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Annual fibre crops

On this page we cover agronomy and the carbon capture potential of flax and hemp

Image source: Elsoms Seeds

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Industrial hemp

Background of hemp in the UK

Hemp, Cannabis sativa, was historically widely grown crop in the UK. Production declined in the 19th Century as imported cotton displaced hemp from textile markets. Industrial hemp can currently be grown in the field under licence to produce fibre, but the harvesting of the leaves, buds, roots and flowers is prohibited in the UK. Hemp is a source of high-value phytochemicals that are used within the health and cosmetic sectors, notably cannabidiol (CBD) extracts and hemp seed oil. Other emerging markets include food (hemp seed), feed and specialist industrial oils.

Hemp agronomy

Hemp is a fast-growing, spring-sown annual crop with a deep taproot, suited to a range of soil types and climates. However, the need for moist field conditions for retting, followed by dry weather before the crop is baled, removed, and stored, presents challenges in very dry or very wet regions. Retting is the process whereby over 3-6 weeks micro-organisms rot away cellular tissues and pectin’s surrounding the fibre bundles, facilitating separation of the fibre from the stem.

Hemp is:

  • competitive with weeds, provided that it establishes quickly

  • has a relatively low requirement for fertilisers

  • Relatively low requirement for plant protection products.

Pulling flax in Scotland 2023.JPG

Flax, Linum usitatissimum, is the same plant species as Linseed, but grown for its fibre rather than its oil. Varieties of flax are specifically bred for fibre quality. There is increasing interest in growing flax in the UK. Uses are shown on the right.

Flax is a spring-sown crop, best suited to free-draining silty or loamy soils. It takes about 100 days to mature, normally in August. At harvest, flax can be pulled or cut depending on the end market. Pulling is for long fibre, for scutching and eventually spinning. This is for high end markets, but does come with high costs - machinery is specialised, and risk is high. Flax is being cut in the same way as Hemp, particularly in the UK. This short and medium fiber goes through the same decortication process as Hemp, it is then used in the same products and applications as Hemp - building materials, mattresses, co-spinning with wool for upholstery and some garments, and composites - woven and non-woven. This cutting and decorticating is a far cheaper option, with less risks, but less financial reward as well.

As with hemp, a mix of moist conditions and dry weather are required to enable the retting process and then baling and removal of the crop.

Flax has a low requirement for nitrogen fertiliser, but potash is important for fibre quality. Effective weed control is vital, typically involving a pre-emergence herbicide treatment. Most disease risks can be controlled through fungicide treatment at flowering. The main pest threat is large flax flea beetle, which can cause damage to cotyledons or first leaves of young plant.

Flax 

Textiles

Insulation

Rope and twine

Aeromotive, automotive composites

Capturing carbon in fibre crops

As biorenewable materials, industrial hemp and flax are an opportunity to increase longer-term carbon capture through crop production. Estimates of carbon capture for industrial hemp range from 8 to 22 tonnes of CO2 per hectare of crop, which may exceed that captured within a year by one hectare of woodland.

Stored carbon dioxide has been estimated at 1,393 kg CO2e per tonne of fibre for industrial hemp and 1,386 kg CO2e per tonne of fibre for flax (Nova, 2019). ‘Farm to factory gate’ carbon footprints in Europe have been estimated at 406 kg CO2e per tonne of intermediate fibre (not final product) for industrial hemp, and 349 kg CO2e per tonne of intermediate fibre for flax (Nova, 2019). This does not include methane or CO2 emitted during retting. Fertiliser and associated nitrous oxide emissions account for around half of the total emissions.

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