thumb|Hop flower in a hop yard in the [[Hallertau, Germany]]
thumb|Cross-section drawing of a hop
alt=Full grown hops .|thumb|upright=1.3|Fully grown [[Vine#Twining vines|hops bines ready for harvest on the Yakama Indian Reservation]]<!--Note to editors: 'bines' here is an actual word, not a typo-->
thumb|Humulus on a house
Hops are the flowers (also called seed cones or strobiles) of the hop plant Humulus lupulus, a member of the Cannabaceae family of flowering plants. They are used primarily as a bittering, flavouring, and stability agent in beer, to which, in addition to bitterness, they impart floral, fruity, or citrus flavours and aromas. Hops are also used for various purposes in other beverages and herbal medicine. The hops plants have separate female and male plants, and only female plants are used for commercial production. The hop plant is a vigorous climbing herbaceous perennial, usually trained to grow up strings in a field called a hopfield, hop garden (in the South of England), or hop yard (in the West Country and United States) when grown commercially. Many different varieties of hops are grown by farmers around the world, with different types used for particular styles of beer.
The first documented use of hops in beer is from the 9th century, though Hildegard of Bingen, 300 years later, is often cited as the earliest documented source. Before this period, brewers used a "gruit", composed of a wide variety of bitter herbs and flowers, including dandelion, burdock root, marigold, horehound (the old German name for horehound, , means 'mountain hops'), ground ivy, and heather. Early documents include mention of a hop garden in the will of Charlemagne's father, Pepin the Short.
Hops are also used in brewing for their antibacterial effect over less desirable microorganisms and for purported benefits including balancing the sweetness of the malt with bitterness and a variety of flavours and aromas.
History
The first documented hop cultivation was in 736, in the Hallertau region of present-day Germany. In 768, hop gardens were left to the Cloister of Saint-Denis in a will of Pepin the Short, the father of Charlemagne. The first mention of hops being used in German brewing was in 1079.
Not until the 13th century did hops begin to start threatening the use of gruit for flavouring. Gruit was used when the nobility levied taxes on hops. Whichever was taxed made the brewer then quickly switch to the other.
In Britain, hopped beer was first imported from Holland around 1400, yet hops were condemned as late as 1519 as a "wicked and pernicious weed".
In Germany, using hops was also a religious and political choice in the early 16th century. There was no tax on hops to be paid to the Catholic church, unlike on gruit. For this reason the Protestants preferred hopped beer.
Hops used in England were imported from France, Holland and Germany and were subject to import duty; it was not until 1524 that hops were first grown in the southeast of England (Kent), when they were introduced as an agricultural crop by Dutch farmers. Consequently, many words used in the hop industry derive from the Dutch language. Hops were then grown as far north as Aberdeen, near breweries for convenience of infrastructure.
According to Thomas Tusser's 1557 Five Hundred Points of Good Husbandry:
In England there were many complaints over the quality of imported hops, the sacks of which were often contaminated by stalks, sand or straw to increase their weight. As a result, in 1603, King James I approved an Act of Parliament banning the practice by which "the Subjects of this Realm have been of late years abused &c. to the Value of £20,000 yearly, besides the Danger of their Healths".
Hop cultivation was begun in the present-day United States in 1629 by English and Dutch farmers. Before prohibition, cultivation was mainly centred around New York, California, Oregon, and Washington state. Problems with powdery mildew and downy mildew devastated New York's production by the 1920s, and California only produces hops on a small scale.