Humulus lupulus (common hop or hops) is a species of flowering plant in the hemp family (Cannabaceae), native to Europe, western Asia and North America. It is a dioecious, perennial, herbaceous climbing plant which sends up new shoots in early spring and dies back to a cold-hardy rhizome in autumn.
Hops are described as bine plants rather than vine because, unlike vines, they have stiff downward facing hairs that provide stability and allow them to climb. These shoots allow H. lupulus to grow anywhere from 4.6 to 6.1 metres (15 to 20 ft). Hops have fragrant, wind-pollinated flowers that attract butterflies.
The female cone shaped fruits from H. lupulus are used by breweries to preserve and flavor beer, and so H. lupulus is widely cultivated for use by the brewing industry. The fragrant flower cones, known as hops, impart a bitter flavor, and also have aromatic and preservative qualities. H. lupulus contains myrcene, humulene, xanthohumol, myrcenol, linalool, tannins, and resin.
The genus name Humulus is a Medieval name that was at some point Latinized after being borrowed from a Germanic source exhibiting the h•m•l consonant cluster, as in Middle Low German homele. According to Soviet Iranist V. Abaev this could be a word of Sarmatian origin which is presented in modern Ossetian language (Ossetian: Хуымæллæг) and derives from proto-Iranian hauma-arayka, an Aryan haoma. From Sarmatian dialects this word spread across Eurasia, thus creating a group of related words in Turkic, Finno-Ugric, Slavic and Germanic languages (see Russian: хмель, Chuvash хăмла, Finnish humala, Hungarian komló, Mordovian комла, Avar хомеллег).
The specific epithet, lupulus, is Latin for small wolf. The name refers to the plant's tendency to strangle other plants, mainly osiers or basket willows (Salix viminalis), like a wolf does a sheep. Hops could be seen growing over these willows so often that it was named the willow-wolf. The English word hop is derived from the Middle Dutch word hoppe, with the same meaning.
H. lupulus can grow to be 10 meters tall and because it is a perennial herbaceous plant it goes through several growing seasons sometimes living up to 20 years. H. lupulus has simple leaves that can be opposite or alternate with 3-5 lobes. The staminate flowers do not have petals, while the pistillate flowers’ petals completely cover the fruit. The cones found on female plants are called strobili. The fruit of H. lupulus is an achene, meaning that the fruit is dry but does not split open at maturity. The achene is surrounded by tepals and lupulin secreting glands are concentrated on the fruit.
H. lupulus grows best in the latitude range of 38°-51° in full sun with moderate amounts of rainfall. It uses the longer summer days as a cue for when to flower which is usually around July/ August.
H. lupulus can cause dermatitis to some who handle them. It is thought that about 1 in 30 people are affected by this.
The five varieties of this species (Humulus lupulus) are:
- H. l. var. lupulus – Europe, western Asia
- H. l. var. cordifolius – eastern Asia
- H. l. var. lupuloides (syn. H. americanus) – eastern North America
- H. l. var. neomexicanus - western North American.
- H. l. var. pubescens – midwestern and eastern North America 
Many cultivated varieties are found in the list of hop varieties. A yellow-leafed ornamental cultivar, Humulus lupulus 'Aureus', is cultivated for garden use. It is also known as golden hop, and holds the Royal Horticultural Society's Award of Garden Merit (AGM).
Use in brewing
The chemical compounds found in H. lupulus are main components in flavoring and bittering beer. Some other compounds help with creating foam in beer. Chemicals such as linalool and aldehydes contribute to the flavor of beer. The main components of bitterness in beer are iso-alpha acids, with many other compounds contributing to the overall bitterness of beer. Until the middle ages, Myrica gale was the most common plant used for brewing beer. H. lupulus took off as a flavoring agent for beer because it contains preserving agents, making the beer viable for longer.
Pests and diseases
- Hop leaf aphid (Phorodon humuli)
- Red spider mite (Tetranychus urticae)
- Japanese Beetle (Popillia japonica)
- Comma butterfly (Polygonia c-album)
- H. lupulus contains xanthohumol, which is converted by large intestine bacteria into the phytoestrogen 8-prenylnaringenin, which may have a relative binding affinity to estrogen receptors  as well as potentiating effects on GABAA receptor activity
- H. lupulus extract is antimicrobial, an activity which has been exploited in the manufacture of natural deodorant.
- Spent H. lupulus extract has also been shown to have antimicrobial and anti-biofilm activities, raising the possibility this waste product of the brewing industry could be developed for medical applications.
- Extracts of the bitter alpha-acids present in H. lupulus have been shown to decrease nocturnal activity, acting as a sleep aide, in certain concentrations.
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