Hemp, well known the world over as a multipurpose crop. Used for thousands of years, the cultivation process has been refined for the particular end-use. Impinging on a balance of biology, environment and economy, the cultivation process for Hemp is next to none. But what is this process, and what are the more important factors to consider? Let’s find out.
Cannabis Sativa L., also known as Hemp, is well known around the world as a multiuse and multifunctional crop which provides raw materials for an incredibly diverse number of industrial applications. This once very important industrial crop declined after World War II, which became unstoppable. In the late 1960s, Hemp was sparse in Western European countries (1). In the early 1990s, a renewed interest in Hemp cultivation began. The reason being that Hemp had a simple cultivation process and was increasingly seen as being sustainable (2). A few decades later between 10,00 to 15,000 hectares in the European Union alone are used for hemp cultivation (3).
The main target of modern Hemp cultivation is to maximize biomass production, however, as we all know, the end-use destination has specific quality requirements in terms of the properties of different components of the Hemp plant (4). These components usually involve the bast fibre, specific characteristics and composition of oils and proteins in the seeds, or the profile of secondary metabolites in the flower head (5).
However, in order to move forward towards modern cultivation best practices, we must look back on the history of this valuable crop and its plant biology.
The history of Hemp cultivation is dated back to at least 6000 years ago by archaeological findings and ancient records in China. Even then, general descriptions of the cultivation techniques were recorded in the book of Si Shengzhi. Although Hemp documents from ancient Europe are relatively scarce, it is known that Scythians brought Hemp to Europe from Asia during their migrations in 1500 B.C (6).
During the 15th Century, documents relating to the use of Hemp as a crop in Europe became well-known. The popularity of Hemp grew substantially after becoming a popular fibre source for a number of products, including, in the production of textiles and ropes. This commercial role grew substantially from the 18th to 19th centuries. Of which, in the 20th century, Hemp cultivations rapidly declined as a result of the progressive inclusion of synthetic fibres in textiles as well as the increasing cost of labour (7).
The quality of Hemp fibre, seeds and heads impinge on the characteristics of the overall plant. This includes the morphology of the fibre bundles, the chemical composition of elementary fibre, fibre surface characteristics, fibre fitness and fibre tensile strength. Because of this, looking at the specific biological characteristics of the plant is valuable (8).
Hemp is an annually reproducing plant, which is normally wind-pollinated. The stem consists of different morphological regions, the stem consists of an external outer layer of cells, much like the skin, followed by fibre cells, referred to as bast fibres. Hemp cultivation targets the production of the primary bast fibres. The quantity and quality of these fibres need to be monitored and controlled in order to meet the requirements of the end-use products (9).
Hemp has a wide range of environmental adaptation, however, several things around its biology mean that different types of the Hemp plant are best suited to different environments. Of particular importance for achieving the best cultivation yields is: 1) Photoperiod; 2) Crop density; 3) Soil fertility; 4) Moisture and, 5) Sowing/Harvesting dates.
Hemp Cultivation Best Practices
In Hemp, a very specific and essential environmental parameter is the photoperiod (day-length). Flowering time is critical in Hemp yield determination, including both quantity and quality. Many investigations have shown that a “sub-optimal” photoperiod results in “pre-flowering”. In Hemp, during the first weeks of emergence, most of the available area/energy is associated with leaf growth. This increase in the biomass of leaves means the plant absorbs more radiation from the sun. This is where you require more sun exposure as a higher amount of radiation intercepted by the plant corresponds with higher biomass. For a plant to flower, the amount of daylight required is roughly 14 hours per day. It is best, if your end-use product is associated with stem yield, to delay flowering until very late in the season, this will ensure more vegetative growth, achieving higher stem biomass (10).
2. Crop Density
Plant density affects fibre content as well as the fibre fitness and amount of associated proteins and oils/ secondary metabolites in the seeds. As such, the plant densities for crops designated for nutritional purposes sit at 15 plants per metre squared for essential oils and 30 to 75 plants per metre squared for seed output only. In plants with end-uses relating to fibre, the optimal densities are from 50 to 750 plants per metre squared, depending on your location (11).
3. Soil Fertility
The fertility of the soil is important for most plant crops. For Hemp, nitrogen appears to be the most critical, compared to other minerals such as phosphorus or potassium. However, the impact of nitrogen depends on the original soil fertility, supplementing the soil with nitrogen in areas with high original nitrogen concentrations will produce negligible results and could potentially be damaging to the crop yield. If the original soil was nitrogen limiting, significant yield increases will correspond with larger levels of nitrogen fertilisation, this can be up to 120 kg of nitrogen per hectare (12).
In drier conditions, stem height, diameter and layer thickness of fibres are significantly lower. In this case, it is obvious moisture, in terms of irrigation or rainfall, is important for crop yield. Investigations have shown that, for optimum yield, hemp requires at least 500-700 mm of moisture, with 250-300 mm needed to be available during the vegetative growth phase of the crops. This will increase in areas of high evaporation (13).
5. Sowing/Harvesting Dates
Sowing date is usually defined on the basis of soil temperature and water availability. In general, early sowing corresponds with higher stem and seed yields. One must keep in mind however, that is southern environments, sowing should be slightly later as early sowing will result in the plant not reaching the required photoperiod, reducing the vegetative growth.
The dates of harvesting are incredibly important for the quality and quantity of yields, however, these are often different depending on the end-use products of the crops (14).
Investigations have shown that when harvests are delayed, there is an increase in biomass and stem yield but a reduction in the yield of bast fibre. For example, in China, bast fibre content was 22.3% 90 days after the first emergence of the plant but reduces to 19.2% at 100 days post-emergence. Ideally, the harvesting date should coincide with the end-use, with seed and flower production greater later in the cropping cycle (15).
The Bottom Line
From photoperiod to when you sow and harvest your crop, there are a number of factors to consider to get the most out of your hemp farm. Depending on your desired end-use, there are numerous factors you must consider before you start your venture. Looking back on history, we have provided a number of best-practice solutions to help you get on your way.