Cannabis sativa ssp.

General info

Hemp is one of the most ancient ‘domesticated’ crops in the world, it originated in temperate Asia but migrated worldwide. It is a naturally herbaceous dioecious plant that can grow between 1 – 6 meters tall and produces woody stalks. Though hybrid varieties have been developed that are monoecious to improve reproduction and uniformity of plants for agricultural plantation purposes. It is among the fastest growing plants on earth alongside bamboo.

Whilst certain strains were bred for their high THC content (1 – 20%) and hallucinogenic properties, other varieties were bred for their excellent fibre potential for utilisation as canvas and cordage (which inherently had lower THC values, max 0.2% in Europe for example). Alongside its well-known and -studied medicinal benefits, Hemp has numerous applications from pulp/papermaking to textiles and bio composites.

Cultivation and agronomy

Yields range from 2 – 18 tonnes ha^-1 yr^-1 depending on cultivars, environment and cultivation techniques. Optimal temperature growth is between 13 – 22 °C making it feasible for growth outdoors in some temperate environments. It has a variety of self-protective properties, making it resistant to diseases and insect damage, reducing biocide requirements. It also requires far less water than crops such as maize and other bioenergy crops. For liquid fuel biodiesel considerations, hemp was demonstrated to outperform canola, soybean, and corn and was only marginally outperformed by oil palm produced biodiesel, with regards to L/ha (hemp – 784 L/ha) it notably had lower sulphur content and a higher flash point temperature than most of these making it a desirable biodiesel. Since the production of biodiesel from hemp doesn’t utilise the straw or seeds, this can act as a desirable production strategy for increased economic benefits.

Despite this, solid fuel considerations of hemp, including briquettes and pellets, are far better with a significantly high value of around 100 GJ ha^-1 yr^-1 which, whilst lower than some competing energy crops, is mitigated by the low growth requirements of hemp and possible co-production benefits. It possesses a higher heating value of 19.24 MJ Kg^-1 comparable to, and exceeding, species such as poplar and willow. The strong fibres are a challenge to cultivation requiring adapted machinery. Post-harvest separation and processing are also required to achieve the highest economic gain from the crop.

Sida hermaphrodita

General info

S. hermaphrodita is a perennial herbaceous forb species native to the eastern United States. It is neither a grass nor a short rotation woody crop, instead it can be defined as a soft woody herbaceous energy crop. The species was introduced from the USA to Europe in the 1930s and was initially trialled as a fodder and fibre plant due to its favourable proteins, amino acids, vitamin C, carotene and other important elements (similar to alfalfa).

More modern breeds have been developed to grow to larger heights than wildtype variants, with up to 4m heights achievable within 4 months in East European studies. It is often cultivated from seeds and subsequently shoots emerge year on year from belowground rhizome structures. The species are known for a deep penetrating root system allowing exploitation of significantly deep levels of soil nutrients and water.

Cultivation and agronomy

In Central and East European research climates, growth of S. hermaphrodita has indicated the potential to harvest 9–17 oven dried tonnes ha-1 yr-1 for crops established from seeds or 20 oven dried tonnes ha-1 yr-1 from root cutting established crops. It has an energy profile comparable to Arundo, switchgrass, poplar and willow at 18.75 MJ Kg-1 and continues to produce biomass at equivalent levels for up to 24 years. Though actual biomass yields achieved are heavily influence by environment and specific management conditions on cultivation.

This harvested biomass is favourable because it is very easily granulated, meaning there is minimal energy consideration associated with the drying process for the production of pellets and briquettes compared to many other bioenergy crops.

This species is adaptable to a range of soil conditions but grows best under wet soils. Despite this, in direct comparison with Salix spp. for example, it is far less susceptible to yield impacts relating to lower moisture and rainfall. These properties make it another bioenergy crop with high potential for utilisation in marginal soils to avoid food production competition (though it is noted to achieve sufficient above ground biomass for efficiency to other crops, fertilisation may be required).

It has been demonstrated in a 6-year study that different dressing of S. hermaphrodita seeds can have significant impacts on subsequent years’ cultivation biomass yields. Stalk bending issues have been observed due to both fungal infections and high wind loads on this species potentially impacting biomass production. Trials to investigate planting densities for agricultural bioenergy planting of this species are currently limited but, in those that have been performed, 4 plants per m2 ­demonstrated the best 3-year returns with regards to tonnes of dry mater ha-1 despite individual year benefits to some of the lower plantation densities.

A key cultivation consideration for this species is the suppression of weeds via herbicides in the establishment period, which may require subsequent research into more environmentally considerate weeding strategies in future. One suggested application in this direction is initial intercropping with legumes to outcompete weeds, whilst also supplying N to the soil for S. hermaphrodita benefit and to prevent some levels of evaporation to provide more moist soils as desired in years of low precipitation. S. hermaphrodita can easily be harvested, with crop choppers as applied to maize, meaning bespoke machinery is not an issue.