One of the great unanswered questions in the history of beer is why it took 9,000 years or so after brewing began for brewers to start using hops.

Today there are very few beers made without hops. They give beer flavour and, most importantly, they keep it from going off. The shelf life for unhopped ale can be as short as a fortnight or so before it starts to spoil and sour. Hopped beer can last for years. But it took many millennia for brewers to discover this, though they had been using a huge range of other plants to flavour their ale in the meanwhile: the bushy, aromatic moorland shrub bog myrtle, for example, the grassland weed yarrow, the hedgerow plant ground-ivy, even rosemary and sage.

The first documented link between hops and brewing comes from Picardy in Northern France, in 822, where Abbot Adalhard of the Benedictine monastery of Corbie, in the Somme valley near Amiens, wrote a series of statutes on how the abbey should be run. The many rules covered areas such as the duties of the abbey’s tenants, which included gathering of firewood and also of hops – implying wild hops, rather than cultivated ones. Adalhard also said that a tithe (or tenth) of all the malt that came in should be given to the porter of the monastery, and the same with the hops. If this did not supply enough hops, the porter should take steps to get more from elsewhere to make sufficient beer for himself: “De humlone … decima ei portio … detur. Si hoc ei non sufficit, ipse … sibi adquirat unde ad cervisas suas faciendas sufficienter habeat.”

It is important that the Corbie statutes should link hops with beer brewing, because hops had other uses they might have been collected for: to make dyes, for example (brown dye from hop sap and yellow dye from the leaves and cones). The stems can also be used to make ropes, sacking and paper. Thus any mentions in old documents of hops being collected from the wild, or even cultivated, does not mean automatically that the hops were going into beer

But Adalhard’s statutes do not say whether the hops were being used to preserve the beer, or merely to flavour it (the way brewers today dry-hop their beers). Proof that hops were being used the way they are today, as a preservative, does not come for three more centuries, at another Benedictine establishment at Rupertsberg, near Bingen, in the Rhineland. About 1150, Abbess Hildegard of Bingen (1098-1179), mystical philosopher and healer, published a book called Physica Sacra, which translates best as “The Natural World”. Book I, Chapter 61, “De Hoppho”, or “Concerning the hop”, says of the plant: “It is warm and dry, and has a moderate moisture, and is not very useful in benefiting man, because it makes melancholy grow in man and makes the soul of man sad, and weighs down his inner organs. But yet as a result of its own bitterness it keeps some putrefactions from drinks, to which it may be added, so that they may last so much longer.”

By itself this does not prove hops were used in beer, just “in drinks” (in potibus in Hildegard’s original Latin). But in a later chapter, on the ash tree, the abbess wrote: “If you also wish to make beer from oats without hops, but just with grusz [gruit], you should boil it after adding a very large number of ash leaves. That type of beer purges the stomach of the drinker, and renders his heart [literally ‘chest’ or ‘breast’] light and joyous.” Clearly Hildegard knew about brewing beer with hops. The passage also suggests that Hildegard knew about boiling wort, without which just adding hops is not much help in keeping away “putrefactions”.

What probably kept the usefulness of hops from being discovered for so long is that the bittering, preserving resins in hop cones are not very soluble, and the hops need boiling for a long time, around 90 minutes, for what is called isomerisation – the physical change in the hop acids to a more soluble form of the molecule – to take place. Nobody would have boiled hops that long, and thus discovered the isomerisation, without a prior good reason (it takes a lot of fuel, a precious commodity when you have to gather wood by hand, to boil quantities of water for an hour and a half). How was it found out that a good long boil improved both the flavouring and the preserving ability of hops? One possibility is that a dyer, boiling hops to dye cloth, made the discovery that the dye water had a pleasant bitter taste, and told her friend the brewer. But this is just a guess.

When exactly hops began to be cultivated for putting into beer, rather than just being gathered wild from forests, is surprisingly unclear. German sources today claim that hop gardens appear in records dating from the second half of the ninth century in and around Hallertau, in Bavaria, Southern Germany, which is still the world’s largest single hop-growing area. However, they do not specify exact documents in which these hop gardens are mentioned, which makes it impossible to rely on their assertions. The best evidence seems to be that commercial hop cultivation happened in Northern Germany first, and not until the 1100s or 1200s, feeding the breweries of the Hansa trading towns, which were exporting hopped beer from at least the 13th century onwards. (Merchant beer brewers in North German cities eventually became rich enough to join the local aristocracy, something not found in Britain until the 18th century).

The buyers of this beer brewed in cities such as Hamburg and Bremen included the richer inhabitants of Flanders and Holland. Local brewers in the Low Countries reacted by brewing hopped beer themselves, and by the 1360s or thereabouts Dutch towns were growing hops to supply their brewers. From around 1390 brewing of hopped beer took off in Holland, with Flanders following a decade or so later.

The first import of Low Countries “beere” into England seems to have come in 1362/63, when James Dodynessone of Amsterdam paid a toll on beer at Great Yarmouth in 1361-62. (There is a reference in the Norwich Leet Roll of 1288-9 to cervesiam flandrensem, or Flanders ale, which “Ricardo Somer”, Richard Summer, was fined 2s for selling occulte, secretly, thus depriving the bailiffs of money due on the ale of ale. However, this was probably too early to be a hopped brew). Further mentions of beer imports followed, gradually increasing in frequency: Henry Vandale (a man with a Dutch-sounding surname) bought four barrels of “beere” in London in 1372. A ship’s captain named Clays Johanson arrived in London in July 1384 with a cargo that included earthenware dishes, Holland linen cloth and beer. Other records of beer imports in the late 14th century come from Newcastle, Scarborough, Lynn, Ipswich, Winchelsea and Sussex. At the end of the 14th century Great Yarmouth was importing 40 to 80 barrels of beer a month, while in 1397-8 Colchester imported 100 barrels of beer.

