunsaturated fats also makes hemp seed oil somewhat unstable and so subject to fairly rapid rancidity unless stored and treated properly. Hemp seed oil can be extracted or expressed and used in cooking, or industrial uses such as paints, varnishes, detergents, cosmetics, and lubrication. The left over seed casings are a rich source of protein which can be ground into flour.
Hemp and the environment
In both its cultivation and uses, hemp is considered an exceptionally environmentally friendly crop. Hemp requires little or no pesticides as it is naturally pest resistant. Hemp is also a natural herbicide known for its ability to smother weeds when grown at a density suitable for producing high quality bast fibre.
Hemp also has a lower net nutrient requirements than other common farm crops, since it can return 60-70% of the nutrients it takes from the soil when dried in the field. However, prior to the nutrient recycling, hemp extracts more nutrients per hectare than grain crops due to its fast biomass production.
In rotation with other crops, its deep root system is also very beneficial as it is effective in preventing erosion, cleaning the ground, providing a disease break, and helping the soil structure by aerating the soil for future crops
Hemp is also a particularly high yield fibre crop. In fact, an acre of hemp produces more biomass than most other crops. As a result, hemp can be used effectively in many applications as an alternative to wood or fossil fuels. For example, hemp can be used as a renewable, low polluting source of biomass fuel, or hemp pulp could easily replace wood pulp in paper making.
Agronomics
There are two potentially viable approaches to growing hemp commercially: growing hemp for fibre or for seed. If hemp is grown for fibre, it is sown very densely (a seed rate of 55-70 kg/ha is standard, though for very high quality textile fibre a much higher seed rate can be used). Since hemp grows so quickly, at this density hemp can effectively out compete weeds, and so weed control measures (herbicides) are not needed.
If hemp is grown for seed, it is grown much less densely (typically 10 -15kg/ha) and is not as effective at suppressing weeds, so herbicides will probably be required. Hemp seed may be drilled or broadcast, though drilling is recommended for uniformity. A standard grain drill or modified alfalfa seeder can be used for sowing.
Pesticides are generally considered unnecessary in the cultivation of hemp. Another positive aspect of the crop is that once planted, no further husbandry is required until harvest, thereby minimising labour costs and energy consumption.