Plant 252

Humulus lupulus L. (Cannabaceae)

There are three species in the northern temperate genus Humulus. The best known species is hops, an herbaceous perennial native to Europe and western Asia but widely grown in temperate regions around the world. Hops are related to cannabis and to the woody genus Celtis. It is a vigorous climbing plant, often seen growing to the top of hedgerow trees in the British Isles. Its shoots, up to nine metres in length, can completely cover other plants, perhaps explaining the specific epithet lupulus, meaning 'little wolf'. Plants are dioecious with separate male and female plants. Male flowers are inconspicuous, arranged in loose panicles, whilst female flowers are arranged in distinctive clusters that resemble cones (strobiles).

Since the ninth century, in north-western Europe, the female flower heads have been used in the brewing of beer; hops were first introduced to Kent from Flanders in the early fifteenth century. Hops, with their bacteriostatic properties, were originally used solely as a preservative in beer; its addition made the distinction between 'ale' and 'beer'. Although it is now an essential ingredient for creating the characteristic taste of beer, the bitterness that it imparts was initially mistrusted and unpopular with consumers. Hops were described to the Parliament of Henry VIII as a 'wicked weed that would spoil the taste of the drink and endanger the people'.

Agricultural production of hops in the British Isles reached its zenith in the 1870s. Hops were cultivated widely in the south-east counties, requiring a large force of seasonal workers until mechanisation was adopted in the late 1950s. Hops are grown up frameworks of wooden poles and wire. Workers maintaining these structures in the nineteenth century would walk on stilts between the plants.

There are many cultivated varieties of hops producing a wide variety of flavours in beer; many modern varieties produce strong citrus flavours. Harvested hops are dried very carefully to avoid moulds at high moisture contents and the loss of volatile oils at high temperatures.

Hop fibre has been used to make high-quality paper and coarse cloth. Young shoots of the plant are often described as a delicacy when eaten like asparagus. In traditional herbal medicine, hops have been used for a wide variety of complaints. In this context, they are most widely used as a sedative and hypnotic. The pollen can irritate human skin; dermatitis on the hands and face was a common ailment among hop pickers.

Further reading

Henning JA et al. 2002. Genetic diversity among world hop accessions grown in the USA. Crop Science 44: 411-417.

Murakami A et al. 2006. Molecular phylogeny of wild hops, Humulus lupulus L. Heredity 97: 66-74.

Small E 1980. The relationships of hop cultivars and wild variants of Humulus lupulus. Canadian Journal of Botany 58: 676-686.

James Penny