Hemp: the next supercrop?

In 1533 Henry VIII made the cultivation of hemp (Cannabis sativa) compulsory by law for all farmers that owned a certain amount of land; he wanted the strong, rot-resistant fibres from the plant for the ropes, sails and clothing of his new British navy. Every ship carried a cargo of hemp seed, and this was the first crop sown in newly colonised lands. Hemp was also commonly used for food, medicine, textiles, fishing nets and animal feed and bedding.

The Industrial Revolution led to the demise of hemp cultivation, however, and it eventually became a Schedule 1 substance at the UN narcotics conference in 1961. Last year, the World Health Organization (WHO) admitted historical misjudgement on the wrongful scheduling of cannabis, and on 20 March the UN votes on WHO's recommendations to remove cannabis from Schedule 1 and hemp from all drug scheduling so farmers can grow the whole plant, which could be good news for them.

Currently, hemp can only be grown in the UK under a Home Office licence for cultivation of cannabis with a low level of THC, the psychoactive compound, as legislation considers the plant's leaves and flowers as class B controlled substances; this is even though industrial hemp cultivars contain less than 0.2 per cent THC, making the plant non-psychoactive. The expense, bureaucracy and difficulty of obtaining a licence puts farmers off growing hemp, but the outcome of the UN vote could change this. There are some generic routes to market for farmers, and work is going on to expand the end-use market.

This is not to say that hemp is not agriculturally viable in the UK. All the signs point towards it being the next big crop, as it ticks the boxes for sustainable agriculture, food security, enhancing soil health and carbon sequestration. Its myriad potential markets could also place hemp in direct competition with fossil fuels for energy generation and storage. It can be turned into a graphene substitute, and can even be made into super capacitors and batteries. It is the next generation renewable energy source that can outcompete lithium batteries.

Hemp is an annual plant that is sown in spring, generally at the beginning of May in the UK, and harvested at the end of summer or beginning of autumn. Although it can be grown as a monoculture crop for several years without depleting the soil, it is advisable to grow it as part of a rotating crop. As with all spring crops, hemp allows a break in the autumn rotation and is useful for reducing weeds such as black grass, brome grass and shepherd's purse that are difficult to control. Hemp can be successfully grown without pesticides or herbicides as the high density and shading of this crop mean it quickly outcompetes weeds, and there are few pest issues.

Shallow seeding at 12.5–25mm in warm soil of 8–10°C or higher and good soil moisture will support fast, uniform emergence. Hemp can be seeded with conventional equipment such as air seed drills, hoe-press drills and disk-press drills. Air delivery systems can crack the seeds, so you may need to adjust the pressure to prevent this. Seed density depends on the variety, and this will correspond with the intended use.

One of the biggest decisions is choosing the right variety: there are currently 66 approved varieties listed on the EU common catalogue. Consider soil type, climatic conditions, latitude and most importantly, how to harvest. Varieties can grow 0.91–4.5m high. Cannabis is naturally dioicous, but most have now been bred to be monoecious – that is, having both sexes on the same plant. Some varieties also have an auto-flowering gene that shortens the vegetative cycle and allows the crop to bloom early.

If growing primarily for seed, you will most likely choose a short auto-flowering variety with a seed density of 100–150 plants per square metre. This density allows the plant to produce side branches, and thus more flowers and seed. These short varieties can also be harvested using a conventional combine, although care must be taken as the strong fibres can wrap around the machinery and even the bearings. If you are growing primarily for seed, organic registration will provide a better price, at around £1,500–£2,000/t and you can expect a yield of up to 1t/ha. With the growth of vegetarianism and veganism, the organic hemp seed market is secure and has great potential.

Crops grown for fibre have a higher seed density and produce tall, straight plants with no branching. Seed density can be up to 300 plants per square metre, but a dual crop is preferable as it can produce multiple products. The tops of the plants can be harvested to collect the flowers and seeds and the stalks can be cut and processed to produce bast fibre and the inner, woody part of the stalk, called the shiv. A dual crop can produce 5–6t/ha of biomass, comprising 3–3.5t shiv, 1.75–2.5t fibre and 0.6–1t of dust, the latter of which can be compressed into biofuel pellets or blocks. Market prices for these depend on the intended use and quality.

Adapted or specialist equipment is needed to harvest the stalks and these need to be retted – where micro-organisms and moisture help separate the bast from surrounding tissues – and turned for several weeks before baling. The straw then needs to be processed by decortication, which in turn separates the bast fibre from the shiv. Currently, it is the lack of infrastructure for harvesting and processing that is the major issue for the UK hemp industry. However, there are plans to develop farmers' cooperatives to share harvesting machinery and research and development into new methods of processing on site, without the need for retting or conventional decortication.

The cultivation of hemp connects to several policy goals in the UK including the 25-year environment plan, climate change and emissions policy, the Clean Growth Strategy, bioeconomy, rural development policy and the Brexit green paper.

Guy Coxall is chairman of the think tank Hemptank UK  info@hemptank.co.uk

Related competencies include: Land use and diversification