Crops Hemp (Cannabis sativa)

Summary Information



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Young plants

The hemp plant is a herbaceous annual, with a deeply-penetrating taproot. The main stem is erect, and may in fibre types attain a height of up to 4 m; the degree of branching varies with the closeness of planting.

Stem base of female plant

The stalked compound leaves each consist of up to 11 leaflets, radiating from a common centre; each leaflet is pointed and has a lightly serrated edge.

Flower head of male plant

Flowers are borne in groups on the upper portions of the plant: male and female flowers may occur together on the same plant, but in many varieties occur on separate unisexual plants. The male flowers are individually small, usually cream or pink-tinged; female flowers are green and unobtrusive. Generally female plants tend to be larger than male ones, the latter ceasing to grow and maturing after flowering.

Seeds

Successful hemp culture requires a deep, well-drained soil and the absence of frost during early growth. Seed should be sown thickly, to establish a population of 150-200 plants per square metre; this encourages straight growth of stems and an absence of branching, improving fibre quality and aiding weed suppression. Nitrogen fertiliser may be required at a rate of up to 125 kg per ha, while phosphate and potash needs may be met by application of 50-75 kg each per ha. No pest or disease problems have yet become apparent.

Retted stem showing fibres

For best-quality fibre the crop should be cut at flowering (usually during August), and swathed so that field retting can take place; usually the swath will need to be turned during the retting process, which loosens the fibres from the central core of the stem, and may take several weeks. Yield of dry stem may be 7-12 t per ha, with a fibre content of approximately 25 percent. Breeding and research work are currently being undertaken in several European countries, including studies into utilisation of the crop. For example, in the Netherlands, research effort is directed towards use of the whole stem for pulping: in the United Kingdom, where the central pith of the stem is being marketed as an absorbent and easily compostable bedding material for horses, the emphasis is on the development of distinct markets for the long fibres in textile mixtures and as reinforcement in building materials.