However, the first brewer of the hopped drink in England does not appear until 1412, when Agnes Smyth, “Dutchman” (sic – and “Dutch” meant “German” at this time, rather than “person from the Netherlands”), was making beer in Colchester. The English beer trade seems to have stayed in the hands of immigrants from the Low Countries for the next century, as the conservative-minded natives stuck to their unhopped ale. As a result, the first beer brewers in England apparently imported all their hops from across the Channel, with no attempt to cultivate the plant here until early in the 16th century.

When exactly the first hops were grown in England is, again, uncertain – dates given by different writers range from 1511 to 1524. But the place where they were first planted was almost certainly Kent: one tradition says the first hop garden was established in 1520, in the parish of Westbere, near Canterbury. By 1569 English hop cultivation was sufficiently advanced for one agricultural writer, the Sussex landowner Leonard Mascall (or Mascal), to claim that “one pound of our hoppes dried and ordered will go as far as two poundes of the best hoppe that come from overseas.”

Five years later, in 1574 the first book in English solely devoted to hop growing was written by a 36-year-old Kentish landowner called Reynolde (or Reginald) Scot. His A Perfite Platform of a Hoppe Garden, filled with woodcut illustrations to aid the less literate Elizabethan farmer, went to three editions in four years. By 1577 hop cultivation had reached Herefordshire, where a “hoppyarde” was running at Whitbourne, near Bromyard. The differences found in the terminology used between the West Midlands and South East England – hop yard for hop garden, hop kiln for oast house, crib rather than bin for the container the hops are picked into, for example – suggest hop-growing was started independently in the two places.

In 1655 hops were being grown in at least 14 English counties, including Somerset, though Kent accounted for a third of the total crop. The use of bitter hop alternatives such as broom and wormwood was banned by Parliament in 1710 to ensure brewers did not try to avoid the new hop tax of a penny a pound. However, although it was reckoned an acre of hops would bring in more profit than 50 acres of arable land in a good year, the hop farmer’s life was more insecure than any other branch of agriculture. An old Kentish rhyme said of hops: “First the flea, then the fly/Then the mould, then they die.” Annual yields swung wildly: 1.57 million pounds of hops in 1726, for example, 20.39 million pounds the following year.

John Banister of Horton Kirby in Kent, in a book called Synopsis of Husbandry, published in 1799, identified a long list of different types of hop, including “the Flemish, the Canterbury, the Goldings, the Farnham etc.” Goldings is still regarded as one of the great English hops, though it now comes in several varieties: it was supposedly propagated from an especially fine plant spotted not long before 1790 by Mr Golding of Malling, who was still alive in 1798.

Stourbridge fair, just south of Cambridge, was the biggest hop mart in England in the late 17th and early 18th century. By the late 18th century Southwark, in London, which was handily placed on the road up from Canterbury, had become the country’s most important hop centre. (When “three-letter” telephone exchange names were introduced in London before the First World War, Southwark’s was HOP – even today, many Southwark telephone numbers still contain the numerical equivalent, 407.)

There were 35,000 acres of hops under cultivation in Britain by 1800, and 50,000 by 1850. Hop farming hit a peak of 71,789 acres in 1878, with hops grown in 40 English counties, though the tiny Scottish hop industry, which operated in just five counties, disappeared in 1871, and Welsh hop growing ended in 1874. Hops from Farnham in Surrey were regarded as the finest, followed by Kentish hops, though some brewers paid a premium for North-Clay hops grown on the stiff clays of Nottinghamshire, which were reckoned to be the best for strong keeping-beers.

New varieties of hop were still appearing: Bramling, an “early” variety of Goldings, named after the hamlet near Canterbury where it was discovered, was introduced about 1865. According to later writers, Richard Fuggle of Brenchley, Kent unveiled the hop variety that still bears his name, the second great English hop, in 1875, though this has since been thrown into doubt. The first Fuggles plant supposedly originated from a seed thrown out with the crumbs of a hop-picker’s lunch at George Stace’s farm in Horsmonden, Kent in 1861. A book on English hops published in 1919 listed more than 30 different hop types.

However, tastes were changing away towards sweeter, less-hopped milds, and at the same time imports of hops from Europe and the United States were increasing. total hop acreage plunged to 51,000 in 1900, a drop of almost 30 per cent in 22 years. The restrictions on brewing of the First World War also hit the industry, and by 1918 there were just 16,000 acres under cultivation.

At the beginning of the 20th century it was realised that the soft resin content in hops, that is, the part that contains the alpha acids, which was first measured in 1888, was the best test of the keeping qualities they would bring to beer. Gradually brewers began to buy hops on their soft resin content, and growers began to plant varieties that contained a higher proportion of soft resins.

Researchers at Wye College, near Ashford in Kent cross-bred English hop varieties with native American hops, which generally have twice the alpha acid content of Europeans, but a “fruity” aroma English brewers had looked down upon. One, Bramling Cross, born in 1927 of a female Bramling hop with a wild male hop from Manitoba in Canada, has become appreciated for its “blackcurrant” nose. Others have had the fruitiness bred out, and the high alpha acid kept in. Many hop varieties in use today in Europe, the United States and Australia are based on hops first developed at Wye College.

However higher alpha acid content in the hops means fewer hops are needed in the beer means fewer hops need be grown. Total hop cultivation in England in 1976 was 17,000 acres. By 1997 it was just 7,500 acres, with 2,400 acres in Kent, 2,300 in Herefordshire, 870 in Worcestershire, 300 in Sussex and tiny amounts in Surrey, Hampshire and Oxfordshire. In 2006 that total had dropped to only 2,400, just two per cent of the worldwide acreage.